UBC Theses and Dissertations

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UBC Theses and Dissertations

Design study for third harmonic flat-topping of the Triumf 520 MeV cyclotron Michelson, David George 1986

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DESIGN STUDY FOR THIRD HARMONIC F L A T - T O P P I N G OF THE TRIUMF 520 MeV CYCLOTRON b y D A V I D G E O R G E M I C H E L S O N B . A . S c , T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1 9 8 2 A THES IS SUBMITTED IN PART IAL FULF ILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPL I ED SC IENCE i n T H E F A C U L T Y O F G R A D U A T E S T U D I E S D e p a r t m e n t o f E l e c t r i c a l E n g i n e e r i n g We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d T H E © U N I V E R S I T Y O F B R I T I S H C O L U M B I A J a n u a r y 1986 D a v i d G e o r g e M i c h e l s o n , 1986 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l l m e n t o f t h e r e q u i r e m e n t s f o r a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e l i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s m a y b e g r a n t e d b y t h e h e a d o f my d e p a r t m e n t o r b y h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t b e a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D a v i d G M I C H E L S O N D e p a r t m e n t o f E l e c t r i c a l E n g i n e e r i n g T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 1 9 5 6 M a i n M a l l V a n c o u v e r , B r i t i s h C o l u m b i a , C a n a d a V 6 T 1Y3 •3 I ^QyrujdO^ju^ R>^ Cg A b s t r a c t T h e a c c e l e r a t i n g v o l t a g e i n t h e 5 2 0 M e V / 2 0 0 u A H -c y c l o t r o n a t T R I U M F i s d e v e l o p e d i n a r e s o n a n t c a v i t y a n d c o n s e q u e n t l y h a s a s i n u s o i d a l t i m e d e p e n d e n c e . I t i s p o s s i b l e t o " f l a t - t o p " t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m b y s u p e r i m p o s i n g a t h i r d h a r m o n i c w a v e f o r m o f t h e a p p r o p r i a t e r e l a t i v e a m p l i t u d e a n d p h a s e o n t h e f u n d a m e n t a l w a v e f o r m . T h i s i n c r e a s e s t h e p h a s e a c c e p t a n c e o f t h e c y c l o t r o n b y r e d u c i n g t h e s p r e a d i n e n e r g y g a i n p e r t u r n n o r m a l l y a s s o c i a t e d w i t h t h e s p r e a d i n p h a s e o f t h e i o n b e a m w i t h r e s p e c t t o t h e p e a k o f t h e R F w a v e f o r m . S l i g h t a d j u s t m e n t s t o t h e s h a p e o f t h e w a v e f o r m c a n i n t r o d u c e l o n g i t u d i n a l b u n c h i n g m e c h a n i s m s w h i c h t e n d t o c o m p e n s a t e f o r s e v e r a l s e c o n d o r d e r e f f e c t s t h a t n o r m a l l y l i m i t t h e c y c l o t r o n ' s p e r f o r m a n c e d u r i n g h i g h c u r r e n t o r h i g h e n e r g y r e s o l u t i o n m o d e s o f o p e r a t i o n . T h e r e l a t i v e s i m p l i c i t y o f T R I U M F ' s d e e g e o m e t r y s u g g e s t s t h a t b o t h t h e f u n d a m e n t a l a n d t h i r d h a r m o n i c m o d e s c o u l d b e e x c i t e d i n t h e s a m e r a t h e r t h a n i n s e p a r a t e a c c e l e r a t o r R F c a v i t i e s . I m p l e m e n t i n g s u c h a s y s t e m w i l l r e q u i r e t h e d e v e l o p m e n t o f new c a v i t y c o u p l i n g a n d t u n i n g m e c h a n i s m s f o r t h e e x i s t i n g r a d i o f r e q u e n c y c a v i t y a n d a new r a d i o f r e q u e n c y c o n t r o l s y s t e m . B e c a u s e a c c e s s t o t h e c y c l o t r o n i s s e v e r e l y r e s t r i c t e d b y s c h e d u l e d b e a m p r o d u c t i o n a n d h i g h r e s i d u a l r a d i o a c t i v i t y i n t h e c y c l o t r o n v a c u u m t a n k , a t e s t c a v i t y b u i l t f r o m i i c o m p o n e n t s i d e n t i c a l t o t h o s e u s e d i n t h e c y c l o t r o n R F c a v i t y w a s a d a p t e d f o r i n i t i a l d e v e l o p m e n t a n d t e s t i n g o f t h i r d h a r m o n i c c a v i t y t u n i n g a n d c o n t r o l s y s t e m s a t o p e r a t i o n a l d e e v o l t a g e s ( 1 0 0 k i l o v o l t s a t 23 M H z , 11 k i l o v o l t s a t 6 9 M H z ) a n d u n d e r h a r d v a c u u m ( 1 0 T o r r ) . T h i s t h e s i s d e s c r i b e s : a . t h e s c o p e a n d t e c h n i c a l o b j e c t i v e s o f t h e t h i r d h a r m o n i c f l a t - t o p p i n g p r o j e c t a t T R I U M F i n c l u d i n g r e q u i r e d i m p r o v e m e n t s t o t h e f u n d a m e n t a l R F s y s t e m ; b . t h e d e s i g n a n d i n i t i a l d e v e l o p m e n t o f c a v i t y t u n i n g a n d c o u p l i n g m e c h a n i s m s f o r t h e R F t e s t c a v i t y ; c . t h e d e m o n s t r a t i o n o f a f l a t - t o p p e d a c c e l e r a t i n g v o l t a g e i n t h e t e s t c a v i t y a t o p e r a t i o n a l v o l t a g e l e v e l s w h i l e u n d e r v a c u u m ; a n d , d . t h e d e s i g n a n d i n i t i a l d e v e l o p m e n t o f a p r o t o t y p e v e r s i o n o f t h e new r a d i o f r e q u e n c y c o n t r o l s y s t e m . T h e r e s u l t s o b t a i n e d s h o w t h a t u n d e r r e a l i s t i c c o n d i t i o n s o f v a c u u m a n d R F i n p u t p o w e r , i t i s p o s s i b l e t o s i m u l t a n e o u s l y e x c i t e t h e f u n d a m e n t a l a n d t h i r d h a r m o n i c m o d e s i n a n R F c a v i t y w i t h m e c h a n i c a l c o n s t r u c t i o n a n d o p e r a t i o n a l c h a r a c t e r i s t i c s s i m i l a r t o t h o s e o f t h e c y c l o t r o n R F c a v i t y . i i i T A B L E O F C O N T E N T S A b s t r a c t i i T a b l e o f C o n t e n t s i v L i s t o f T a b l e s i x L i s t o f F i g u r e s x L i s t o f S y m b o l s xv A c k n o w l e d g e m e n t s x v i i CHAPTER ONE - INTRODUCTION 1.1 A M E T H O D F O R I M P R O V I N G T H E O P E R A T I N G C H A R A C T E R I S T I C S O F I S O C H R O N O U S C Y C L O T R O N S 1 1 . 1 . 1 G e n e r a l 1 1 . 1 . 2 H i s t o r i c a l N o t e s 3 1 . 2 S O M E P R I N C I P L E S O F C Y C L O T R O N O P E R A T I O N 6 1 . 3 T H E T R I U M F 5 2 0 M e V C Y C L O T R O N 12 1 . 4 U P G R A D I N G T R I U M F ' S R A D I O F R E Q U E N C Y S Y S T E M 17 1 . 5 T H E S I S O U T L I N E 19 R e f e r e n c e s 21 CHAPTER TWO - MODIFICATION AND CONTROL OF THE ACCELERATING VOLTAGE WAVEFORM 2 . 1 I N T R O D U C T I O N 2 5 2 . 1 . 1 T h e A c c e l e r a t i n g G a p i n t h e T R I U M F C y c l o t r o n 28 2 . 1 . 2 S t a g e s D u r i n g A c c e l e r a t i o n o f t h e I o n B e a m . . 31 2 . 2 T H E S H A P E O F T H E A C C E L E R A T I N G V O L T A G E W A V E F O R M . . . 33 2 . 2 . 1 P h a s e A c c e p t a n c e 3 5 2 . 2 . 2 P h a s e A c c e p t a n c e i n t h e C e n t r a l R e g i o n 38 i v 2 . 2 . 3 M i n i m i z i n g t h e E f f e c t i v e R a d i a l W i d t h o f t h e I o n B e a m P a c k e t 40 2 . 2 . 4 M i n i m i z i n g t h e E f f e c t o f L o n g i t u d i n a l S p a c e C h a r g e F o r c e s 42 2 . 2 . 5 P h a s e H i s t o r y o f t h e M a i n M a g n e t i c F i e l d . . . . 43 2 . 2 . 6 R F W a v e f o r m M o d i f i c a t i o n f o r S e l e c t e d M o d e s o f O p e r a t i o n 44 2 . 3 A M P L I T U D E A N D P H A S E S T A B I L I T Y O F T H E A C C E L E R A T I N G V O L T A G E W A V E F O R M 47 2 . 3 . 1 A c c e l e r a t i o n i n S e p a r a t e d T u r n s 4 7 2 . 3 . 2 E s t i m a t e o f t h e T o l e r a n c e s o n t h e S t a b i l i t y o f t h e A c c e l e r a t i n g V o l t a g e W a v e f o r m 4 9 2 . 4 T H E A C C E L E R A T I N G V O L T A G E P R O F I L E 59 R e f e r e n c e s 62 CHAPTER THREE - DEMONSTRATION OF A FLAT-TOPPED ACCELERATING VOLTAGE IN THE RF SYSTEMS TEST FACILITY CAVITY 3 . 1 I N T R O D U C T I O N 6 5 3 . 2 T H E R F S Y S T E M S T E S T F A C I L I T Y 70 3 . 2 . 1 G e n e r a l D e s c r i p t i o n 70 3 . 2 . 2 D e s c r i p t i o n o f t h e R a d i o F r e q u e n c y C a v i t y . . . 7 9 3 . 3 M E C H A N I S M S F O R T U N I N G T H E R A D I O F R E Q U E N C Y C A V I T Y 88 3 . 3 . 1 I n t r o d u c t i o n 88 3 . 3 . 2 S e l e c t i o n C r i t e r i a f o r C a v i t y T u n i n g S c h e m e s 92 3 . 3 . 3 A n a l y s i s o f t h e D e e L i n e r D e f l e c t i o n T u n i n g S c h e m e 96 3 . 3 . 4 I m p l e m e n t a t i o n o f t h e D e e L i n e r D e f l e c t i o n T u n i n g S c h e m e 104 3 . 4 C O U P L I N G POWER I N T O T H E R A D I O F R E Q U E N C Y C A V I T Y . . . 113 3 . 4 . 1 I n t r o d u c t i o n 113 3 . 4 . 2 D e s i g n C r i t e r i a - T h e C o u p l i n g L o o p 116 v 3 . 4 . 3 D e s i g n C r i t e r i a - L o o p L o c a t i o n 118 3 . 5 D E M O N S T R A T I O N O F A F L A T - T O P P E D A C C E L E R A T I N G V O L T A G E 129 3 . 5 . 1 A u t o m a t i c S t a r t - u p a n d C a v i t y T u n i n g P r o c e d u r e 131 3 . 6 T H E R F S Y S T E M U P G R A D E 142 R e f e r e n c e s 143 C H A P T E R F O U R - C O N C E P T U A L D E S I G N A N D I N I T I A L D E V E L O P M E N T O F T H E NEW R A D I O F R E Q U E N C Y C O N T R O L S Y S T E M 4 . 1 I N T R O D U C T I O N 148 4 . 1 . 1 O r g a n i z a t i o n o f t h e New C o n t r o l S y s t e m 155 4 . 2 S E Q U E N T I A L C O N T R O L O F T H E R F S Y S T E M 177 4 . 2 . 1 P u l s e d M o d e a n d S p a r k D e t e c t i o n a n d R e c o v e r y 182 4 . 2 . 2 S e l f - e x c i t e d M o d e 185 4 . 2 . 3 C l o s i n g t h e A m p l i t u d e F e e d b a c k C o n t r o l L o o p 190 4 . 2 . 4 D r i v e n M o d e 193 4 . 2 . 5 F l a t - t o p p e d M o d e 1 9 5 4 . 2 . 6 T h e R F C o n s o l e a n d t h e L o c a l C o n t r o l P r o c e s s o r 197 4 . 3 T H E P R O T O T Y P E R A D I O F R E Q U E N C Y C O N T R O L S Y S T E M 201 4 . 3 . 1 P h y s i c a l D e s c r i p t i o n 201 4 . 3 . 2 T h e R F S o u r c e a n d M o d u l a t o r S u b - S y s t e m s 2 0 5 4 . 3 . 3 A n a l o g C o n t r o l l e r s 2 1 5 4 . 4 T H E R F D E T E C T I O N S U B - S Y S T E M 2 2 7 4 . 4 . 1 M e a s u r e m e n t o f t h e A c c e l e r a t i n g F i e l d b y I t s E f f e c t o n t h e I o n B e a m 2 2 8 4 . 4 . 2 O r g a n i z a t i o n a n d T o p o l o g y o f t h e New R F D e t e c t i o n S u b - S y s t e m 231 4 . 4 . 3 R F F i l t e r s 241 v i 4 . 4 . 4 T h e R F A m p l i t u d e D e t e c t o r 2 4 2 4 . 4 . 4 R F P h a s e D e t e c t o r s 2 4 5 4 . 5 D E V E L O P M E N T O F T H E F I N A L R F C O N T R O L S Y S T E M D E S I G N 251 R e f e r e n c e s 2 5 6 C H A P T E R F I V E - SUMMARY A N D C O N C L U S I O N S 5 . 1 SUMMARY 261 5 . 2 S T A T U S O F T H E T H I R D H A R M O N I C F L A T - T O P P I N G P R O J E C T P R I O R T O T H E C O M M E N C E M E N T O F T H I S D E S I G N S T U D Y . . . 2 6 3 5 . 3 R E S U L T S O F T H I S D E S I G N S T U D Y 2 6 5 5 . 3 . 1 D e m o n s t r a t i o n o f a F l a t - t o p p e d A c c e l e r a t i n g V o l t a g e i n t h e R F S y s t e m s T e s t F a c i l i t y 2 6 5 5 . 3 . 2 T h e New R F C o n t r o l S y s t e m 2 6 6 5 . 4 F U R T H E R WORK 2 6 7 5 . 4 . 1 P a r a s i t i c M o d e s i n t h e C y c l o t r o n B e a m G a p . . . 2 6 7 5 . 4 . 2 M e c h a n i c a l D e s i g n o f M o d i f i c a t i o n s t o t h e C y c l o t r o n R F C a v i t y 2 6 7 5 . 4 . 3 A S e c o n d P r o t o t y p e R F C o n t r o l S y s t e m 2 6 8 5 . 4 . 4 R F D e t e c t i o n 2 6 9 5 . 5 C O N C L U S I O N S 2 7 0 R e f e r e n c e s 2 7 0 A P P E N D I X A - T H E T R I U M F M E S O N F A C I L I T Y - 1985 271 A P P E N D I X B - T H E T R I U M F C Y C L O T R O N ' S R A D I O F R E Q U E N C Y S Y S T E M B . 1 I N T R O D U C T I O N 2 8 8 B . 2 T H E C Y C L O T R O N R A D I O F R E Q U E N C Y C A V I T Y .' 2 9 0 B . 2 . 1 G e n e r a l D e s c r i p t i o n 2 9 0 B . 2 . 2 T h e R F R e s o n a t o r R e p l a c e m e n t P r o g r a m 2 9 7 v i i B . 3 P R O P O S E D R F S T R U C T U R E S F O R F L A T - T O P P I N G A T T R I U M F 3 0 0 B . 3 . 1 S e p a r a t e C a v i t i e s f o r F u n d a m e n t a l a n d T h i r d H a r m o n i c M o d e s 3 0 0 B . 3 . 2 D e e W i t h i n A D e e 3 0 2 B . 3 . 3 H a r m o n i c a l l y R e s o n a n t R a d i o F r e q u e n c y C a v i t y 3 0 6 B . 4 R E - D E S I G N O F T H E R A D I O F R E Q U E N C Y C A V I T Y 3 0 9 B . 5 SUMMARY O F T H E T E C H N I C A L O B J E C T I V E S O F T H E R A D I O F R E Q U E N C Y S Y S T E M U P G R A D E 3 1 3 B . 5 . 1 R a d i o F r e q u e n c y C a v i t y 3 1 3 B . 5 . 2 R a d i o F r e q u e n c y C o n t r o l S y s t e m 3 1 4 R e f e r e n c e s 3 1 6 APPENDIX C - CALCULATION AND MEASUREMENT OF THE PROPERTIES OF ACCELERATOR RF CAVITIES C . 1 I N T R O D U C T I O N 3 1 9 C . 2 D E S I G N O F T H E R F C A V I T Y 3 1 9 C . 3 C A L C U L A T I O N A N D M E A S U R E M E N T O F T H E P R O P E R T I E S O F T H E C A V I T Y C O U P L I N G N E T W O R K 3 2 2 R e f e r e n c e s 3 2 3 APPENDIX D - OBSTACLES TO SUCCESSFUL EXCITATION OF ACCELERATOR RF CAVITIES D . 1 I N T R O D U C T I O N 3 2 6 D . 2 E X C I T A T I O N O F A N A C C E L E R A T O R R F C A V I T Y I N A I R 331 D . 3 E X C I T A T I O N O F A N A C C E L E R A T O R R F C A V I T Y U N D E R V A C U U M 3 3 5 D . 4 M U L T I P A C T O R I N G 3 3 8 D . 5 O B S E R V A T I O N S O F M U L T I P A C T O R I N G I N T H E T R I U M F R F S Y S T E M S T E S T F A C I L I T Y 3 4 9 D . 6 X - R A Y E M I S S I O N B Y A C C E L E R A T O R R F C A V I T I E S 3 5 3 R e f e r e n c e s 3 5 4 v i i i L I S T O F T A B L E S T a b l e I F o u r G e n e r a t i o n s o f C y c l o t r o n s 9 T a b l e I I I n t e r n a l B e a m C h a r a c t e r i s t i c s a n d C y c l o t r o n D e s i g n P a r a m e t e r s 16 T a b l e I I I P a r a m e t e r s o f t h e S i m p l i f i e d M o d e l o f A c c e l e r a t i o n i n t h e 5 2 0 M e V C y c l o t r o n . . . 5 5 T a b l e I V T o l e r a n c e s R e q u i r e d f o r S e p a r a t e d T u r n A c c e l e r a t i o n 57 T a b l e V E f f e c t o f A m p l i t u d e a n d P h a s e M o d u l a t i o n o f t h e A c c e l e r a t i n g V o l t a g e o n t h e E n e r g y o f t h e I o n B e a m 60 T a b l e V I E x p r e s s i o n s f o r C a v i t y T u n i n g F a c t o r f o r T u n i n g b y D e e L i n e r D e f l e c t i o n 100 T a b l e V I I G a i n a n d R e s p o n s e S e t t i n g s - P r o t o t y p e A n a l o g C o m p e n s a t i o n A m p l i f i e r s 2 2 5 T a b l e V I I I L i n e a r C o e f f i c i e n t o f E x p a n s i o n o f V a r i o u s M e t a l s 3 3 4 i x L I S T O F F I G U R E S F i g . 1.1 R o s s i ' s C y c l o t r o n S q u a r e W a v e R F S y s t e m 4 F i g . 1 . 2 T R I U M F C y c l o t r o n - I n j e c t i o n , A c c e l e r a t i o n , a n d E x t r a c t i o n o f t h e I o n B e a m 13 F i g . 1 . 3 T R I U M F C y c l o t r o n - A r t i s t ' s C o n c e p t i o n 15 F i g . 2 . 1 T R I U M F C y c l o t r o n - C r o s s - s e c t i o n T h r o u g h D e e G a p 29 F i g . 2 . 2 E n e r g y G a i n e d b y t h e I o n B e a m D u r i n g t h e A c c e l e r a t i o n P r o c e s s 32 F i g . 2 . 3 E f f e c t s o f a T h i r d H a r m o n i c C o m p o n e n t o n t h e A c c e l e r a t i n g V o l t a g e W a v e f o r m 36 F i g . 2 . 4 P r i n c i p l e o f A c c e l e r a t i o n w i t h a F l a t - t o p p e d A c c e l e r a t i n g V o l t a g e W a v e f o r m 37 F i g . 2 . 5 E x p e c t e d P h a s e A c c e p t a n c e f o r V a r i o u s S e l e c t i o n C r i t e r i a 3 9 F i g . 2 . 6 E f f e c t o f E n e r g y D r i f t a n d E n e r g y D i s p e r s i o n o n t h e E f f e c t i v e R a d i a l W i d t h o f a B e a m P a c k e t . . 41 F i g . 2 . 7 R F W a v e f o r m s O p t i m i z e d f o r S e l e c t e d M o d e s o f O p e r a t i o n 4 5 F i g . 2 . 8 T R I U M F C y c l o t r o n - C h a r a c t e r i s t i c s o f t h e I o n B e a m D u r i n g A c c e l e r a t i o n 50 F i g . 2 . 9 T R I U M F C y c l o t r o n - T r a j e c t o r y o f t h e I o n B e a m D u r i n g A c c e l e r a t i o n 51 F i g . 2 . 1 0 O r b i t S i m u l a t i o n S h o w i n g R e d u c t i o n o f E f f e c t i v e B e a m W i d t h b y F l a t - t o p p i n g t h e A c c e l e r a t i n g V o l t a g e 58 F i g . 3 . 1 R a d i o F r e q u e n c y S y s t e m s T e s t F a c i l i t y 72 F i g . 3 . 2 R F S y s t e m s T e s t F a c i l i t y - P l a n V i e w 73 F i g . 3 . 3 R F S y s t e m s T e s t F a c i l i t y - R F C a v i t y 74 F i g . 3 . 4 C o m p a r i s o n B e t w e e n t h e T e s t F a c i l i t y C a v i t y a n d t h e C y c l o t r o n R F C a v i t y 7 5 x F i g . 3 . 5 R F S y s t e m s T e s t F a c i l i t y - R F P o w e r S o u r c e s . . 77 F i g . 3 . 6 A T r a n s m i s s i o n L i n e M o d e l o f t h e T e s t F a c i l i t y R F C a v i t y 80 F i g . 3 . 7 S o l u t i o n o f ( 3 . 3 ) , t h e E i g e n v a l u e E q u a t i o n o f t h e T e s t F a c i l i t y R F C a v i t y 82 F i g . 3 . 8 A n E q u i v a l e n t C i r c u i t M o d e l f o r L o o p C o u p l i n g t o t h e T e s t F a c i l i t y R F C a v i t y 8 5 F i g . 3 . 9 V a r i a t i o n o f C a v i t y C o u p l i n g P a r a m e t e r s A s s o c i a t e d w i t h a C a v i t y T u n i n g E r r o r 87 F i g . 3 . 1 0 A P e r t u r b a t i o n M o d e l f o r C a v i t y T u n i n g b y D e e L i n e r D e f l e c t i o n 98 F i g . 3 . 1 1 C a v i t y T u n i n g F a c t o r a s a F u n c t i o n o f D e f l e c -t i o n P o s i t i o n - P o i n t - L i k e D e f l e c t i o n 101 F i g . 3 . 1 2 C a v i t y T u n i n g F a c t o r a s a F u n c t i o n o f D e f l e c -t i o n P o s i t i o n - S t e p - L i k e D e f l e c t i o n ( F u n d a -m e n t a l M o d e ) 102 F i g . 3 . 1 3 C a v i t y T u n i n g F a c t o r a s a F u n c t i o n o f D e f l e c -t i o n P o s i t i o n - S t e p - L i k e D e f l e c t i o n ( T h i r d H a r m o n i c M o d e ) 103 F i g . 3 . 1 4 R F C a v i t y G e o m e t r y - D e e T i p R e g i o n 106 F i g . 3 . 1 5 M e c h a n i s m f o r D e e - L i n e r D e f l e c t i o n 108 F i g . 3 . 1 6 T e s t F a c i l i t y C a v i t y T u n i n g R a n g e - L o w W a t e r P r e s s u r e 110 F i g . 3 . 1 7 T e s t F a c i l i t y C a v i t y T u n i n g R a n g e - H i g h W a t e r P r e s s u r e 111 F i g . 3 . 1 8 A n E q u i v a l e n t C i r c u i t M o d e l f o r T r a n s m i s s i o n T h r o u g h t h e T e s t F a c i l i t y C a v i t y 120 F i g . 3 . 1 9 T r a n s m i s s i o n T h r o u g h a C r i t i c a l l y C o u p l e d R F C a v i t y a s a F u n c t i o n o f t h e C a v i t y C o u p l i n g F a c t o r o f t h e S e c o n d P o r t 121 F i g . 3 . 2 0 O p t i m u m L o c a t i o n s f o r F u n d a m e n t a l a n d T h i r d H a r m o n i c C o u p l i n g L o o p s 128 F i g . 3 . 2 1 C a v i t y D e e V o l t a g e D u r i n g P u l s e d O p e r a t i o n . . 133 F i g . 3 . 2 2 D e m o n s t r a t i o n o f a T h i r d H a r m o n i c F l a t - t o p p e d A c c e l e r a t i n g V o l t a g e i n t h e R F S y s t e m s T e s t F a c i l i t y 134 F i g . 3 . 2 3 T e s t F a c i l i t y R F C a v i t y - T u n i n g C h a r t 140 x i F i g . 4 . 1 T R I U M F R F S y s t e m - R F S y s t e m C o n t r o l l e r ( 1 9 7 3 t o p r e s e n t ) 151 F i g . 4 . 2 A S i m p l e R a d i o F r e q u e n c y C o n t r o l S y s t e m 1 5 6 F i g . 4 . 3 I n c o r p o r a t i o n o f F e e d b a c k C o n t r o l L o o p s i n t o a S i m p l e R a d i o F r e q u e n c y C o n t r o l S y s t e m 161 F i g . 4 . 4 S i m p l i f i e d M o d e l o f t h e New R F C o n t r o l L o o p . 164 F i g . 4 . 5 T h e L o c a l C o n t r o l . P r o c e s s o r a n d i t s D i g i t a l I n t e r f a c e B u s 169 F i g . 4 . 6 G e n e r a l C o n f i g u r a t i o n a n d O u t l i n e o f t h e New R F C o n t r o l S y s t e m 171 F i g . 4 . 7 New R F C o n t r o l S y s t e m - P r o v i s i o n f o r O p e r a t i o n i n S e l f - E x c i t e d M o d e 173 F i g . 4 . 8 New R F C o n t r o l S y s t e m - O p e r a t i n g M o d e s 178 F i g . 4 . 9 ( a ) R F S y s t e m O p e r a t i o n D u r i n g S t a r t - U p 179 F i g . 4 . 9 ( b ) R F S y s t e m O p e r a t i o n i n D r i v e n M o d e 180 F i g . 4 . 1 0 P r o g r a m m e r ' s M o d e l o f t h e P r o t o t y p e R F C o n t r o l S y s t e m 181 F i g . 4 . 1 1 R F S y s t e m s T e s t F a c i l i t y - R F C o n t r o l s R a c k . 2 0 2 F i g . 4 . 1 2 P r o t o t y p e R F C o n t r o l S y s t e m - A n a l o g S i g n a l P r o c e s s i n g C r a t e s 2 0 3 F i g . 4 . 1 3 P r o t o t y p e R F C o n t r o l S y s t e m - C A M A C I n t e r f a c e 2 0 4 F i g . 4 . 1 4 R F S y s t e m s T e s t F a c i l i t y - S a f e t y S y s t e m . . . . 2 0 6 F i g . 4 . 1 5 R F S y s t e m s T e s t F a c i l i t y - C a v i t y T u n i n g C o n t r o l l e r 2 0 7 F i g . 4 . 1 6 P r o t o t y p e R F C o n t r o l S y s t e m - R F S o u r c e 2 0 9 F i g . 4 . 1 7 New R F C o n t r o l S y s t e m - R F M o d u l a t o r C h a i n . . 2 1 0 F i g . 4 . 1 8 C o m p o n e n t s o f t h e R F M o d u l a t o r C h a i n 2 1 2 F i g . 4 . 1 9 A m p l i t u d e a n d P h a s e T r a n s f e r C h a r a c t e r i s t i c s -S R A - 1 B a l a n c e d M i x e r / A m p l i t u d e M o d u l a t o r . . . . 2 1 3 F i g . 4 . 2 0 A m p l i t u d e a n d P h a s e T r a n s f e r C h a r a c t e r i s t i c s -P S E S - 4 E l e c t r o n i c P h a s e S h i f t e r 2 1 4 F i g . 4 . 2 1 P r o t o t y p e R F C o n t r o l S y s t e m - R F A m p l i t u d e M o d u l a t o r D r i v e r s 2 1 6 x i i 4 . 2 2 P r o t o t y p e R F C o n t r o l S y s t e m - R F P h a s e M o d u l a t o r D r i v e r s 2 1 7 4 . 2 3 P r o t o t y p e R F C o n t r o l S y s t e m - E r r o r S i g n a l G e n e r a t i o n a n d C o m p e n s a t i o n 2 1 8 4 . 2 4 ( a ) S i m p l i f i e d D i a g r a m o f A n a l o g C o n t r o l l e r -C o m p e n s a t i o n A m p l i f i e r s 2 2 2 4 . 2 4 ( b ) A n a l o g C o n t r o l l e r - C o m p e n s a t i o n A m p l i f i e r s 2 2 4 4 . 2 5 E q u i v a l e n t C i r c u i t - R F T u n i n g E r r o r a n d A c c e l e r a t i n g V o l t a g e D e t e c t i o n 2 3 2 4 . 2 6 S o u r c e s o f E r r o r i n M e a s u r e m e n t o f t h e A c c e l e r a t i n g V o l t a g e 2 3 3 4 . 2 7 V a r i a t i o n o f t h e D i e l e c t r i c C o n s t a n t o f K a p t o n P o l y i m i d e F i l m w i t h F r e q u e n c y a n d T e m p e r a t u r e 2 3 4 4 . 2 8 C o m p a r i s o n o f T i m e - o f - F I i g h t a n d D e e V o l t a g e P e r t u r b a t i o n S p e c t r a 2 3 7 4 . 2 9 New R F C o n t r o l S y s t e m - R F D e t e c t i o n S y s t e m 2 3 8 4 . 3 0 R F D e t e c t o r / R F C o n t r o l S y s t e m I n t e r f a c e . . . 2 3 9 4 . 3 1 P r o t o t y p e R F C o n t r o l S y s t e m - R F A m p l i t u d e D e t e c t o r .- 2 4 7 4 . 3 2 P r o t o t y p e R F C o n t r o l S y s t e m - S p a r k D e t e c t o r 2 4 7 4 . 3 3 P r o t o t y p e R F C o n t r o l S y s t e m - R F P h a s e D e t e c t o r s 2 4 8 4 . 3 4 R F L i m i t e r b a s e d o n E C L Q u a d L i n e R e c e i v e r . . 2 4 9 4 . 3 5 T r a n s f e r C h a r a c t e r i s t i c s - P r o t o t y p e R F P h a s e D e t e c t o r s 2 5 0 A . 1 T R I U M F - S u m m a r y o f B e a m P r o d u c t i o n ( 1 9 7 5 -1 9 8 4 ) 2 7 3 A . 2 T R I U M F - B e a m l i n e s a n d E x p e r i m e n t a l F a c i l i t i e s 2 7 6 A . 3 T R I U M F 5 2 0 M e V C y c l o t r o n - P l a n V i e w 2 7 8 x i i i F i g . A . 4 T R I U M F 5 2 0 M e V C y c l o t r o n - C r o s s - s e c t i o n a l V i e w 2 7 9 F i g . A . 5 C y c l o t r o n V a u l t a n d S e r v i c e A n n e x - C r o s s -s e c t i o n a l V i e w 2 8 0 F i g . B . 1 C o m p o n e n t s o f T R I U M F ' s R a d i o F r e q u e n c y S y s t e m 2 8 9 F i g . B . 2 T R I U M F R F S y s t e m - 23 M H z P o w e r S o u r c e 291 F i g . B . 3 E v o l u t i o n o f t h e T R I U M F R F C a v i t y f r o m a Q u a r t e r - W a v e S t u b 2 9 6 F i g . B . 4 T h e F l a t - t o p p i n g S y s t e m o f t h e 5 9 0 M e V R i n g C y c l o t r o n a t S I N 3 0 3 F i g . B . 5 A T h i r d H a r m o n i c F l a t - t o p p i n g C a v i t y f o r t h e T R I U M F C y c l o t r o n 3 0 4 F i g . B . 6 A T h i r d H a r m o n i c D e e w i t h i n t h e F u n d a m e n t a l D e e 3 0 5 F i g . B . 7 T h e F u n d a m e n t a l a n d T h i r d H a r m o n i c A c c e l e r a t i n g M o d e s i n T R I U M F ' s R a d i o F r e q u e n c y C a v i t y 3 0 7 F i g . B . 8 V o l t a g e P r o f i l e a l o n g t h e C y c l o t r o n A c c e l e r a t i n g G a p 311 F i g . B . 9 M e a s u r e m e n t s o f t h e F i e l d D i s t r i b u t i o n i n P a r a s i t i c M o d e s E x c i t e d i n t h e C y c l o t r o n B e a m G a p 3 1 2 F i g . D . 1 A S i m p l i f i e d M o d e l o f M u l t i p a c t o r i n g 3 4 5 F i g . D . 2 M u l t i p a c t o r i n g T h r e s h o l d V o l t a g e a s a F u n c t i o n o f S t a r t i n g P h a s e 3 4 7 F i g . D . 3 M u l t i p a c t o r i n g T h r e s h o l d V o l t a g e a s a F u n c t i o n o f G a p L e n g t h a n d R F W a v e l e n g t h . . . . 3 4 8 F i g . D . 4 O b s e r v a t i o n s o f M u l t i p a c t o r i n g i n t h e T R I U M F R F S y s t e m s T e s t F a c i l i t y C a v i t y 3 5 0 x i v LIST OF SYMBOLS Symbols are defined as follows unless otherwise noted: f3 = v/c where v i s the v e l o c i t y of the mass and c i s the v e l o c i t y of l i g h t Y = 1//1 - 3Z = the ion r o t a t i o n or cyclotron resonance frequency = the resonant frequency of a c i r c u i t or structure co = the frequency of the RF accelerating voltage, also r e f e r r e d to as the driven or system frequency r = the radius of a p o s i t i o n within the cyclotron magnetic f i e l d 6 = the azimuth of a p o s i t i o n within the cyclotron magnetic f i e l d B (r,6) = the v e r t i c a l component of the cyclotron magnetic f i e l d as a function of radius and azimuth <B (r)> = the average value of the v e r t i c a l component of the cyclotron z m o magnetic f i e l d as a function of radius the cyclotron magnet s p i r a l angle VV3 p e r m i t t i v i t y of a d i e l e c t r i c m = mass -31 = mass of an electron, 9.11 x 10 kg -27 rest mass of a proton, 1.67 x 10 kg q = e l e c t r i c charge -19 e = the e l e c t r o n i c charge, 1.602 x 10 C A<() = the l o n g i t u d i n a l phase length of an ion beam packet measured i n radio frequency degrees AR = the r a d i a l width of an ion beam packet xv 3 V a c c = the energy gained by the ion beam during a single pass through the dee gap dE/dn = the energy gained by the ion beam during a single turn N = the total number of turns completed by the ion beam n = the turn number or turn index E T = the total energy gained by the ion beam after N turns E = the maximum energy that the ion beam could gain after max « 6 N turns AE = E — E max T = the RF voltage measured from the tip of the dee to the dee liner - the dee voltage V = the potential that the ion beam f a l l s through during a single pass through the dee gap - the accelerating voltage = the amplitude of the fundamental component of the accelerating voltage <|> = the phase of the fundamental component of the accelerating voltage relative to the passage of the ion beam through the dee gap = the amplitude of the third harmonic component of the accelerating voltage 6 = the phase of the third harmonic component of the accelerating voltage relative to the fundamental component £ = the distance from the cavity to a shorting plane to a given point T = the total volume of the RF cavity AT = the volume of the RF cavity displaced by the cavity tuning mechanism capacitance the cavity tuning factor: AU/OJ = C AT/T X V i A c k n o w l e d g e m e n t M a n y p e o p l e a t T R I U M F a n d t h e D e p a r t m e n t o f E l e c t r i c a l E n g i n e e r i n g m a d e c o m p l e t i o n o f t h i s t h e s i s a n i n t e r e s t i n g a n d r e w a r d i n g a c t i v i t y . I n p a r t i c u l a r , I w i s h t o t h a n k my t h e s i s s u p e r v i s o r s : D r . E . V . J u l l D r . G e r a r d o D u t t o D r . D . A . D o h a n M r . R . E . W o r s h a m D r . T . A . E n e g r e n D r . L u c D u r i e u P r o f e s s o r , D e p a r t m e n t o f E l e c t r i c a l E n g i n e e r i n g , U B C H e a d , C y c l o t r o n D i v i s i o n L e a d e r , C y c l o t r o n D e v e l o p m e n t G r o u p L e a d e r , R F S y s t e m s G r o u p R F S y s t e m s E n g i n e e r C o n t r o l S y s t e m s E n g i n e e r , C E R N ( S P S D i v i s i o n ) - o n t e m p o r a r y l e a v e t o T R I U M F 1 9 8 3 - 8 4 a n d m a n y o t h e r m e m b e r s o f t h e T R I U M F s t a f f , i n c l u d i n g D r . K . S . K . F o n g , D r . J . V . P a c a k , M r . R . B u r g e , M r . M . M . M o u a t , a n d M r . E . K n a p e o f t h e C y c l o t r o n D e v e l o p m e n t G r o u p , M r . R . P o i r i e r , M r . N . C a r l s o n , M r . A . M a n n e s , M r . B . C h o w , M r . R . S h a n k s , a n d M r . H . B . W h i t e s i d e o f t h e R F S y s t e m s G r o u p , M r . R . E . L a x d a l o f t h e A c c e l e r a t o r R e s e a r c h D i v i s i o n , M r . P . B e n n e t t a n d M r . B . E v a n s o f t h e T e c h n o l o g y a n d A d m i n i s t r a t i o n D i v i s i o n . T h e i r i n t e r e s t i n a n d s u p p o r t o f my t h e s i s a c t i v i t i e s w e r e f r e e l y a n d g e n e r o u s l y g i v e n , d e s p i t e t h e i r d e m a n d i n g d u t i e s a n d r e s p o n s i b i l i t i e s e l s e w h e r e . D r . D u r i e u , i n p a r t i c u l a r , p r o v i d e d me w i t h t r e m e n d o u s s u p p o r t d u r i n g h i s s i x t e e n m o n t h s t a y a t T R I U M F a n d d e s e r v e s s p e c i a l m e n t i o n . T h e a s s i s t a n c e o f M i s s A n g e l a C h o i , M r . W a r d C a r t i e r , a n d t h e T R I U M F D r a w i n g O f f i c e i n t h e p r e p a r a t i o n o f m a n y o f t h e f i g u r e s i n t h i s t h e s i s i s m o s t a p p r e c i a t e d . F i n a n c i a l s u p p o r t f r o m t h e N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a , t h r o u g h T R I U M F , d u r i n g t h e c o u r s e o f my s t u d i e s i s g r a t e f u l l y a c k n o w l e d g e d . x v i i " I f t h e r e i s o n e t h i n g t h a t e x p e r i e n c e t e a c h e s u s , i t i s t h a t h o w e v e r w e l l c o n c e i v e d t h e d e s i g n o f t h e r . f . s y s t e m i n i t i a l l y i s , v a r i o u s p r o b l e m s w i l l m a n i f e s t t h e m s e l v e s w h e n i t i s r e q u i r e d t o p e r f o r m . . . T h e b e s t t h a t o n e c a n h o p e f o r i s t h a t t h e r . f . s y s t e m w i l l n o t s e l f d e s t r u c t o n f i r s t t u r n o n ! " J . R i e d e l " R . F . S y s t e m s " IEEE Trans N S - 2 6 ( 2 ) p 2 1 3 5 ( A p r i l 1 9 7 9 ) x v i i i THE CYCLOTRON AS SEEN BY THE EXPERIMENTAL PHYSICIST Drawing by Ron MacKenzie One of a well-known s e r i e s of drawings f i r s t p r esented at the I n t e r n a t i o n a l Conference on Isochronous C y c l o t r o n s , G a t l i n b u r g , Tennessee (2-5 May 1966) xix CHAPTER 1 INTRODUCTION i 1.1 A METHOD FOR IMPROVING THE OPERATING CHARACTERISTICS OF ISOCHRONOUS CYCLOTRONS 1.1.1 General I n p a r t i c l e a c c e l e r a t o r s , t h e a c c e l e r a t i n g f i e l d s a r e o f t e n d e v e l o p e d w i t h i n r e s o n a n t c a v i t i e s a n d c o n s e q u e n t l y h a v e a s i n u s o i d a l t i m e d e p e n d e n c e . I n a c c e l e r a t o r s s u c h a s s y n c h r o c y c l o t r o n s a n d s y n c h r o t r o n s t h a t r e l y o n V e k s l e r a n d M a c M i l l a n ' s " p r i n c i p l e o f p h a s e s t a b i l i t y " [ 1 ] [ 2 ] f o r p r o p e r o p e r a t i o n , t h e s i n u s o i d a l s h a p e o f t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m i s c l o s e t o t h e o p t i m u m s h a p e . I n t h e c a s e o f i s o c h r o n o u s c y c l o t r o n s , h o w e v e r , t h e o p t i m u m s h a p e m o r e c l o s e l y r e s e m b l e s a s q u a r e w a v e . W i t h t h e a d d i t i o n o f a t h i r d h a r m o n i c c o m p o n e n t o f t h e a p p r o p r i a t e r e l a t i v e a m p l i t u d e a n d p h a s e , i t w o u l d b e p o s s i b l e t o m o d i f y t h e s h a p e o f t h e a c c e l e r a t i n g v o l t a g e i n t h e T R I U M F 5 2 0 M e V c y c l o t r o n s o t h a t : • t h e s p r e a d i n e n e r g y n o r m a l l y a s s o c i a t e d w i t h t h e s p r e a d i n p h a s e o f t h e i o n b e a m w i t h r e s p e c t t o t h e m i d p o i n t o f t h e R F c y c l e w o u l d b e r e d u c e d . T h i s w o u l d i n c r e a s e t h e c y c l o t r o n ' s p h a s e a c c e p t a n c e , r e d u c e t h e e f f e c t i v e r a d i a l w i d t h o f t h e i o n b e a m , a n d w o u l d t h e r e f o r e i n c r e a s e t h e e f f e c t i v e s e p a r a t i o n b e t w e e n a d j a c e n t t u r n s d u r i n g a c c e l e r a t i o n ; a n d , 1 • s e c o n d o r d e r e f f e c t s , s u c h a s l o n g i t u d i n a l s p a c e c h a r g e f o r c e s i n t h e i o n b e a m ( a s o u r c e o f d i f f i c u l t y w h e n l a r g e b e a m c u r r e n t s a r e a c c e l e r a t e d ) a n d d e v i a t i o n s f r o m i s o c h r o n i s m i n t h e m a i n m a g n e t ' s p h a s e h i s t o r y , w o u l d b e c o m p e n s a t e d f o r b y v a r i o u s m e c h a n i s m s w h i c h w o u l d t e n d t o l o n g i t u d i n a l l y b u n c h t h e i o n b e a m . T h e s e i m p r o v e m e n t s w o u l d m a k e i t e a s i e r t o e x t r a c t : • i n t e r m e d i a t e e n e r g y p r o t o n b e a m s w i t h h i g h e n e r g y r e s o l u t i o n f o r p r e c i s e e x p e r i m e n t s i n n u c l e a r p h y s i c s ; • i n t e r m e d i a t e e n e r g y p r o t o n b e a m s w i t h h i g h c u r r e n t d e n s i t i e s f o r c o p i o u s p r o d u c t i o n o f s e c o n d a r y b e a m s o f m e s o n s a n d o t h e r p a r t i c l e s ; o r , • s i n g l e t u r n s o f n e g a t i v e h y d r o g e n i o n s e i t h e r f o r i n j e c t i o n b y c h a r g e e x c h a n g e i n t o a p o s t a c c e l e r a t o r s u c h a s t h e T R I U M F K A O N f a c t o r y o r f o r p r o d u c t i o n o f n e u t r a l h y d r o g e n b e a m s f o r a p p l i e d r e s e a r c h . " F l a t - t o p p i n g " t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m i n T R I U M F r e q u i r e s t h a t a s e c o n d r a d i o f r e q u e n c y s y s t e m o p e r a t i n g a t t h e t h i r d h a r m o n i c o f t h e m a i n R F s y s t e m b e d e s i g n e d a n d i n s t a l l e d . T h i s t h e s i s i s c o n c e r n e d w i t h : • t h e t o l e r a n c e s o n t h e s h a p e a n d s t a b i l i t y o f t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m r e q u i r e d b y t h e s e b e a m p r o d u c t i o n o b j e c t i v e s ; • d e m o n s t r a t i o n o f new t u n i n g a n d c o u p l i n g m e c h a n i s m s f o r t h e T R I U M F c y c l o t r o n ' s R F c a v i t y ; a n d , • i n i t i a l d e v e l o p m e n t o f a new R F c o n t r o l s y s t e m f o r t h e T R I U M F c y c l o t r o n . 2 1.1.2 H i s t o r i c a l N o t e s S o m e o f t h e a d v a n t a g e s o f f l a t - t o p p i n g t h e a c c e l e r a t i n g v o l t a g e i n i s o c h r o n o u s c y c l o t r o n s w e r e r e c o g n i z e d a s e a r l y a s 1 9 3 9 b y G . B . R o s s i w h o w a s l a t e r ( 1 9 5 7 ) a w a r d e d a p a t e n t [ 3 ] f o r a p o s s i b l e m e t h o d o f i m p l e m e n t i n g s u c h a s y s t e m i n a c l a s s i c a l c y c l o t r o n ( s e e f i g . 1 . 1 ) . T h e r e a r e r e f e r e n c e s i n t h e l i t e r a t u r e t o e a r l y w o r k b y G o o d m a n [ 4 ] o f O a k R i d g e N a t i o n a l L a b o r a t o r y i n 1 9 5 9 b u t t h e w o r k w a s a p p a r e n t l y a s s i g n e d a r a t h e r l o w p r i o r i t y [ 5 ] a n d o n l y a m o c k - u p o f a f l a t - t o p p e d c y c l o t r o n R F s y s t e m w a s c o n s t r u c t e d . T h e f i r s t d e t a i l e d p r o p o s a l s t o b u i l d f l a t - t o p p e d i s o c h r o n o u s c y c l o t r o n s a p p e a r e d i n t h e l a t e 1 9 6 0 ' s , a p p a r e n t l y f o l l o w i n g M . M . G o r d o n ' s s u g g e s t i o n [ 6 ] t h a t i t m i g h t b e d e s i r a b l e t o d e s i g n t h e S e p a r a t e d O r b i t C y c l o t r o n [ 7 ] , e s s e n t i a l l y a s p i r a l l i n a c o p e r a t i n g a t a l o w R F f r e q u e n c y , a s a " s e p a r a t e d t u r n i s o c h r o n o u s c y c l o t r o n " . He s u g g e s t e d t h a t f l a t - t o p p i n g c o u l d p l a y a m a j o r r o l e i n i m p r o v i n g t h e e x t r a c t i o n e f f i c i e n c y o f c y c l o t r o n s t h a t a r e u s e d t o a c c e l e r a t e h i g h c u r r e n t i o n b e a m s t o i n t e r m e d i a t e e n e r g i e s ( s u c h a s t h e m e s o n f a c t o r i e s t h e n b e i n g d e s i g n e d ) b y i n c r e a s i n g t h e e f f e c t i v e s e p a r a t i o n b e t w e e n a d j a c e n t t u r n s a t t h e e x t r a c t i o n r a d i u s . G o r d o n ' s p a p e r [ 6 ] a n d - a s u b s e q u e n t p u b l i c a t i o n [ 8 ] e x t o l l e d t h e v i r t u e s o f f l a t -t o p p i n g b u t a d d r e s s e d o n l y t h e b e a m d y n a m i c s i s s u e s . G o r d o n [ 8 ] c o n c l u d e d t h a t , " D e s p i t e the d i f f i c u l t i e s o u t l i n e d i n t he p r e c e d i n g p a r a g r a p h s , the p o t e n t i a l a d v a n t a g e s o f t he v o l t a g e f l a t -t o p p i n g t e c h n i q u e a r e s u f f i c i e n t l y a t t r a c t i v e t o j u s t i f y the e f f o r t r e q u i r e d t o s u c c e s s f u l l y implement t h i s t e c h n i q u e . " 3 Jan. 22, 1957 a B R o s s i 2,778,937 CYCLOTRON SQUARE 1AVE RF SYSTEM ttlad April 22. 1954 ATTORNEY. F i g . 1.1 Rossi's Cyclotron Square Wave RF System T h e p r o c e e d i n g s o f t h e F i f t h I n t e r n a t i o n a l C y c l o t r o n C o n f e r e n c e ( 1 9 6 9 ) r e c o r d s i x p r o p o s a l s [ 9 ] - [ 1 4 ] t o b u i l d c y c l o t r o n s w i t h f l a t - t o p p e d a c c e l e r a t i n g v o l t a g e s . T h e i n c r e a s e d p h a s e a c c e p t a n c e g a i n e d b y f l a t - t o p p i n g w a s p a r t i c u l a r l y a t t r a c t i v e f o r h e a v y i o n a c c e l e r a t o r s [ 1 0 ] [ 1 1 ] s i n c e t h e h e a v y i o n s o u r c e s t h a t w e r e a v a i l a b l e a t t h a t t i m e w e r e r a t h e r w e a k . M o s t o f t h e p r o p o s a l s d e a l t w i t h c y c l o t r o n s i n t e n d e d t o d e l i v e r i o n b e a m s w i t h v e r y f i n e e n e r g y r e s o l u t i o n [ 1 2 ] - [ 1 4 ] , r e f e r r e d t o b y s o m e w r i t e r s a s " s p e c t r o m e t r i c " o r " m o n o e n e r g e t i c " c y c l o t r o n s [ 1 2 ] [ 1 3 ] . H . G . B l o s s e r [ 1 5 ] a d d r e s s e d t h e i s s u e o f f l a t - t o p p i n g a t t h e n e x t c y c l o t r o n c o n f e r e n c e i n 1 9 7 2 i n a n i n v i t e d p a p e r c o n c e r n i n g t h e a p p l i c a t i o n o f new t e c h n i q u e s a n d t e c h n o l o g i e s t o f u t u r e c y c l o t r o n s . H e l a m e n t e d t h a t , "In s p i t e of ... e a r l y e f f o r t s and i n s p i t e of a number of obvious advantages of f l a t - t o p p i n g , no o p e r a t i n g c y c l o t r o n has yet u t i l i z e d such a system." A l t h o u g h m a n y c y c l o t r o n f a c i l i t i e s [ 9 ] - [ 1 8 ] h a v e i n d i c a t e d t h e i r i n t e r e s t i n f l a t - t o p p i n g , i n c l u d i n g : a . I U C F , t h e I n d i a n a U n i v e r s i t y C y c l o t r o n F a c i l i t y [ 1 6 ] ; b . G A N I L , t h e F r e n c h N a t i o n a l H e a v y I o n L a b o r a t o r y n e a r C a e n [ 1 7 ] ; a n d , C. T R I U M F [ 9 ] ; a s o f 1 9 8 5 , o n l y t h e 5 9 0 M e V r i n g c y c l o t r o n [ 1 8 ] [ 1 9 ] a n d i t s 72 M e V i n j e c t o r I I c y c l o t r o n [ 2 0 ] a t t h e S w i s s I n s t i t u t e f o r N u c l e a r R e s e a r c h ( S I N ) h a v e b e e n s u c c e s s f u l l y " f l a t -t o p p e d " . 5 T h e f i r s t d i s c u s s i o n s o f f l a t - t o p p i n g t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m i n t h e T R I U M F c y c l o t r o n w e r e p r e s e n t e d i n 1 9 6 9 i n R i c h a r d s o n a n d C r a d d o c k ' s [ 2 1 ] [ 2 2 ] a n a l y s i s o f t h e a c h i e v a b l e e n e r g y r e s o l u t i o n f r o m t h e c y c l o t r o n , a n d E r d m a n ' s ( e t a l ) [ 9 ] d e s c r i p t i o n o f p l a n s f o r d e v e l o p m e n t o f a " s q u a r e w a v e " R F s y s t e m . U r g e n t p r i o r i t i e s e l s e w h e r e s h i f t e d a t t e n t i o n a w a y f r o m t h i r d h a r m o n i c f l a t - t o p p i n g d u r i n g t h e 1 9 7 0 ' s , h o w e v e r . D e v e l o p m e n t s a t T R I U M F d u r i n g t h e p a s t f i v e y e a r s ( s e e s e c t i o n 1 . 4 ) h a v e r e n e w e d i n t e r e s t i n t h i r d h a r m o n i c f l a t - t o p p i n g a n d h a v e l e d t o t h e c u r r e n t d e s i g n a n d d e v e l o p m e n t e f f o r t . 1.2 SOME PRINCIPLES OF CYCLOTRON OPERATION T h e c y c l o t r o n w a s t h e f i r s t , a n d i s p e r h a p s t h e b e s t k n o w n , o f t h e s o - c a l l e d " c i r c u l a r a c c e l e r a t o r s " u s e d t o i m p a r t h i g h v e l o c i t i e s t o s t a b l e c h a r g e d p a r t i c l e s f o r u s e i n n u c l e a r a n d p a r t i c l e p h y s i c s r e s e a r c h . I t i s a m a g n e t i c r e s o n a n c e a c c e l e r a t o r [ 2 3 ] [ 2 4 ] i n w h i c h a s t a t i c m a g n e t i c f i e l d i s u s e d t o : a . s t e e r t h e i o n s b e i n g a c c e l e r a t e d i n t o c i r c u l a r o r b i t s ; a n d , b . p r o v i d e v e r t i c a l f o c u s i n g t o k e e p t h e i o n b e a m i n t h e m e d i a n p l a n e ( a n d a w a y f r o m t h e w a l l s o f t h e v a c u u m t a n k a n d o t h e r s t r u c t u r e s ) . T h e e l e c t r i c f i e l d w h i c h a c c e l e r a t e s t h e i o n b e a m i s d e v e l o p e d a c r o s s t h e d e e g a p ( a c c e l e r a t i n g g a p ) b y t h e c y c l o t r o n ' s r a d i o f r e q u e n c y s y s t e m . I n m o d e r n c y c l o t r o n s , t h i s p o t e n t i a l , d r o p i s u s u a l l y a f e w h u n d r e d t h o u s a n d v o l t s . 6 T h e R F s y s t e m f r e q u e n c y i s s y n c h r o n i z e d t o t h e i o n r o t a t i o n p e r i o d s o t h a t t h e a c c e l e r a t i n g v o l t a g e p e a k s w h e n t h e i o n b e a m p a s s e s t h r o u g h t h e a c c e l e r a t i n g g a p . I n c y c l o t r o n s , t h e i o n b e a m p a s s e s t h r o u g h a t l e a s t t w o ( a n d p e r h a p s s e v e r a l ) a c c e l e r a t i n g g a p s d u r i n g e a c h o r b i t . M o d e r n c y c l o t r o n R F s y s t e m s i n c l u d e t h e f o l l o w i n g c o m p o n e n t s : a . a m a s t e r o s c i l l a t o r - a n R F s i g n a l s o u r c e w i t h l o w p h a s e n o i s e a n d v e r y g o o d f r e q u e n c y s t a b i l i t y , e . g . a c r y s t a l - c o n t r o l l e d f r e q u e n c y s y n t h e s i z e r ; b . a r a d i o f r e q u e n c y p o w e r a m p l i f i e r ( o r t r a n s m i t t e r ) t h a t b o o s t s t h e s i g n a l f r o m t h e m a s t e r o s c i l l a t o r f r o m l e s s t h a n a w a t t t o t h e f e w h u n d r e d t o f e w m i l l i o n w a t t s n e c e s s a r y t o e x c i t e t h e d e s i r e d a c c e l e r a t i n g v o l t a g e i n t h e R F c a v i t y ; c . a t r a n s m i s s i o n l i n e a n d m a t c h i n g s e c t i o n - u s e d t o t r a n s f e r R F p o w e r f r o m t h e p o w e r a m p l i f i e r t o t h e c a v i t y c o u p l i n g e l e m e n t ; d . o n e o r m o r e R F c a v i t i e s ( a l s o r e f e r r e d t o a s a c c e l e r a t i n g c a v i t i e s , r e s o n a t o r s , o r d e e s t r u c t u r e s ) , c o m p l e t e w i t h c o u p l i n g e l e m e n t a n d t u n i n g m e c h a n i s m , t h a t s u p p o r t t h e a c c e l e r a t i n g v o l t a g e b e t w e e n t h e " d e e s " ; a n d , e . a f e e d b a c k c o n t r o l l o o p a n d a s s o c i a t e d c o n t r o l s t r u c t u r e s t h a t s t a b i l i z e t h e a m p l i t u d e a n d o f t e n , b u t n o t a l w a y s , t h e p h a s e o f t h e a c c e l e r a t i n g v o l t a g e . 7 T h e c y c l o t r o n r e s o n a n c e , w h i c h i s t h e f u n d a m e n t a l p r i n c i p l e o f c y c l o t r o n o p e r a t i o n , w a s d i s c o v e r e d b y E r n e s t 0 . L a w r e n c e [ 2 5 ] i n 1 9 2 9 . S i n c e t h e n , c y c l o t r o n s h a v e e v o l v e d i n t o a f a m i l y o f a c c e l e r a t o r s ( s e e T a b l e I ) t h a t i n c l u d e : a . t h e f i r s t g e n e r a t i o n " c l a s s i c a l " o r f i x e d f r e q u e n c y c y c l o t r o n s w h o s e m a x i m u m e n e r g y i s l i m i t e d t o a f e w t e n s o f M e V b y t h e r e l a t i v i s t i c i n c r e a s e i n t h e m a s s o f h i g h e n e r g y i o n s a n d t h e c o n s e q u e n t l o s s o f c y c l o t r o n r e s o n a n c e ; b . t h e s e c o n d g e n e r a t i o n s y n c h r o c y c l o t r o n s ( o r f r e q u e n c y - m o d u l a t e d c y c l o t r o n s ) , t h e f i r s t c i r c u l a r a c c e l e r a t o r s t o a c c e l e r a t e p a r t i c l e s p a s t t h e r e l a t i v i s t i c m a s s i n c r e a s e b a r r i e r ( t o s e v e r a l h u n d r e d M e V ) b y t a k i n g a d v a n t a g e o f t h e p r i n c i p l e o f p h a s e s t a b i l i t y [ 1 ] [ 2 ] b u t w i t h a v e r y l o w m a c r o d u t y c y c l e , h e n c e b e a m i n t e n s i t y ; a n d , c . t h e t h i r d a n d f o u r t h g e n e r a t i o n s e c t o r - f o c u s e d i s o c h r o n o u s c y c l o t r o n s [ 2 6 ] , o r i g i n a l l y p r o p o s e d b y L . H . T h o m a s i n t h e l a t e 1 9 3 0 ' s a n d " r e d i s c o v e r e d " i n t h e 1 9 5 0 ' s , t h a t p e r m i t i o n s t o b e a c c e l e r a t e d t o r e l a t i v i s t i c e n e r g i e s w i t h a 100 p e r c e n t m a c r o d u t y c y c l e b y s t e e r i n g t h e m i n t o i s o c h r o n o u s o r b i t s b u t a t t h e e x p e n s e o f r e q u i r i n g a r e l a t i v e l y c o m p l i c a t e d m a i n m a g n e t d e s i g n w i t h a n a z i m u t h a l l y v a r y i n g f i e l d . 8 T A B L E I FOUR GENERATIONS OF CYCLOTRONS 1 9 3 0 t o t h e p r e s e n t G e n e r a t i o n T y p e C o n s t r u c t i o n C o m m e n t s F i r s t C l a s s i c a l o r N o n - r e l a t i v i s t i c ( N R ) S e c o n d F r e q u e n c y m o d u l a t e d o r ' S y n c h r o ' c y c l o t r o n s T h i r d S e c t o r - f o c u s e d ( G e n e r a l P u r p o s e ) F o u r t h S e c t o r - f o c u s e d ( S p e c i a l P u r p o s e ) S e c t o r - f o c u s e d ( S u p e r - c o n d u c t i n g ) 1 9 3 0 - 1 9 5 0 ' s 1 9 4 5 - 1 9 5 0 ' s 1 9 5 0 - p r e s e n t 1 9 6 0 - 1 9 7 0 ' s o b s o l e t e 11 s t i l l i n o p e r a t i o n w o r l d - w i d e f i n a l e n e r g y < 70 M e V e . g . T R I U M F a n d S I N 1 9 8 0 ' s - p r e s e n t h e a v y i o n a c c e l e r a t i o n o r a c c e l e r a t i o n t o h i g h e n e r g i e s ( m u l t i - G e V ) 9 T h e b e a m o r b i t s i n i s o c h r o n o u s c y c l o t r o n s a r e d i f f e r e n t f r o m t h o s e i n o t h e r c i r c u l a r a c c e l e r a t o r s , s u c h a s s y n c h r o t r o n s , i n t w o w a y s : a . T h e o r b i t a l r a d i u s i n c r e a s e s a s t h e v e l o c i t y a n d k i n e t i c e n e r g y o f t h e i o n s i n c r e a s e ; and, b . T h e o r b i t a l ( o r " i o n r o t a t i o n " ) p e r i o d i s t h e same f o r i o n s o f a l l e n e r g i e s , i . e . t h e o r b i t s a r e i s o c h r o n o u s . A s a r e s u l t o f t h e l a s t p r o p e r t y , i s o c h r o n o u s c y c l o t r o n s c a n a c c e p t b e a m p a c k e t s f o r a c c e l e r a t i o n w h i l e o t h e r p a c k e t s a r e s t i l l i n t h e p r o c e s s o f b e i n g a c c e l e r a t e d . T h i s " p i p e l i n e d " a c c e l e r a t i o n o f m a n y i o n b e a m p a c k e t s s i m u l t a n e o u s l y g i v e s c y c l o t r o n s a u n i q u e c a p a b i l i t y : N o o t h e r c i r c u l a r a c c e l e r a t o r c a n a c c e l e r a t e i o n b e a m s w i t h a 100 p e r c e n t m a c r o d u t y c y c l e . S i n c e t h e e a r l y 1 9 6 0 ' s , t h e n u c l e a r a n d p a r t i c l e p h y s i c s c o m m u n i t i e s h a v e i d e n t i f i e d a n d p u r s u e d f o u r m a j o r d e s i g n o b j e c t i v e s f o r i s o c h r o n o u s c y c l o t r o n s : a . s i m p l i c i t y - f o r u s e i n m e d i c a l o r i n d u s t r i a l a p p l i c a t i o n s ; b . v e r s a t i l i t y - f o r a c c e l e r a t i o n o f h e a v y i o n s ; c . e x c e l l e n t b e a m p r o p e r t i e s - f o r p r e c i s e e x p e r i m e n t s i n n u c l e a r p h y s i c s ; a n d , d . h i g h e r e n e r g i e s a n d i n t e n s i t i e s - f o r c o p i o u s p r o d u c t i o n o f s e c o n d a r y p a r t i c l e s . P u r s u i t o f t h e f o u r t h o b j e c t i v e l e d t o t h e d e s i g n a n d c o n s t r u c t i o n o f t h e t w o m o s t p o w e r f u l c y c l o t r o n s e v e r b u i l t : 10 a . t h e 5 2 0 M e V / 2 0 0 u A H - s p i r a l - s e c t o r c y c l o t r o n a t T R I U M F , l o c a t e d o n t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a c a m p u s i n V a n c o u v e r ; a n d , b . t h e 5 9 0 M e V / 6 0 0 u A H+ r i n g c y c l o t r o n a t t h e S w i s s I n s t i t u t e f o r N u c l e a r R e s e a r c h ( S I N ) n e a r Z u r i c h . A l t h o u g h t h e r e a r e o v e r o n e h u n d r e d o p e r a t i n g c y c l o t r o n s w o r l d - w i d e i n 1 9 8 5 , m o s t o p e r a t e o n a r e l a t i v e l y s m a l l s c a l e . A s u b s t a n t i a l f r a c t i o n a r e p r i m a r i l y u s e d t o p r o d u c e r a d i o i s o t o p e s o r n e u t r o n b e a m s f o r u s e i n m e d i c a l , i n d u s t r i a l , o r s c i e n t i f i c a p p l i c a t i o n s . M a n y a r e u s e d t o a c c e l e r a t e h e a v y i o n s f o r u s e i n b o t h p u r e a n d a p p l i e d r e s e a r c h . O n l y t h r e e c y c l o t r o n s ( S I N - 5 9 0 M e V , T R I U M F -5 2 0 M e V , a n d I U C F - 2 1 5 M e V ) c a n a c c e l e r a t e p r o t o n s b e y o n d a b o u t 90 M e V , h o w e v e r . A l t h o u g h I U C F r e s e m b l e s i t s l o w e r e n e r g y b r e t h r e n i n m a n y w a y s , t h e m a m m o t h a c c e l e r a t o r s a t T R I U M F a n d S I N a r e i n a c l a s s o f t h e i r own i n t e r m s o f b o t h s c a l e a n d c o m p l e x i t y c o m p a r e d t o t h e m a j o r i t y o f t h e w o r l d ' s c y c l o t r o n s . T R I U M F a n d S I N a r e o f t e n r e f e r r e d t o a s " m e s o n f a c t o r i e s " . D u r i n g f u l l e n e r g y a n d m a x i m u m i n t e n s i t y o p e r a t i o n , t h e i r p r i m a r y p r o t o n b e a m s a r e d i r e c t e d a t s p e c i a l w a t e r - c o o l e d " p r o d u c t i o n t a r g e t s " . T h e h i g h l y e n e r g e t i c p r i m a r y p r o t o n b e a m i n t e r a c t s w i t h t h e n u c l e o n s i n t h e p r o d u c t i o n t a r g e t a n d g i v e s u p s o m e o f i t s e n e r g y i n t h e f o r m o f s e c o n d a r y b e a m s o f p i - m e s o n s ( p i o n s ) a n d t h e i r d e c a y p r o d u c t s , m u - m e s o n s ( m u o n s ) , t h a t a r e h u n d r e d s o f t i m e s m o r e 11 i n t e n s e t h a n a r e a v a i l a b l e a t m o s t o t h e r a c c e l e r a t o r f a c i l i t i e s . A t h i r d m e s o n f a c t o r y , w h i c h i s b a s e d o n a n 8 0 0 M e V p r o t o n l i n e a r a c c e l e r a t o r , i s l o c a t e d a t L o s A l a m o s N a t i o n a l L a b o r a t o r y i n New M e x i c o . 1.3 THE TRIUMF 520 MeV CYCLOTRON T R I U M F ' s 5 2 0 M e V c y c l o t r o n h a s p r o v i d e d e x p e r i m e n t a l p h y s i c i s t s w i t h i n t e r m e d i a t e e n e r g y p r o t o n b e a m s s i n c e l a t e 1 9 7 4 [ 2 7 ] f o r u s e i n : a . s t u d i e s o f n u c l e o n - n u c l e o n i n t e r a c t i o n s a t i n t e r m e d i a t e e n e r g i e s ( i . e . b e t w e e n 2 0 0 M e V a n d 5 0 0 M e V ) ; b . t h e p r o d u c t i o n o f s e c o n d a r y b e a m s o f p i o n s , m u o n s , f a s t n e u t r o n s , a n d t h e r m a l n e u t r o n s f o r u s e i n b o t h p u r e a n d a p p l i e d r e s e a r c h ; a n d , c . t h e p r o d u c t i o n o f r a d i o i s o t o p e s . I n j e c t i o n , a c c e l e r a t i o n , a n d e x t r a c t i o n o f t h e i o n b e a m f r o m t h e T R I U M F 5 2 0 M e V c y c l o t r o n a r e d e p i c t e d i n f i g . 1 . 2 . A l t h o u g h h i g h l y s i m p l i f i e d , t h e s k e t c h c l e a r l y i l l u s t r a t e s t h e b a s i c o p e r a t i o n o f t h e c y c l o t r o n a n d m a n y o f i t s i m p o r t a n t f e a t u r e s i n c l u d i n g : a . a x i a l i n j e c t i o n o f t h e i o n b e a m a t a r e l a t i v e l y h i g h e n e r g y ( 3 0 0 k e V ) f r o m a n e x t e r n a l i o n s o u r c e ; b . t h e g e o m e t r y o f t h e d e e s ; a n d , c . e x t r a c t i o n b y e l e c t r o n i c s t r i p p i n g . T h e T R I U M F 5 2 0 M e V c y c l o t r o n a n d p l a n s f o r i t s d e v e l o p m e n t h a v e b e e n d e s c r i b e d i n a n u m b e r o f r e c e n t 12 F i g . 1.2 TRIUMF Cyclotron - Injection, Acceleration, and Extraction of the Ion Beam This highly s i m p l i f i e d sketch shows the H" ion beam during (1) a x i a l i n j e c t i o n at 300 keV, (2) acceleration through the dee gap, ( 3 ) electronic stripping, and ( 4 ) extraction from the cyclotron. Certain d e t a i l s of the stripping and extraction sequence have been altered for sake of i l l u s t r a t i o n - see f i g . 2.9 r e p o r t s and p u b l i c a t i o n s such as the Report t o the NRC Review Committee on TRIUMF [ 2 8 ] , the t u t o r i a l paper c o n c e r n i n g KAON F a c t o r y s t u d i e s by Mackenzie [29] and, a r e p o r t on the s t a t u s of TRIUMF by Baartman e t a l [ 3 0 ] . An a r t i s t ' s c o n c e p t i o n of the c y c l o t r o n i s p r e s e n t e d i n f i g . 1.3. S e l e c t e d i n t e r n a l beam c h a r a c t e r i s t i c s and c y c l o t r o n d e s i g n parameters a r e p r e s e n t e d i n T a b l e I I . For r e f e r e n c e , a b r i e f d e s c r i p t i o n of the c y c l o t r o n and the f a c i l i t y i s p r e s e n t e d i n Appendix A. The TRIUMF c y c l o t r o n i n c o r p o r a t e s s e v e r a l fundamental improvements t o the c l a s s i c a l c y c l o t r o n c o n c e p t : 1. An a z i m u t h a l l y - v a r y i n g main magnetic f i e l d i s used t o p r o v i d e b o t h : a. the r a d i a l i n c r e a s e of the mean f l u x d e n s i t y of the main magnetic f i e l d t h a t i s r e q u i r e d t o compensate f o r the r e l a t i v i s t i c i n c r e a s e i n the mass of h i g h energy p a r t i c l e s : ' o o <B ( r ) > = ; and, z q b. v e r t i c a l f o c u s i n g of the i o n beam as r e q u i r e d f o r s u c c e s s f u l a c c e l e r a t i o n of beam t h r o u g h many o r b i t s . 2. The a c c e l e r a t i o n of n e g a t i v e ( r a t h e r than p o s i t i v e ) i o n s p e r m i t s : 14 Key to Figure i 1 Beam line I* — Proton Hall 2 Primary Support Structure 3 Magnet Sector Number 6 l» Main Magnet Excitation Coils 5 Graphite Shielding Ring 6 Concrete Shielding 7 RADIO FREQUENCY CAVITY -Resonator Segments 8 ACCELERATING GAP or DEE GAP 9 Beam line from Ion Source 10 Upper Centre Structure 11 Lower Centre Supports 12 Secondary Support Structure 13 Upper Tie Rods 1U Lower Tie Rods 15 Magnet Trim Coils 16 Vacuum Tank 17 Magnet Sector Number 2 18 Beam Exit Port 19 Combination Magnet 20 Elevating Jack 21 Support Column 22 Beam line 1 — Meson Hall 23 Magnet Support F i g . 1.3 TRIUMF Cyclotron - A r t i s t ' s Conception Table II - Internal Beam Char a c t e r i s t i c s and Cyclotron Design Parameters Property Aim Achieved Energy range 165-500 MeV 183-520 MeV Current (unpolarized) 100 yA (500 MeV) 200 uA (500 MeV) 400 yA (450 MeV) 10-20 yA (average scheduled current) Current (polarized) 60 nA 250 nA Polarization (reversible) 80% 70% Split ratio (Line 4/Line 1) 1/1 to 1/2000 1/1 to 1/10,000 (±25%) Duty factor - maximum ( 11% 11% (5 nsec/43 nsec) ( 20% (3rd harmonic) no - minimum ( 1% ( s l i t s ) 4% (chopped) Transmission (5-500 MeV) 86% 85% Fraction of dc beam to 500 MeV 10% 10% 50% (1st and 2nd harmonic bunchers on) Vertical centering ±6 mm ±4 mm Isochronism (sin 0) ±0.02 ±0.4 Energy spread (10% peak) 1.8 MeV 2.0 MeV 0.5 MeV ( s l i t s ) 0.4 MeV ( s l i t s ) 0.1 MeV (3rd harmonic) no Radial emittance (90% beam) 3 ir mm— mrad 5 ir mm-mrad Vertical emittance - internal 1.2 ir mm-mrad 5 TT mm-mrad (90% beam) - external 2.4 ir mm-mrad 3 TT mm-mrad Spot size at BL1AT2 2 x 10 mm2 3 x 14 mm2 Cyclotron Design Parameters Design Value Achieved Dee voltage 100 kV peak 85 kV Energy gain per turn 400 keV 340 keV Voltage s t a b i l i t y ±2.5 parts in 105 ± 1 part in I0h RF frequency (nominal) 23.05 MHz 5th harmonic of ion rotation frequency Frequency s t a b i l i t y ± 7.5 parts in 10s yes RF cavity tuning range ± 4 kHz yes RF cavity quality factor greater than 6000 yes Power dissipated at 100 kilovolt 1.2 MW yes Maximum anticipated beam load 300 kW 50 kW Magnetic s t a b i l i t y 3 ppm 0.7 ppm 16 a . t h e i o n b e a m t o b e e x t r a c t e d b y e l e c t r o n i c s t r i p p i n g r a t h e r t h a n e l e c t r o s t a t i c d e f l e c t i o n ) ; a n d , b . t h e s i m u l t a n e o u s e x t r a c t i o n o f i o n b e a m s w h o s e e n e r g i e s a n d i n t e n s i t i e s a r e e a s i l y a n d i n d e p e n d e n t l y v a r i e d . 3 . A n e x t e r n a l i o n s o u r c e a n d i n j e c t i o n s y s t e m i s u s e d t o : a . r e d u c e c o n t a m i n a t i o n o f t h e v a c u u m i n t h e c y c l o t r o n b y i o n s o u r c e e m i s s i o n s ; a n d , b . p e r m i t t h e u s e o f s p e c i a l p o l a r i z e d a n d h i g h i n t e n s i t y i o n s o u r c e s . D e s i g n a n d i n s t a l l a t i o n o f a t h i r d h a r m o n i c r a d i o f r e q u e n c y s y s t e m w i l l b e t h e f o u r t h f u n d a m e n t a l i m p r o v e m e n t t o t h e c l a s s i c a l c y c l o t r o n c o n c e p t a p p l i e d t o T R I U M F . 1 . 4 U P G R A D I N G T R I U M F ' S R A D I O F R E Q U E N C Y S Y S T E M T h e t h i r d h a r m o n i c f l a t - t o p p i n g p r o j e c t i s p a r t o f a l a r g e r p r o g r a m t o u p g r a d e t h e T R I U M F c y c l o t r o n ' s r a d i o f r e q u e n c y s y s t e m . U n d e r t h e t e r m s o f t h e R F R e s o n a t o r R e p l a c e m e n t P r o g r a m [ 3 1 ] , m a j o r s t r u c t u r a l i m p r o v e m e n t s w i l l b e m a d e t o t h e R F c a v i t y . T h o s e f e a t u r e s r e q u i r e d b y t h e t h i r d h a r m o n i c R F s y s t e m w i l l b e i n c o r p o r a t e d i n t o t h e new d e s i g n . A t t h e s a m e t i m e , t h e c y c l o t r o n ' s R F c o n t r o l s y s t e m i s b e i n g r e d e s i g n e d t o i m p r o v e t h e a m p l i t u d e a n d p h a s e s t a b i l i t y o f t h e a c c e l e r a t i n g v o l t a g e . P r o v i s i o n f o r c o n t r o l l i n g t h e t h i r d h a r m o n i c R F s y s t e m w i l l b e b u i l t i n t o t h e new c o n t r o l s y s t e m . 17 A n u m b e r o f r e c e n t d e v e l o p m e n t s a t T R I U M F w i l l b e n e f i t g r e a t l y f r o m t h e i m p r o v e d o p e r a t i n g c h a r a c t e r i s t i c s t h a t t h i r d h a r m o n i c f l a t - t o p p i n g c a n o f f e r : H i g h e r B e a m C u r r e n t a . d e v e l o p m e n t o f h i g h i n t e n s i t y i o n s o u r c e s ( p o l a r i z e d a n d u n p o l a r i z e d ) a t T R I U M F [ 3 2 ] [ 3 3 ] ; b . t h e r e c e n t u p g r a d e t o B e a m l i n e 1 A ' s b e a m d u m p , t h e T h e r m a l N e u t r o n F a c i l i t y , w h i c h i n c r e a s e d i t s c a p a c i t y f r o m 50 k i l o w a t t s o f b e a m p o w e r ( c o r r e s p o n d i n g t o a p p r o x i m a t e l y 170 m i c r o a m p e r e s o f b e a m c u r r e n t ) t o 125 k i l o w a t t s ( c o r r e s p o n d i n g t o a p p r o x i m a t e l y 4 0 0 m i c r o a m p e r e s o f b e a m c u r r e n t ) [ 3 0 ] ; c . p l a n s t o u s e t h e 5 2 0 M e V c y c l o t r o n a s t h e i n j e c t o r f o r t h e r e c e n t l y p r o p o s e d I S O L ( I s o t o p e S e p a r a t i o n O n L i n e ) r a d i o a c t i v e b e a m a c c e l e r a t o r ; H i g h e r E n e r g y R e s o l u t i o n d . p r o p o s a l s f o r d e v e l o p m e n t o f a h i g h r e s o l u t i o n s p e c t r o m e t e r f o r n u c l e o n - n u c l e o n i n t e r a c t i o n s t u d i e s [ 3 4 ] ; A c c e l e r a t i o n i n S e p a r a t e d T u r n s e . r e c e n t s u c c e s s a t r a i s i n g t h e s h o r t t e r m s t a b i l i t y o f t h e m a i n m a g n e t i c f i e l d t o 0 . 7 p a r t s p e r m i l l i o n ( f r o m 5 p a r t s p e r m i l l i o n ) b y R e i n i g e r , D o h a n , a n d B a u m a n n [ 3 5 ] a n d d e v e l o p m e n t o f v e r y h i g h r e s o l u t i o n NMR p r o b e s f o r r a i s i n g t h e l o n g t e r m s t a b i l i t y o f t h e f i e l d t o s i m i l a r l e v e l s b y 18 Dohan, Burge, and Dennison [36] which together with f l a t - t o p p i n g w i l l s a t i s f y the key requirements f o r a c c e l e r a t i n g the ion beam i n separated turns near the e x t r a c t i o n r a d i u s [37]; f. e f f o r t s d i r e c t e d towards d e v e l o p i n g a mechanism fo r e x t r a c t i n g n e gative hydrogen ions ( i n s t e a d of protons) from the c y c l o t r o n [38]; g. p l a n s to use the 520 MeV c y c l o t r o n as the i n j e c t o r f o r the TRIUMF KAON F a c t o r y [29] [39] [40], a f a s t - c y c l i n g s ynchrotron p o s t - a c c e l e r a t o r which w i l l boost a 100 - microampere proton beam from 450 m i l l i o n e l e c t r o n v o l t s (MeV) to n e a r l y 30 b i l l i o n e l e c t r o n v o l t s (GeV). 1 . 5 T H E S I S O U T L I N E The purpose of t h i s t h e s i s p r o j e c t i s to demonstrate the f e a s i b i l i t y of i n c o r p o r a t i n g p r o v i s i o n f o r o p e r a t i o n with a f l a t - t o p p e d a c c e l e r a t i n g v o l t a g e i n t o the TRIUMF RF system by demonstrating: a. a f l a t - t o p p e d a c c e l e r a t i n g v o l t a g e i n a t e s t c a v i t y t hat i s m e c h a n i c a l l y s i m i l a r to the c y c l o t r o n RF c a v i t y ; and, b. the o p e r a t i o n of a pro t o t y p e v e r s i o n of the new RF c o n t r o l system. 19 T h e i m p o r t a n c e o f t h e s h a p e o f t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m , i t s a m p l i t u d e a n d p h a s e s t a b i l i t y , a n d i t s r a d i a l d i s t r i b u t i o n i n d e t e r m i n i n g t h e c h a r a c t e r i s t i c s o f t h e a c c e l e r a t e d i o n b e a m a r e d e s c r i b e d i n c h a p t e r t w o . D e v e l o p m e n t o f t h i r d h a r m o n i c c a v i t y t u n i n g a n d c o u p l i n g m e c h a n i s m s a n d a d e m o n s t r a t i o n o f t h i r d h a r m o n i c f l a t - t o p p i n g o f t h e a c c e l e r a t i n g v o l t a g e i n t h e T R I U M F r a d i o f r e q u e n c y t e s t f a c i l i t y a t o p e r a t i o n a l v o l t a g e l e v e l s ( 1 0 0 k V - f u n d a m e n t a l , 11 k V - t h i r d h a r m o n i c ) w h i l e u n d e r v a c u u m a r e d e s c r i b e d i n c h a p t e r t h r e e . T h e i n i t i a l s p e c i f i c a t i o n a n d d e v e l o p m e n t o f a new r a d i o f r e q u e n c y c o n t r o l s y s t e m t o : a . s t e p t h e R F s y s t e m t h r o u g h t h e s t a r t - u p p r o c e d u r e ; b . k e e p t h e R F c a v i t y t u n e d t o b o t h t h e f u n d a m e n t a l a n d t h i r d h a r m o n i c d r i v i n g f r e q u e n c i e s ; c . r e g u l a t e t h e a m p l i t u d e a n d p h a s e o f t h e f u n d a m e n t a l ( 2 3 M H z ) a n d t h i r d h a r m o n i c ( 6 9 M H z ) a c c e l e r a t i n g m o d e s ; a n d , d . e n s u r e t h a t t h e R F s y s t e m o p e r a t e s i n a s a f e a n d r e l i a b l e m a n n e r w i t h e i t h e r a f u n d a m e n t a l o r a f l a t - t o p p e d a c c e l e r a t i n g v o l t a g e ; a r e d e s c r i b e d i n c h a p t e r f o u r . T h e m a j o r p o i n t s o f t h e t h e s i s a n d t h e r e s u l t s o b t a i n e d d u r i n g t h i s s t u d y a r e s u m m a r i z e d i n c h a p t e r f i v e . F o u r a p p e n d i c e s a r e a t t a c h e d . A b r i e f d e s c r i p t i o n o f t h e 5 2 0 M e V c y c l o t r o n a n d t h e T R I U M F m e s o n f a c i l i t y i s 20 p r e s e n t e d i n A p p e n d i x A . T h e s c o p e a n d t e c h n i c a l o b j e c t i v e s o f t h e t h i r d h a r m o n i c f l a t - t o p p i n g p r o j e c t i n c l u d i n g r e q u i r e d i m p r o v e m e n t s t o t h e f u n d a m e n t a l R F s y s t e m a r e d e s c r i b e d i n A p p e n d i x B . T e c h n i q u e s f o r c a l c u l a t i n g o r m e a s u r i n g t h e p r o p e r t i e s o f l o o p - c o u p l e d R F c a v i t i e s , i n c l u d i n g u s e o f a n R F n e t w o r k a n a l y z e r t o m e a s u r e c a v i t y c o u p l i n g a n d q u a l i t y f a c t o r s , a r e o u t l i n e d i n A p p e n d i x C . T h e n a t u r e o f t h e p r o b l e m s t h a t a r e o f t e n e n c o u n t e r e d d u r i n g t h e o p e r a t i o n o f a c c e l e r a t o r R F s y s t e m s , s u c h a s d r i f t s i n t h e r e s o n a n t f r e q u e n c y o f t h e a c c e l e r a t o r R F c a v i t y w i t h t e m p e r a t u r e , s p a r k i n g , a n d m u l t i p a c t o r i n g o r r e s o n a n t s e c o n d a r y e l e c t r o n e m i s s i o n a r e r e v i e w e d i n A p p e n d i x D . R e f e r e n c e s : [ 1 ] E . P e r s i c o , E . F e r r a r i , a n d S . E . S e g r e . " P r i n c i p l e s o f P a r t i c l e A c c e l e r a t o r s . " New Y o r k : W . A . B e n j a m i n , I n c . p p 1 8 - 2 4 ( 1 9 6 8 ) . [ 2 ] J . J . L i v i n g o o d . " P r i n c i p l e s o f C y c l i c P a r t i c l e A c c e l e r a t o r s . " P r i n c e t o n : D . V a n N o s t r a n d C o . I n c . p p 7 6 - 1 0 1 ( 1 9 6 1 ) . [ 3 ] G . B . R o s s i . " C y c l o t r o n S q u a r e W a v e R F S y s t e m . " U . S . P a t e n t N o . 2 , 7 7 8 , 9 3 7 d a t e d 22 J a n u a r y 1 9 5 7 . [ 4 ] C D . G o o d m a n . O a k R i d g e N a t i o n a l L a b o r a t o r y R e p o r t O R N L - 2 4 0 3 ( 1 9 5 7 ) . [ 5 ] R . E . W o r s h a m , T R I U M F . P r i v a t e c o m m u n i c a t i o n ( 1 9 8 4 ) . [ 6 ] M . M . G o r d o n . " D e s i g n C o n s i d e r a t i o n s f o r a S e p a r a t e d T u r n I s o c h r o n o u s C y c l o t r o n . " Nucl Inst Meth 5 8 : 2 4 5 - 2 5 2 ( 1 9 6 8 ) . [ 7 ] J . A . M a r t i n . " T h e S e p a r a t e d O r b i t C y c l o t r o n . " IEEE Trans N S - 1 3 ( 4 ) : 2 8 8 - 2 9 2 ( 1 9 6 6 ) . 21 [ 8 ] M . M . G o r d o n . " I m p r o v i n g t h e E n e r g y R e s o l u t i o n a n d D u t y F a c t o r o f I s o c h r o n o u s C y c l o t r o n s . " P a r t i c l e Accel erat or s 2 : 2 0 3 - 2 0 9 ( 1 9 7 1 ) . [ 9 ] K . L . E r d m a n , A . P r o c h a z k a , O . K . F r e d r i k s s o n , R . T h o m a s , a n d W . A . G r u n d m a n . " A S q u a r e - W a v e R F S y s t e m D e s i g n f o r T R I U M F . " Proc 5th Int Cyclotron Conf, p p 1 0 5 - 1 1 0 ( 1 9 6 9 ) . [ 1 0 ] T . K . K h o e , J . J . L i v i n g o o d , a n d W . J . R a m l e r . " P r o p o s e d M i d w e s t T a n d e m C y c l o t r o n . " Proc 5th Int Cyclotron Conf, p p 3 0 - 4 0 ( 1 9 6 9 ) . [ 1 1 ] J . A . M a r t i n , E . D . H u d s o n , S . W . M o s k o , L . N . H o w e l l , a n d M . L . M a l l o r y . " A H e a v y - I o n A c c e l e r a t o r F a c i l i t y F e a t u r i n g a S e p a r a t e d S e c t o r I s o c h r o n o u s C y c l o t r o n . " Proc 5th Int Cyclotron Conf, p p 4 1 - 4 9 ( 1 9 6 9 ) . [ 1 2 ] I. Y a . B a r i t , G . A . V a s i l y e v , V . N . K a n u n n i k o v , L . N . K a t s a u r o v , A . A . K o l o m e n s k y , E . M . M o r o z , L . N . N e t c h a e v a , A . P . F a t a e v , Y u . K . K h o k h l o v , I . V . S h t r a n i k h , Y u . G . B a s a r g i n , O . A . G u s e v , R . N . L i t u n o v s k y , I . F . M a l i s h e v , O . A . M i n y a e v , a n d M . P . S v i n y i n . " S p e c t r o m e t r i c I s o c h r o n o u s C y c l o t r o n . " Proc 5th Int Cyclotron Conf, p p 3 7 5 - 3 7 9 ( 1 9 6 9 ) . [ 1 3 ] V . P . D m i t r i e v s k y , V . V . K o l g a , a n d N . I . P o l u m o r d v i n o v a . " P e c u l i a r i t i e s o f C h a r g e d P a r t i c l e M o t i o n i n t h e M o n o e n e r g e t i c C v c l o t r o n . " Proc 5th Int Cyclotron Conf, p p 3 7 0 - 3 7 4 ( 1 9 6 9 ) . [ 1 4 ] M . E . R i c k e y , M . B . S a m p s o n , a n d B . M . B a r d i n . " T h e I n d i a n a U n i v e r s i t y 2 0 0 M e V C y c l o t r o n P r o j e c t . " Proc 5th Int Cyclotron Conf, p p 2 4 - 2 9 ( 1 9 6 9 ) . [ 1 5 ] H . G . B l o s s e r . " F u t u r e C y c l o t r o n s . " Proc 6th Int Cyclotron Conf, p p 1 6 - 3 2 ( 1 9 7 2 ) . [ 1 6 ] M . E . R i c k e y , M . B . S a m p s o n , a n d B . M . B a r d i n . " G e n e r a l D e s i g n F e a t u r e s o f t h e I n d i a n a U n i v e r s i t y 2 0 0 M e V C y c l o t r o n . " IEEE Trans N S - 1 6 ( 3 ) : 3 9 7 - 4 0 4 ( D e c e m b e r 1 9 6 8 ) . [ 1 7 ] C . B i e t h , A . J o u b e r t , G , R a s t o i x , J . R i e d e l . " T h e G a n i l A c c e l e r a t i n g S y s t e m . " Proc 7th Int Conf on Cyclotrons and t h e i r Applications, p p 1 6 3 - 1 6 6 ( 1 9 7 5 ) . [ 1 8 ] B . B i s c h o f . " T h e R F - S y s t e m o f t h e F l a t t o p - A c c e l e r a t i o n S t r u c t u r e i n t h e S I N 5 9 0 - M e V - R i n g - C y c l o t r o n . " IEEE Trans N S - 2 6 ( 2 ) : 2 1 8 6 - 2 1 8 9 ( A p r i l 1 9 7 9 ) . [ 1 9 ] S . A d a m , W. J o h o , P . L a n z , H . L e b e r , N . S c h m i d , U . S c h r y b e r . " F i r s t O p e r a t i o n o f a F l a t - t o p A c c e l e r a t i n g S y s t e m i n a n I s o c h r o n o u s C y c l o t r o n . " IEEE Trans N S -2 8 ( 3 ) : 2 7 2 1 - 2 7 2 3 ( J u n e 1 9 8 1 ) -22 [ 2 0 ] W. J o h o , S . A d a m , B . B e r k e s , T . B l u m e r , M . H u m b e l l , G . I r m i n g e r , P . L a n z , C . M a r k o v i t s , A . M e z g e r , M . O l i v o , L . R e z z o n i c o , U . S c h r y b e r , P . S i g g . " C o m m i s s i o n i n g o f t h e New H i g h I n t e n s i t y 72 M e V I n j e c t o r I I f o r t h e S I N R i n g C y c l o t r o n . " IEEE Trans N S - 3 2 ( 5 ) : 2 6 6 6 - 2 6 6 8 ( O c t o b e r 1 9 8 5 ) . [ 2 1 ] J . R . R i c h a r d s o n . " E n e r g y R e s o l u t i o n i n a 5 0 0 M e V H -C y c l o t r o n . " T R I U M F R e p o r t T R I - 6 9 - 6 ( J u n e 1 9 6 9 ) . [ 2 2 ] J . R . R i c h a r d s o n a n d M . K . C r a d d o c k . " B e a m Q u a l i t y a n d E x p e c t e d E n e r g y R e s o l u t i o n f r o m t h e T R I U M F C y c l o t r o n . " Proc 5th Int Cyclotron Conf, p p 8 5 - 9 4 ( 1 9 6 9 ) . [ 2 3 ] E . P e r s i c o , E . F e r r a r i , a n d S . E . S e g r e . " P r i n c i p l e s o f P a r t i c l e A c c e l e r a t o r s . " New Y o r k : W . A . B e n j a m i n , I n c . p p 1 1 - 1 4 , 1 6 0 - 1 7 7 ( 1 9 6 8 ) . [ 2 4 ] J . J . L i v i n g o o d . " P r i n c i p l e s o f C y c l i c P a r t i c l e A c c e l e r a t o r s . " P r i n c e t o n : D . V a n N o s t r a n d C o . I n c . p p 7 -1 1 , 1 0 2 - 1 5 3 ( 1 9 6 1 ) . [ 2 5 ] E . O . L a w r e n c e a n d N . F . E d l e f s e n . " O n t h e P r o d u c t i o n o f H i g h S p e e d P r o t o n s . " Science 7 2 : 3 7 6 - 3 7 7 ( 1 9 3 0 ) . [ 2 6 ] J . R . R i c h a r d s o n . " S e c t o r - F o c u s i n g C y c l o t r o n s . " Progr Nucl Tech Instr 1 : 5 7 - 1 5 8 ( 1 9 6 5 ) . [ 2 7 ] J . R . R i c h a r d s o n , E . W . B l a c k m o r e , G . D u t t o , C . J . K o s t , G . H . M a c K e n z i e , a n d M . K . C r a d d o c k . " P r o d u c t i o n o f S i m u l t a n e o u s V a r i a b l e E n e r g y B e a m s f r o m t h e T R I U M F C y c l o t r o n . " IEEE Trans N S - 2 2 ( 3 ) : 1 4 0 2 - 1 4 0 7 ( J u n e 1 9 7 5 ) . [ 2 8 ] E . W . V o g t , e d . " R e p o r t t o t h e N R C C o m m i t t e e o n T R I U M F . " ( A p r i l 1 9 8 3 ) . [ 2 9 ] G . H . M a c k e n z i e . " S t u d i e s f o r a T R I U M F K A O N F a c t o r y . " I N S K I K U C H I W i n t e r S c h o o l o n A c c e l e r a t o r s f o r N u c l e a r P h y s i c s , 2 9 J a n - 2 F e b 1 9 8 4 . [ 3 0 ] R . B a a r t m a n , E . W . B l a c k m o r e , J . C a r e y , D . D o h a n , G . D u t t o , D . G u r d , R . E . L a x d a l , G . H . M a c k e n z i e , D . P e a r c e , R . P o i r i e r , a n d P . W . S c h m o r . " S t a t u s R e p o r t o n t h e T R I U M F C y c l o t r o n . " Proc 10th 1nl Conf on Cyclotrons and t h e i r Applications, p p 2 0 3 - 2 0 6 ( 1 9 8 4 ) . [ 3 1 ] T R I U M F F a c i l i t y D e v e l o p m e n t P l a n . " R F R e s o n a t o r R e p l a c e m e n t P r o g r a m . " ( 1 2 J u l y 1 9 8 4 ) . [32] P.W. Schmor, R. Baartman, J.W. Carey, D. Dohan, G. Dutto, and G.H. Mackenzie. "Progress Towards Higher I n t e n s i t i e s and Improved Beam S t a b i l i t y at TRIUMF." IEEE Trans NS-30(4).:2092-2095 (August 1983). 23 [ 3 3 ] P . S c h m o r . " H i g h I n t e n s i t y S o u r c e s o f P o l a r i z e d P r o t o n s . " IEEE Trans N S - 3 2 ( 5 ) : 1 7 1 3 - 1 7 1 7 ( 1 9 8 5 ) . [ 3 4 ] J . M . C a m e r o n , P . K i t c h i n g , a n d D . A . H u t c h e o n , e d . " P r o s p e c t s f o r H i g h R e s o l u t i o n S t u d i e s w i t h A P r o t o n B e a m B e t w e e n E p = 2 0 0 - 5 0 0 M e V . " T R I U M F R e p o r t T R I - 8 0 - 2 ( O c t o b e r 1 9 8 0 ) . [ 3 5 ] K . R e i n i g e r , H . B a u m a n n , D . D o h a n , a n d D . P e a r c e . " S t a b i l i t y I m p r o v e m e n t s i n t h e T R I U M F C y c l o t r o n M a g n e t i c F i e l d t o . + / - 0 . 7 p p m . " J de Physique 4 5 ( C 1 ) : 2 2 5 - 2 2 8 ( J a n u a r y 1 9 8 4 ) . [ 3 6 ] R . B u r g e , D . D o h a n , a n d G . D e n n i s o n . " P r e c i s i o n NMR M e a s u r e m e n t o f t h e T R I U M F C y c l o t r o n M a g n e t i c F i e l d . " Proc 10th Int Conf on Cyclotrons and t h e i r Applications, p p 8 2 - 8 4 ( 1 9 8 4 ) . [ 3 7 ] E . W . B l a c k m o r e , D . A . D o h a n , G . H . M a c K e n z i e , a n d R . P o i r i e r . " D e v e l o p m e n t s T o w a r d S e p a r a t e d T u r n s a t T R I U M F . " Proc 9th Int Conf on Cyclotrons and t h e i r Appl i cati ons, p p 4 8 5 - 4 8 7 ( 1 9 8 1 ) . [ 3 8 ] M . Z a c h , G . D u t t o , R . E . L a x d a l , G . H . M a c k e n z i e , J . R . R i c h a r d s o n , R . T r e l l e , R . E . W o r s h a m . " T h e H - H i g h I n t e n s i t y B e a m E x t r a c t i o n S y s t e m f o r T R I U M F . " IEEE Trans N S - 3 2 ( 5 ) : 3 0 4 2 - 3 0 4 4 ( 1 9 8 5 ) . [ 3 9 ] M . K . C r a d d o c k , R . B a a r t m a n , J . B e v e r i d g e , E . W . B l a c k m o r e , J . I . M . B o t m a n , W. C a m e r o n , J . D o o r n b o s , T . A . H o d g e s , D . E . L o b b , G . H . M a c k e n z i e , D . R a p a r i a , P . A . R e e v e , J . R . R i c h a r d s o n , G . S t i n s o n , R . E . W o r s h a m , W . K . L a c e y , C . W . P l a n n e r , H . O . S c h o n a u e r , T . S u z u k i , C . Y a m a g u c h i . " T h e T R I U M F K A O N F a c t o r y . " IEEE Trans N S -32 ( 5 ) : 1 7 0 7 - 1 7 0 9 ( 1 9 8 5 ) . [ 4 0 ] E . W . V o g t , A . A s t b u r y , M . . K . C r a d d o c k , E . W . B l a c k m o r e , G . R . R i t c h i e , W . K . L a c e y , e d . " K A O N F a c t o r y P r o p o s a l . " T R I U M F M e s o n R e s e a r c h F a c i l i t y , V a n c o u v e r , C a n a d a . ( S e p t e m b e r 1 9 8 5 ) . 2k CHAPTER 2 MODIFICATION AND CONTROL OF THE ACCELERAT ING VOLTAGE WAVEFORM 2.1 INTRODUCTION A t p r e s e n t , c y c l o t r o n o p e r a t o r s c a n " t u n e " t h e T R I U M F c y c l o t r o n t o m a x i m i z e b e a m t r a n s m i s s i o n ( t h e p e r c e n t a g e o f t h e b e a m t h a t i s s u c c e s s f u l l y a c c e l e r a t e d f r o m i n j e c t i o n t o e x t r a c t i o n ) , c e n t e r t h e b e a m o r b i t s , a n d i m p r o v e t h e e n e r g y r e s o l u t i o n , m i c r o d u t y c y c l e a n d e m i t t a n c e o f t h e e x t r a c t e d b e a m b y : • m a k i n g f i n e a d j u s t m e n t s t o t h e " s h a p e " o f t h e c y c l o t r o n ' s m a g n e t i c f i e l d u s i n g t h e 5^ c i r c u l a r t r i m a n d 78 ( 1 3 i n e a c h o f 6 m a g n e t s e c t o r s ) h a r m o n i c c o i l p a i r s t h a t a r e b r a z e d t o t h e t o p a n d b o t t o m o f t h e v a c u u m t a n k ; • m i n i m i z i n g t h e e f f e c t s o f m i s a l i g n m e n t o f t h e c e n t r a l r e g i o n d e e s e g m e n t s w i t h a s e t o f e l e c t r o s t a t i c c o r r e c t i o n p l a t e s ; a n d , - • r e j e c t i n g u n d e s i r a b l e c o m p o n e n t s o f t h e i n j e c t e d b e a m u s i n g a s y s t e m o f c h o p p e r s , p u l s e r s , d e f i n i n g s l i t s a n d f l a g s i n t h e c y c l o t r o n ' s i n j e c t i o n l i n e a n d c e n t r a l r e g i o n ; a s d e s c r i b e d b y B l a c k m o r e e t a l [ 1 ] , R a w n s l e y , M a c k e n z i e , a n d O r a m [ 2 ] , a n d o t h e r s . A l t h o u g h t h e p r i m a r y f u n c t i o n o f t h e a c c e l e r a t i n g v o l t a g e d e v e l o p e d b y t h e c y c l o t r o n ' s r a d i o f r e q u e n c y s y s t e m i s t o i m p a r t k i n e t i c e n e r g y t o t h e i o n b e a m , o p t i m i z i n g t h e s h a p e o f t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m a n d i m p r o v i n g i t s 25 a m p l i t u d e a n d p h a s e s t a b i l i t y p l a y i m p o r t a n t r o l e s i n i n c r e a s i n g t h e p h a s e a c c e p t a n c e o f t h e c y c l o t r o n , i n c r e a s i n g t h e s e p a r a t i o n b e t w e e n a d j a c e n t t u r n s d u r i n g t h e a c c e l e r a t i o n p r o c e s s , a n d g e n e r a l l y i m p r o v i n g t h e e m i t t a n c e , m i c r o d u t y c y c l e , a n d e n e r g y r e s o l u t i o n o f t h e e x t r a c t e d b e a m . A n u m b e r o f i m p o r t a n t s e c o n d o r d e r e f f e c t s t h a t a d v e r s e l y a f f e c t t h e b e a m , i n c l u d i n g t h e e f f e c t s o f d e v i a t i o n s f r o m i s o c h r o n i s m i n t h e c y c l o t r o n ' s p h a s e h i s t o r y , l o n g i t u d i n a l s p a c e - c h a r g e e f f e c t s d u r i n g h i g h c u r r e n t o p e r a t i o n , a n d e n e r g y d i s p e r s i o n r e s u l t i n g f r o m t h e s p r e a d i n p h a s e o f i o n s w i t h r e s p e c t t o t h e m i d - p o i n t o f t h e R F c y c l e , c a n b e c o m p e n s a t e d f o r b y s u i t a b l y m o d i f y i n g t h e s h a p e o f t h e R F w a v e f o r m . B y p r o v i d i n g c y c l o t r o n o p e r a t o r s w i t h c o n t r o l o f t h e r e l a t i v e a m p l i t u d e a n d p h a s e o f a s u p p l e m e n t a r y t h i r d h a r m o n i c c o m p o n e n t o f t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m , a n o t h e r i m p o r t a n t d i m e n s i o n i s a d d e d t o t h e p r o c e s s o f t u n i n g t h e c y c l o t r o n f o r o p t i m u m p e r f o r m a n c e . T h e o p t i m u m s h a p e a n d m i n i m u m s t a b i l i t y r e q u i r e m e n t s f o r t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m a r e d e p e n d e n t o n t h e c y c l o t r o n ' s o p e r a t i n g m o d e : a . h i g h b e a m c u r r e n t s (200 - 400 u A ) w i t h l o w e n e r g y r e s o l u t i o n (+ 600 k e V ) ; b . h i g h e n e r g y r e s o l u t i o n (+ 50 K e V w i t h l o w b e a m c u r r e n t s (10 - 20 n A ) ; o r , i n t h e f u t u r e , c . s i n g l e t u r n e x t r a c t i o n o f e i t h e r p r o t o n s o r n e g a t i v e h y d r o g e n i o n s a t m o d e r a t e (30 u A ) t o h i g h (100 u A ) b e a m c u r r e n t s . 26 P r e l i m i n a r y s p e c i f i c a t i o n s f o r t h e s h a p e o f t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m , i t s ' a m p l i t u d e a n d p h a s e s t a b i l i t y , a n d t h e a c c e p t a b l e ( o r d e s i r a b l e ) v a r i a t i o n c f i t s a m p l i t u d e w i t h r a d i a l p o s i t i o n a l o n g t h e a c c e l e r a t i n g g a p a r e a s s e m b l e d ( f r o m v a r i o u s s o u r c e s ) i n t h i s c h a p t e r . I n b r i e f , h i g h c u r r e n t o p e r a t i o n w i l l r e q u i r e t h e g r e a t e s t t h i r d h a r m o n i c a c c e l e r a t i n g v o l t a g e ( 0 . 2 4 o f t h e f u n d a m e n t a l v o l t a g e ) o f t h e t h r e e o p e r a t i n g m o d e s w h i l e s i n g l e t u r n e x t r a c t i o n w i l l r e q u i r e t h e h i g h e s t a m p l i t u d e a n d p h a s e s t a b i l i t y (+ 8 0 p p m - f u n d a m e n t a l a m p l i t u d e , + 0 . 1 2 d e g r e e s - t h i r d h a r m o n i c p h a s e r e l a t i v e t o t h e f u n d a m e n t a l ) . T h e s t a b i l i t y s p e c i f i c a t i o n s p r e s e n t e d i n t h i s c h a p t e r a r e b a s e d o n a h i g h l y s i m p l i f i e d m o d e l o f t h e a c c e l e r a t i o n p r o c e s s i n t h e c y c l o t r o n . B e c a u s e v e r i f i c a t i o n o f s u c h m o d e l s b y e x p e r i m e n t i s d i f f i c u l t , t h e r e s u l t s p r e s e n t e d s h o u l d b e r e g a r d e d a s o r d e r o f m a g n i t u d e e s t i m a t e s o n l y . I t s h o u l d a l s o b e n o t e d t h a t t h e e a s e o f t u n i n g a c y c l o t r o n d e p e n d s a s m u c h o n t h e q u a l i t y o f t h e b e a m d i a g n o s t i c p r o b e s a v a i l a b l e t o c y c l o t r o n o p e r a t o r s a s i t d o e s o n t h e n u m b e r o f p a r a m e t e r s t h e o p e r a t o r s c a n a d j u s t . A n e a r l y r e v i e w ( 1 9 6 6 ) o f c y c l o t r o n b e a m d e v e l o p m e n t a n d d i a g n o s t i c t o o l s w a s p r e s e n t e d b y C l a r k [ 2 ] . A r e v i e w o f b e a m d i a g n o s t i c e q u i p m e n t f o r i n t e r m e d i a t e e n e r g y c y c l o t r o n s w a s p r e s e n t e d m o r e r e c e n t l y ( 1 9 7 5 ) b y O l i v o [ 3 ] , P r o b l e m s w i t h m e c h a n i c a l d e s i g n , R F p i c k u p , "pressure to put the machine i n t o operation q u i c k l y , or a shortage of funds" [ 3 ] 27 h a v e , a t m a n y f a c i l i t i e s , r e s u l t e d i n d i a g n o s t i c p r o b e s t h a t f a i l t o m e e t d e s i g n e x p e c t a t i o n s o r o p e r a t i o n a l r e q u i r e m e n t s . O l i v o n o t e d t h a t " e x p e r i e n c e a t m a n y l a b o r a t o r i e s i n d i c a t e s t h a t e x c e l l e n t d i a g n o s t i c e l e m e n t s a r e a m u s t t o e x t r a c t t h e b e s t p e r f o r m a n c e f r o m t h e a c c e l e r a t o r . " D e v e l o p m e n t o f b e a m d i a g n o s t i c e q u i p m e n t i s a n i m p o r t a n t a n d o n g o i n g a c t i v i t y a t T R I U M F , a s d e s c r i b e d i n R e f s . [ 1 ] , [ 2 ] , a n d o t h e r s . 2.1.1 T h e A c c e l e r a t i n g G a p i n t h e T R I U M F C y c l o t r o n D u r i n g n o r m a l o p e r a t i o n , t h e T R I U M F c y c l o t r o n s u p p o r t s a d e e v o l t a g e o f 8 5 k i l o v o l t s . I n p r i n c i p l e , e a c h i o n c o u l d g a i n 170 k e V o f k i n e t i c e n e r g y e v e r y t i m e t h e b e a m ' s s p i r a l t r a j e c t o r y t a k e s i t t h r o u g h t h e d e e g a p , b u t i n p r a c t i c e , d e v i a t i o n s f r o m i s o c h r o n i s m c o m b i n e d w i t h t h e s i n u s o i d a l s h a p e o f t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m h a v e l i m i t e d t h e a v e r a g e e n e r g y g a i n t o a p p r o x i m a t e l y 135 k e V a s c o m p u t e d b y c o m p a r i n g t h e f i n a l b e a m e n e r g y E ^ w i t h t h e b e a m ' s t i m e o f f l i g h t [ 5 ] : D i v i d i n g t h e b e a m ' s t i m e o f f l i g h t t ^ b y t h e i o n r o t a t i o n p e r i o d T g i v e s t h e n u m b e r o f t u r n s N r e q u i r e d t o g a i n s u f f i c i e n t e n e r g y f o r e x t r a c t i o n . D i v i d i n g t h e e n e r g y g a i n e d d u r i n g a c c e l e r a t i o n E ^ - E ^ b y t h e n u m b e r o f t u r n s N g i v e s t h e a v e r a g e e n e r g y g a i n p e r t u r n < d E / d n > . < d E / d n > = ( E f - E ^ T / t f ( 2 . 1 ) - 4 q < v * o o c o s <t» ( 2 - 2 ) 28 . 2.1 TRIUMF Cyclotron - Cross-section Through Dee Gap T h e l i n e i n t e g r a l o f t h e e l e c t r i c f i e l d e n c o u n t e r e d b y t h e b e a m d u r i n g i t s p a s s a g e t h r o u g h t h e d e e g a p , m u l t i p l i e d b y t h e i o n ' s n e t e l e c t r i c c h a r g e , g i v e s t h e e n e r g y g a i n e d A E = 1 / 2 ( d E / d n ) , a s s h o w n i n f i g . 2 . 1 . T h i s i n t e g r a l i s t h e o n l y e x a c t m e a s u r e o f t h e e n e r g y g a i n e d b y t h e i o n b e a m , a q u a n t i t y w h i c h m u s t b e h i g h l y r e g u l a t e d f o r p r o d u c t i o n o f h i g h q u a l i t y p r o t o n b e a m s . U n f o r t u n a t e l y , b y i t s n a t u r e , i t i s v e r y d i f f i c u l t t o m e a s u r e t h i s v a l u e p r e c i s e l y , e s p e c i a l l y u s i n g a c o n v e n t i o n a l c a p a c i t i v e v o l t a g e d i v i d e r m o u n t e d n e a r t h e d e e g a p , s u c h a s s h o w n i n f i g . 2 . 1 . T h i s c a n m a k e i t v e r y d i f f i c u l t t o r e g u l a t e A E t o t h e r e q u i r e d p r e c i s i o n u s i n g f e e d b a c k c o n t r o l ( s e e s e c t i o n 4 . 4 ) . T h e t e r m " a c c e l e r a t i n g v o l t a g e " V r e f e r s t o t h e a m p l i t u d e o f t h e p o t e n t i a l d i f f e r e n c e b e t w e e n o p p o s i t e s i d e s o f t h e d e e g a p . M u l t i p l y i n g V b y t h e i o n ' s n e t c h a r g e q g i v e s t h e m a x i m u m p o s s i b l e e n e r g y g a i n p e r g a p c r o s s i n g . M u l t i p l y i n g t h i s q u a n t i t y b y c o s <f>, w h e r e 4> r e p r e s e n t s t h e p h a s e d i f f e r e n c e b e t w e e n t h e p e a k o f t h e R F w a v e f o r m a n d t h e p a s s a g e o f t h e b e a m t h r o u g h t h e d e e g a p , g i v e s a n a p p r o x i m a t e m e a s u r e o f t h e e n e r g y a c t u a l l y g a i n e d b y t h e i o n b e a m . T h e t e r m " d e e v o l t a g e " V ^ e g r e f e r s t o t h e a m p l i t u d e o f t h e p o t e n t i a l d i f f e r e n c e b e t w e e n t h e t i p o f t h e d e e ( a c c e l e r a t i n g e l e c t r o d e ) a n d t h e d e e l i n e r . T h e s e r e l a t i o n s h i p s a r e s u m m a r i z e d b e l o w : q ( 2 . 3 ) c 30 _1 dE 2q dn (2.4) V acc cos <f> (2.5) - 2 V dee cos </> (2.6) where: £ r e p r e s e n t s the e l e c t r i c f i e l d encountered by the beam; and, c i s the t r a j e c t o r y taken by the ion beam through the dee gap; 2.1.2 S t a g e s D u r i n g A c c e l e r a t i o n o f t h e I o n B e a m For the purposes of d e v e l o p i n g numerical models of the a c c e l e r a t i o n process, i t i s convenient to d i v i d e the process i n t o t hree stages as suggested by f i g . 2.2: a. Ion Source and I n j e c t i o n System: 0 < E < 300 keV; b. C y c l o t r o n C e n t r a l Region: 300 keV < E < ~ 3 MeV; c. C y c l o t r o n : ~ 3 MeV < E < 450 to 520 MeV; The H- beam i s a c c e l e r a t e d e l e c t r o s t a t i c a l l y from 0 to 300 keV i n the ion source and i n j e c t i o n system. The net charge accepted f o r a c c e l e r a t i o n by the c y c l o t r o n i s i n c r e a s e d by the high e f f i c i e n c y l o n g i t u d i n a l bunching system i n the i n j e c t i o n l i n e t h a t was d e s c r i b e d by Baartman, Dutto, and Schmor [ 6 ] . The phase acceptance and i n t e r n a l beam q u a l i t y of a c y c l o t r o n are determined p r i m a r i l y i n i t s c e n t r a l r e gion where the ions are i n j e c t e d and begin a c c e l e r a t i o n . Numerical s t u d i e s by Craddock, L o u i s , and R e i s e r [7] and subsequent refinement by Dutto, Kost, Mackenzie, and 31 T I M E F i g . 2.2 Energy Gained by the Ion Beam During the Acceleration Process C r a d d o c k [ 8 ] h a v e s h o w n t h a t t h e p h a s e a c c e p t a n c e o f t h e c y c l o t r o n i s s u b s t a n t i a l l y i m p r o v e d w h e n t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m i s f l a t - t o p p e d ( s e e s e c t i o n 2 . 2 . 4 ) . T h e c e n t r a l r e g i o n i s u s u a l l y d e f i n e d a s t h e r e g i o n w h e r e e l e c t r i c f o r c e s i n t h e d e e g a p p l a y a g r e a t e r r o l e i n a f f e c t i n g t h e a x i a l m o t i o n o f t h e i o n b e a m t h a n t h e c y c l o t r o n ' s m a g n e t i c f i e l d d o e s . T h e c e n t r a l r e g i o n d e s i g n p r o b l e m s i m p o s e d b y s p a c e - c h a r g e e f f e c t s , b y t h e p h a s e d e p e n d e n c e o f t h e s t r o n g e l e c t r i c a l f o r c e s , a n d o f o r b i t c e n t e r i n g , a r e w e l l k n o w n . B e c a u s e t h e i o n b e a m p a s s e s o u t o f t h e c e n t r a l r e g i o n a f t e r o n l y 10 t o 15 o f t h e 1 5 0 0 t o 1 8 0 0 t u r n s r e q u i r e d f o r t h e i o n b e a m t o a c q u i r e a t l e a s t 4 5 0 M e V o f e n e r g y , i t i s p o s s i b l e t o . n e g l e c t t h e s p e c i a l c h a r a c t e r i s t i c s o f t h e c e n t r a l r e g i o n i n t h e s i m p l i f i e d m o d e l o f a c c e l e r a t i o n i n t h e c y c l o t r o n t h a t i s d e s c r i b e d i n s e c t i o n 2 . 3 . T h e m o d e l i s u s e d t o e s t i m a t e t h e t o l e r a n c e s o n t h e a m p l i t u d e a n d p h a s e s t a b i l i t y o f t h e a c c e l e r a t i n g v o l t a g e r e q u i r e d t o a c h i e v e s e p a r a t e d t u r n s a t t h e e x t r a c t i o n r a d i u s a n d s o m e o f t h e e f f e c t s o f a n o n - u n i f o r m t h i r d h a r m o n i c a c c e l e r a t i n g v o l t a g e p r o f i l e a l o n g t h e a c c e l e r a t i n g g a p . 2.2 T H E S H A P E O F T H E A C C E L E R A T I N G V O L T A G E W A V E F O R M B y i n c r e a s i n g t h e m a g n i t u d e o f t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m , i t i s p o s s i b l e t o : a . i n c r e a s e t h e e n e r g y g a i n p e r t u r n d E / d n ; 33 b . i n c r e a s e t h e r a d i u s g a i n p e r t u r n d R / d n a n d , t h e r e f o r e , i n c r e a s e t h e s e p a r a t i o n b e t w e e n a d j a c e n t t u r n s ; c . d e c r e a s e t h e t i m e o f f l i g h t t ^ a n d t h e r e f o r e d e c r e a s e t h e f r a c t i o n o f t h e b e a m l o s t t o e l e c t r o m a g n e t i c s t r i p p i n g a n d c o l l i s i o n s w i t h r e s i d u a l g a s m o l e c u l e s . B y a l t e r i n g t h e s h a p e o f t h e a c c e l e r a t i n g v o l t a g e w a v e f o r m [ 9 ] , i t i s p o s s i b l e t o c o m p e n s a t e f o r t h e f o l l o w i n g s e c o n d o r d e r e f f e c t s : a . e n e r g y d i s p e r s i o n i n t h e b e a m p a c k e t a n d t h e c o n s e q u e n t i n c r e a s e i n t h e p a c k e t ' s e f f e c t i v e r a d i a l w i d t h ; b . l i m i t a t i o n s o n t h e a m o u n t o f b e a m a c c e p t e d b y t h e c y c l o t r o n f o r a c c e l e r a t i o n i m p o s e d b y t h e n e e d f o r t h e i o n s t o c l e a r t h e c y c l o t r o n ' s s t r u c t u r a l c e n t r e p o s t o n t h e f i r s t t u r n a n d t o b e v e r t i c a l l y a c c e p t e d , a n d t h e r e q u i r e m e n t s t h a t i o n s m u s t e n d u p i n o r b i t s c e n t e r e d t o 0 . 0 4 i n c h e s , a n d t h e r a d i a l - l o n g i t u d i n a l c o u p l i n g a m p l i t u d e s h o u l d b e l e s s t h a n 0 . 0 1 i n c h e s [ 1 1 ] ; c . l o n g i t u d i n a l s p a c e c h a r g e e f f e c t s i n i o n b e a m s w i t h h i g h c u r r e n t d e n s i t i e s ; a n d , d . d e v i a t i o n s f r o m i s o c h r o n i s m i n t h e m a i n m a g n e t ' s p h a s e h i s t o r y . 34 2.2.1 P h a s e A c c e p t a n c e T h e new R F w a v e f o r m s t h a t c a n b e s y n t h e s i z e d b y c o m b i n i n g t h e f u n d a m e n t a l a n d t h i r d h a r m o n i c w a v e f o r m s a r e s u g g e s t e d b y t h e t r a c e s p r e s e n t e d i n f i g . 2 . 3 . I n f i g . 2 . 3 ( a ) , t h e f u n d a m e n t a l a n d t h i r d h a r m o n i c w a v e f o r m s a r e s u b t r a c t i n g v a r i o u s a m o u n t s o f t h i r d h a r m o n i c a r e s h o w n . I n f i g . 2 . 3 ( b ) , t h e r e l a t i v e a m p l i t u d e o f t h e t w o c o m p o n e n t s i s f i x e d i n t h e e x a c t r a t i o o f o n e t o n i n e b u t t h e p h a s e d i f f e r e n c e b e t w e e n t h i r d h a r m o n i c a n d f u n d a m e n t a l i s a l l o w e d t o s h i f t . O f p a r t i c u l a r i n t e r e s t i s t h e c o m b i n a t i o n o f t h e t h i r d h a r m o n i c a n d f u n d a m e n t a l a c c e l e r a t i n g v o l t a g e s o u t o f p h a s e i n t h e a p p r o x i m a t e r a t i o o f o n e t o n i n e w h i c h g i v e s a c o m p o s i t e w a v e f o r m t h a t h a s a g r e a t e r p h a s e a c c e p t a n c e t h a n d o e s t h e f u n d a m e n t a l a l o n e . T h i s i s r e f e r r e d t o a s " f l a t -t o p p i n g " t h e R F w a v e f o r m [ 1 0 ] . P h a s e a c c e p t a n c e ( A 0 i n f i g . 2 . 4 ) i s t h e t i m e i n t e r v a l , m e a s u r e d i n r a d i o f r e q u e n c y d e g r e e s , o v e r w h i c h a n i o n m a y b e a c c e l e r a t e d w i t h o u t p a y i n g a s p e c i f i e d p e n a l t y i n r e d u c e d e n e r g y g a i n ( A d E / d n ) w i t h r e s p e c t t o i o n s t h a t a r e a c c e l e r a t e d a t t h e p e a k o f t h e R F w a v e f o r m . F l a t - t o p p i n g c a n i m p r o v e t h e o p e r a t i n g c h a r a c t e r i s t i c s o f a c y c l o t r o n b y i n c r e a s i n g t h e p h a s e a c c e p t a n c e f r o m : c o m b i n e d 180 d e g r e e s o u t o f p h a s e . T h e e f f e c t s o f E ( 2 . 7 ) 35 -90° -45° 0° 45° 90° PHASE OF FUNDAMENTAL (degrees) F i g . 2 . 3 E f f e c t s of a Third Harmonic Component on the Accelerating Voltage Waveform 36 ~o 2 0 0 kV > - J < Z UJ O lOOkV Q_ (D Z < cr l±J _ i UJ OkV o o < - 9 0 ° FUNDAMENTAL ACCELERATING POTENTIAL (100 % ) 1 i THIRD HARMONIC ACCELERATING POTENTIAL (IM % ) - 4 5 ° 0 ° 4 5 ° PHASE OF FUNDAMENTAL (degrees) 90 F i g . 2.4 P r i n c i p l e of Acceleration with a Flat-topped Accelerating . Voltage Waveform ! to 101 < 24 A E E ( 2 . 8 ) Flat-topping reduces the energy dispersion (and the consequent spread in radius) of the ion beam as i t s components d r i f t in phase with respect to the peak of the RF waveform. For example, i f the allowable A E from the energy dispersion mechanism i s 50 keV out of 5 0 0 MeV, then the phase acceptance i s less than 0 . 8 1 degrees for a sinusoidal waveform but jumps to less than 7.32 degrees for. a flat-topped waveform. 2 . 2 . 2 P h a s e A c c e p t a n c e i n t h e C e n t r a l R e g i o n The problems usually encountered in the design of the central region are a l l e v i a t e d in the TRIUMF cyclotron by three features: a. i n j e c t i o n at r e l a t i v e l y high energy ( 3 0 0 keV) from an external ion source; b. strong magnetic a x i a l focusing; and, in the near future, c. flat-topping the accelerating voltage waveform. The expected phase acceptance for various selection c r i t e r i a in both fundamental and high current RF modes are shown in f i g . 2 . 5 . Note that optimizing the RF waveform to produce a broad phase acceptance i s quite d i f f e r e n t from the condition required for separated turn acceleration [ 1 1 ] . The optimum r e l a t i v e t h i r d harmonic amplitude and phase for the two 38 60 40 -20 FUNDAMENTAL RF MODE OmA < SPACE CHARGE EFFECT <^4mA < "FLAT-TOPPED" RF MODE € = 0.24 5 = -25° < c~<: <:<z -60 -40 20 J _ 40 _l_ > -20 20 <7J(deg ) > > > > 40 60 I CENTRE POST CLEARANCE CENTERING TO + 0 004in-r-0 COUPLING AMPLITUDE VERTICAL ACCEPTANCE TOTAL PHASE ACCEPTANCE 1 CENTRE POST CLEARANCE CENTERING TO + 0 ,04in r-4> COUPLING' AMPLITUDE VERTICAL ACCEPTANCE TOTAL PHASE ACCEPTANCE 1 60 F i g . 2.5 Expected Phase Acceptance f o r Various Selection C r i t e r i a (after [11] p 138) c a s e s a r e g i v e n i n e q u a t i o n ( 2 . 9 ) . = V, [ c o s < £ - e c o s ( 3 0 + 6 ) ] ( 2 . 9 ) • CC I e = 0 . 1 1 , 6 = 0 d e g r e e s f o r s e p a r a t e d t u r n a c c e l e r a t i o n ; a n d , e = 0 . 2 4 , 6 = - 2 5 d e g r e e s f o r b r o a d p h a s e a c c e p t a n c e . 2 . 2 . 3 M i n i m i z i n g T h e E f f e c t i v e R a d i a l W i d t h o f t h e I o n B e a m T h e e f f e c t i v e r a d i a l w i d t h o f t h e i o n b e a m i s a f u n c t i o n o f t h r e e f a c t o r s , a s s u g g e s t e d b y f i g . 2 . 6 : a . t h e i n s t a n t a n e o u s s i z e o f t h e b e a m p a c k e t ; b . t h e r a d i a l d r i f t o f t h e c e n t r o i d o f t h e b e a m p a c k e t c a u s e d b y d r i f t s i n t h e a v e r a g e e n e r g y g a i n e d p e r t u r n , i . e . f l u c t u a t i o n s i n t h e a m p l i t u d e o f t h e a c c e l e r a t i n g v o l t a g e ; a n d , c . d i s t o r t i o n o f t h e s h a p e o f t h e b e a m p a c k e t c a u s e d b y t h e s p r e a d i n p h a s e o f t h e i o n s w i t h r e s p e c t t o t h e p e a k i n t h e a c c e l e r a t i n g v o l t a g e a n d t h e c o n s e q u e n t s p r e a d i n e n e r g y a n d r a d i a l p o s i t i o n . A f o u r t h f a c t o r , r a d i a l c o h e r e n t ( b e t a t r o n ) o s c i l l a t i o n s o f t h e b e a m a t t h e e x t r a c t i o n r a d i u s , i s m a d e w o r s e b y t h e f i r s t h a r m o n i c c o m p o n e n t o f t h e m a g n e t i c f i e l d a n d a s y m m e t r i e s i n t h e a c c e l e r a t i n g v o l t a g e p r o f i l e [ 1 2 ] . I n p r a c t i c e , s u c h p r o b l e m s a r e n o r m a l l y d e a l t w i t h b y c e n t e r i n g t h e b e a m o r b i t s a n d a d j u s t i n g t h e m a i n m a g n e t ' s 40 EFFECTIVE RADIAL WIDTH OF A BEAM PACKET BEAM T •< >•', / PACKET . . / ' J ' I >• ' y 1 DIRECTION % ^ L y / OF TRAVEL , • ' / \ ' A E E * 7 PHASE FRONTS H H IDEAL ENERGY DRIFT ENERGY DISPERSION COMBINATION • ^ p - r a d i a l width A<p phase width F i g . 2.6 E f f e c t of Energy D r i f t and Energy Dispersion on the Effe c t i v e Radial Width of a Beam Packet h a r m o n i c c o r r e c t i o n c o i l s a n d a r e n o t w i t h i n t h e s c o p e o f t h i s s t u d y . A l t h o u g h a d d i t i o n o f a t h i r d h a r m o n i c c o m p o n e n t t o t h e R F w a v e f o r m c a n , b y i n c r e a s i n g t h e p h a s e a c c e p t a n c e o f t h e c y c l o t r o n , r e d u c e t h e i n c r e a s e i n t h e e f f e c t i v e r a d i a l w i d t h o f t h e i o n b e a m c a u s e d b y e n e r g y d i s p e r s i o n , i t c a n n o t r e d u c e t h e d r i f t i n e n e r g y , h e n c e r a d i a l p o s i t i o n , c a u s e d b y f l u c t u a t i o n s i n t h e a m p l i t u d e o f t h e a c c e l e r a t i n g v o l t a g e . T h u s , i f t h e p u r p o s e o f f l a t - t o p p i n g t h e a c c e l e r a t i n g v o l t a g e i s t o r e d u c e t h e e f f e c t i v e w i d t h o f t h e b e a m p a c k e t a t t h e e x t r a c t i o n r a d i u s t o f a c i l i t a t e s i n g l e t u r n e x t r a c t i o n , i t w i l l b e i n e f f e c t i v e u n l e s s t h e a m p l i t u d e o f t h e a c c e l e r a t i n g v o l t a g e i s s u f f i c i e n t l y w e l l r e g u l a t e d . 2 . 2 . 4 M i n i m i z i n g t h e E f f e c t o f L o n g i t u d i n a l S p a c e C h a r g e F o r c e s W h e n h i g h b e a m c u r r e n t s a r e a c c e l e r a t e d , p e r h a p s a f e w h u n d r e d m i c r o a m p e r e s i n t h e c a s e o f T R I U M F , l o n g i t u d i n a l s p a c e c h a r g e f o r c e s c a n c a u s e t h e e f f e c t i v e p h a s e w i d t h o f t h e i o n b e a m t o i n c r e a s e w h i c h , i n t u r n , t e n d s t o d e c r e a s e t h e e f f e c t i v e p h a s e a c c e p t a n c e o f t h e c y c l o t r o n . U l t i m a t e l y t h i s t e n d s t o d e s t r o y t u r n s e p a r a t i o n b y i n c r e a s i n g t h e e n e r g y s p r e a d w i t h i n a t u r n . W e l t o n [ 1 3 ] , a n d l a t e r M u l l e r a n d M a h r t [ 1 4 ] , s h o w e d t h a t t h i s e f f e c t c a n b e a l l e v i a t e d b y a c c e l e r a t i n g t h e b e a m o f f t h e p e a k o f t h e R F w a v e f o r m o r , i n t h e c a s e o f a f l a t - t o p p e d w a v e f o r m , b y p h a s e s h i f t i n g t h e t h i r d h a r m o n i c c o m p o n e n t b y a f e w d e g r e e s . M . M . G o r d o n [ 1 5 ] p r e s e n t e d a n e x t e n s i v e a n a l y s i s o f t h e l o n g i t u d i n a l s p a c e 42 c h a r g e e f f e c t w i t h f o r m u l a s f o r c a l c u l a t i n g t h e r e s u l t a n t e n e r g y s p r e a d w i t h i n a t u r n u n d e r c e r t a i n c o n d i t i o n s . M o r e r e c e n t r e s u l t s h a v e b e e n r e p o r t e d b y S . A d a m [ 1 6 ] . M o d i f i c a t i o n o f t h e s h a p e o f t h e R F w a v e f o r m t o c o m b a t l o n g i t u d i n a l s p a c e c h a r g e e f f e c t s w a s i n v e s t i g a t e d b y W . J o h o [ 1 7 ] i n c o n n e c t i o n w i t h i s o c h r o n o u s s t o r a g e r i n g s . T h e v a l i d i t y o f t h e c o n c e p t h a s b e e n c o n f i r m e d b y o p e r a t i o n a l e x p e r i e n c e a t S I N [18]. 2 . 2 . 5 P h a s e H i s t o r y o f t h e M a g n e t i c F i e l d T h e p h a s e h i s t o r y o f T R I U M F ' s m a i n m a g n e t i s o f s o m e c o n c e r n s i n c e , a s B l o s s e r [ 1 9 ] n o t e d , a f l a t - t o p p e d c y c l o t r o n m u s t u s u a l l y h a v e a p e r f e c t l y i s o c h r o n o u s p h a s e h i s t o r y . U n f o r t u n a t e l y , a f t e r t h e f i r s t s e r i e s o f s u r v e y s o f T R I U M F ' s m a g n e t i c f i e l d ^ur^ey'sl w e r e m a d e i n e a r l y 1 9 7 3 , i t b e c a m e o b v i o u s t h a t t h e r e w e r e u n e x p e c t e d d i f f e r e n c e s b e t w e e n t h e m a g n e t i c p r o p e r t i e s o f t h e s t e e l u s e d i n t h e m o d e l m a g n e t a n d t h e s t e e l u s e d i n t h e m a i n m a g n e t . T h e s e d i f f e r e n c e s c a u s e d l a r g e d e v i a t i o n s f r o m i s o c h r o n i s m w h i c h w e r e l a r g e l y c o r r e c t e d d u r i n g a y e a r o f v e r y i n t e n s i v e m a g n e t s h i m m i n g . C r a d d o c k , B l a c k m o r e , D u t t o , K o s t , M a c k e n z i e , a n d S c h m o r [ 2 0 ] r e p o r t e d t h a t b e c a u s e o f t h e l i m i t e d t i m e a v a i l a b l e t o s h i m t h e m a i n m a g n e t , p h a s e o s c i l l a t i o n s o f u p t o + 20 d e g r e e s w e r e l e f t n e a r c e r t a i n r a d i i w h i c h g r e a t l y e x c e e d s t h e + 1 . 1 d e g r e e t o l e r a n c e o n d e v i a t i o n s f r o m i s o c h r o n i s m s p e c i f i e d b y R i c h a r d s o n a n d C r a d d o c k [ 2 2 ] . 43 C r a d d o c k e t a l [ 2 0 ] r e p o r t e d t h a t n u m e r i c a l s i m u l a t i o n s h a v e s h o w n t h a t b y s l i g h t l y " o v e r f l a t - t o p p i n g " t h e a c c e l e r a t i n g v o l t a g e s o t h a t t w o p e a k s o n e i t h e r s i d e o f z e r o d e g r e e s w e r e c r e a t e d , i t m i g h t b e p o s s i b l e t o m i n i m i z e t h e e f f e c t i v e r a d i a l w i d t h o f t h e b e a m p a c k e t a t t h e e x t r a c t i o n r a d i u s w i t h o u t r e q u i r i n g t h a t t h e m a i n m a g n e t i c f i e l d h a v e a p e r f e c t l y i s o c h r o n o u s p h a s e h i s t o r y . I t w a s r e p o r t e d t h a t t h i s t e c h n i q u e i s a p p l i c a b l e t o v a r i o u s p h a s e o s c i l l a t i o n s i n c l u d i n g a l i n e a r p h a s e r a m p , a s u c c e s s i o n o f a l t e r n a t i n g r a m p s w i t h a c o m m o n a m p l i t u d e , a n d p u r e l y s i n u s o i d a l p h a s e o s c i l l a t i o n s . N u m e r i c a l s i m u l a t i o n s s h o w e d t h a t f o r A</> = 20 d e g r e e s , a n d w i t h a f l a t - t o p p e d a c c e l e r a t i n g v o l t a g e o f t h e f o r m : V a c c = V 1 [ c o s < / > - e c o s ( 3 d > + 6 ) ] ( 2 . 1 0 ) v a l u e s o f e = 0 . 1 2 0 a n d 8 = 0 . 0 d e g r e e s w o u l d l i m i t A E d u e t o e n e r g y d i s p e r s i o n t o l e s s t h a n 52 k e V a t e x t r a c t i o n e n e r g i e s b e t w e e n 4 5 0 a n d 5 0 0 M e V . 2 . 2 . 6 R F W a v e f o r m M o d i f i c a t i o n f o r S e l e c t e d M o d e s o f O p e r a t i o n T h e t r a c e s i n f i g . 2 . 7 s h o w s e v e n R F w a v e f o r m s o p t i m i z e d f o r s e l e c t e d m o d e s o f o p e r a t i o n . I n f i g . 2 . 7 ( a ) , f o u r b a s i c w a v e f o r m s s u i t a b l e f o r a c c e l e r a t i n g s m a l l c u r r e n t s i n a p e r f e c t l y i s o c h r o n o u s c y c l o t r o n a r e s h o w n . A t p r e s e n t , a 170 k V f u n d a m e n t a l a c c e l e r a t i n g v o l t a g e i s e x c i t e d i n t h e c y c l o t r o n . I n s t a l l a t i o n o f b e t t e r c a n t i l e v e r e d p a n e l s u p p o r t s ( s t r o n g b a c k s ) , b e t t e r a l i g n m e n t 44 F i g . 2.7 RF Waveforms Optimized f o r Selected Modes Operation 4 5 o f t h e c a n t i l e v e r e d p a n e l s , a n d b e t t e r s u p p r e s s i o n o f p a r a s i t i c m o d e s w i l l p e r m i t t h e a c c e l e r a t i n g v o l t a g e t o b e r a i s e d t o 2 0 0 k V f u n d a m e n t a l . I n s t a l l a t i o n o f new c a v i t y t u n i n g m e c h a n i s m s f o r f u n d a m e n t a l a n d t h i r d h a r m o n i c a n d m o d i f i c a t i o n o f t h e c y c l o t r o n c e n t r a l r e g i o n a n d f l u x g u i d e s w i l l p e r m i t e x c i t a t i o n o f a 22 k V t h i r d h a r m o n i c f l a t -t o p p i n g v o l t a g e w h i c h w i l l g i v e a n e t 178 k V f l a t - t o p p e d a c c e l e r a t i n g v o l t a g e . A s l i g h t i n c r e a s e i n p o w e r s u p p l i e d t o e a c h m o d e w i l l y i e l d t h e d e s i g n o b j e c t i v e : a 2 0 0 k V f l a t -t o p p e d a c c e l e r a t i n g v o l t a g e . I n f i g . 2 . 7 ( b ) , t h r e e w a v e f o r m s s u i t a b l e f o r a c c e l e r a t i n g m e d i u m t o h i g h c u r r e n t b e a m s i n a c y c l o t r o n w i t h p e r h a p s s o m e d e v i a t i o n s f r o m i s o c h r o n i s m i n i t s p h a s e h i s t o r y a r e s h o w n . F o r m a x i m u m p h a s e a c c e p t a n c e , a 2 0 5 k V f u n d a m e n t a l a c c e l e r a t i n g v o l t a g e i s c o m b i n e d w i t h a 4 9 . 2 k V t h i r d h a r m o n i c v o l t a g e t h a t i s p h a s e - s h i f t e d b y - 2 5 d e g r e e s . T h i s s c h e m e d o e s n o t , h o w e v e r , p r o v i d e m a x i m u m e n e r g y r e s o l u t i o n . A s m e n t i o n e d a b o v e , W e l t o n [ 1 3 ] s h o w e d t h a t l o n g i t u d i n a l s p a c e - c h a r g e e f f e c t s i n a m a x i m u m e n e r g y r e s o l u t i o n m o d e o f o p e r a t i o n c a n b e d e a l t w i t h b y i n t r o d u c i n g a v e r y s l i g h t p h a s e s h i f t i n t h e t h i r d h a r m o n i c v o l t a g e . C r a d d o c k e t a l [ 2 0 ] h a v e s u g g e s t e d t h a t f l a t -t o p p i n g f o r m a x i m u m e n e r g y r e s o l u t i o n c a n b e i m p l e m e n t e d i n a c y c l o t r o n w i t h s o m e d e v i a t i o n s f r o m i s o c h r o n i s m i n t h e p h a s e h i s t o r y o f i t s m a i n m a g n e t b y a d d i n g s l i g h t l y m o r e t h i r d h a r m o n i c t h a n i s n e c e s s a r y t o f l a t - t o p t h e w a v e f o r m . 46 2.3 A M P L I T U D E A N D P H A S E S T A B I L I T Y O F T H E A C C E L E R A T I N G V O L T A G E W A V E F O R M 2.3.1 A c c e l e r a t i o n i n S e p a r a t e d t u r n s I f , at a pa r t i c u l a r radius, the e f f e c t i v e r a d i a l width of the beam packet i s less than the increment in radius i t gained during the previous turn, adjacent beam packets appear d i s t i n c t . There i s a space between the packets within which there i s no beam. When a packet of ions remains ph y s i c a l l y isolated from adjacent beam packets throughout i t s acceleration history and emerges as a pulse of ions with a spread in energy A E less than dE/dn, the beam i s said to have been "accelerated in separated turns". Although separated turns are routinely observed at low r a d i i or at low energies in cyclotrons, they are d i f f i c u l t to achieve at or near the maximum beam energy. Blackmore, Dohan, MacKenzie, and P o i r i e r [5] have observed separated turns in TRIUMF out to 200 MeV,well short of the beam energy required for meson production or inj e c t i o n into a post accelerator. I f , however, the e f f e c t i v e r a d i a l width of the beam i s greater than the r a d i a l gain per turn, then the t a i l s of the beam packet w i l l merge and the beam packets w i l l no longer appear ph y s i c a l l y d i s t i n c t to the extraction mechanism. At present, t h i s i s the only way that turns can be configured beyond about 200 MeV in the 520 MeV cyclotron. This has two consequences: a. The energy resolution ( A E / E ) of the extracted beam i s quite low because the ions encountered by 47 the stripping f o i l at a given radius could be composed of portions of several packets each having d i f f e r e n t energies; and, b. There i s no beam-free space in which to mount a septum for an e l e c t r o s t a t i c deflector for extraction of negative hydrogen ions. Achievement of acceleration in separated turns at the extraction radius i s a major design objective for TRIUMF. Its r e a l i z a t i o n depends on two factors: a. m i n i m i z i n g the e f f e c t i v e r a d i a l width of the ion beam packet by flat-topping the accelerating voltage waveform with a t h i r d harmonic RF system and improving the amplitude and phase s t a b i l i t y of the accelerating vcltage; and, b. maximizing the increment in radius achieved by the ion beam during the f i n a l turns with RF booster c a v i t i e s and augmented at the extraction radius by a precessional technique using the v = 3/2 resonance [21]. Character i s t i c s o f the I o n Beam D u r i n g Acceleration A beam packet must complete between 1500 and 1800 orbit s (3000 to 3600 passes across the dee gap) in order to acquire 500 MeV of kinetic energy in the TRIUMF cyclotron. The r a d i i of the f i r s t and f i n a l o rbits (25 centimetres and 792 centimetres) suggest an average separation between turns of just over four millimetres but in practice, turn 48 separations at the outer r a d i i are much smaller (less than two millimetres) than in the central regions of the cyclotron. The radius gain per turn dR/dn, the r a d i a l incoherent width V? x occupied by 1 ir - mm mrad at the azimuths of maximum and minimum extent and the r a d i a l width A , R for central ray beams with + 2 and + 5 degree phase bands are shown in f i g . 2 . 8 . The dramatic decrease in the r a d i a l separation between o r b i t s at higher energies i s graphically i l l u s t r a t e d in f i g . 2 . 9 . The ion beam t r a j e c t o r i e s are traced every 10 MeV - about every t h i r t i e t h turn. The t r a j e c t o r i e s taken by stripped beam (protons) as they reverse their radius of curvature and pass out through the beam exit horn are also shown. 2 . 3 . 2 E s t i m a t i o n o f t h e T o l e r a n c e s o n t h e R e g u l a t i o n o f t h e A c c e l e r a t i n g V o l t a g e The advantages of superimposing a t h i r d harmonic component on the accelerating voltage - a smaller energy spread in the extracted beam and the p o s s i b i l i t y of accelerating the ion beam in separated turns - can only be re a l i z e d i f the accelerating voltage i s s u f f i c i e n t l y stable. The tolerances on the phase and amplitude s t a b i l i t y of the accelerating voltage required for separated turn acceleration, the most demanding of the operating modes proposed for t h i r d harmonic enhancement of the accelerating voltage, were discussed during the design and commissioning of the cyclotron ( 1 9 6 8 - 1 9 7 4 ) by Richardson and Craddock h9 0 100 200 3 0 0 4 0 0 5 0 0 E ( M e V ) 0 100 150 200 250 275 3 0 0 R ( i n ) F i g . 2.8 TRIUMF Cyclotron - Characteristics of the Ion Beam During Acceleration (af t e r [21] p 233) F i g . 2.9 Beam Ex i t Horn The trajectory followed by ions being accelerated i n the 520 MeV TRIUMF cyclotron i s shown. The ion beam tra v e l s counter-clockwise as viewed from above. The path of the ions i s traced every t h i r t i e t h turn, i . e . approximately every 10 MeV. Note the rapid decrease i n turn separation at higher energies, p a r t i c u l a r l y greater than 250 MeV. [22], Richardson [23], and Brackhaus [24]. With renewed interest in RF flat-topping (1983) these tolerances were re-examined by Durieu [25] and Laxdal [26]. A S i m p l i f i e d M o d e l o f A c c e l e r a t i o n i n t h e 5 2 0 M e V C y c l o t r o n A t r u l y f a i t h f u l model of acceleration in a cyclotron would be extremely complex given the large number of parameters that can af f e c t the beam during acceleration. The purpose of th i s s i m p l i f i e d model of acceleration in the cyclotron i s to predict the e f f e c t of small changes or fluctuations in the accelerating voltage or cyclotron magnetic f i e l d on the energy resolution of the ion beam and to estimate the s t a b i l i t y requirements for acceleration of the ion beam in separated turns at the extraction radius. T r a d i t i o n a l l y , such models are based solely on considerations of the beam phase width and the magnitude and shape of the accelerating voltage waveform [27]. Such models ignore the longitudinal space charge and the phase compression-phase expansion e f f e c t s which are recognized as being major factors in determining the ultimate performance of high current/intermediate energy separated turn isochronous cyclotrons such as TRIUMF and SIN. In the case of TRIUMF, the accuracy of the model i s further degraded by i t s f a i l u r e to account for the effects of the large deviations from isochronism present in the TRIUMF cyclotron's phase history [20], Although i t neglects most second order e f f e c t s , the 52 s i m p l i f i e d model takes complete account of a l l f i r s t order e f f e c t s and, perhaps most importantly, the model's predictions can be expressed in closed a n a l y t i c a l form. This complements the expensive and time-consuming numerical simulations that are normally run to account for the various second order e f f e c t s . Of p a r t i c u l a r interest i s the derivation of closed form expressions for the eff e c t s of amplitude and phase modulation of the accelerating voltage on the energy resolution of the ion beam at extraction. It should also be recognized that the results of a f i r s t order model are usually only useful to f i r s t order. The results obtained from t h i s model should be regarded as order of magnitude estimates only. The t o t a l energy E T gained by the ion beam i s given by: N N E T = dE/dn = 2q £ v a C c ( e ' 5 ) (2.11) n=l n=l where: V (e,5) = V, [cos 0 - e cos(3<£ + 8)] (2.12) 3 C C I If the amplitude and phase of the accelerating voltage are constant with time, then: E T = 2 q N V a c c ( e f 5 , t ) (2.13) If the amplitude and phase of the accelerating voltage are time-varying, then: / f E T = 2 q/T J V a c c ( t ) dt (2.14) where T and t f are defined i n Table I I I . 53 T h e m a x i m u m p o s s i b l e b e a m e n e r g y i s g i v e n b y : E m a x - 2 q N V , (1 - € ) ( 2 . 1 5 ) T h e p e n a l t y i n r e d u c e d e n e r g y g a i n d u e t o d e v i a t i o n o f t h e a c c e l e r a t i n g v o l t a g e a m p l i t u d e a n d p h a s e f r o m t h e i r o p t i m u m v a l u e s i s g i v e n b y : A E " E m a x * E T ( 2 ' 1 6 ) T h e m o r e c o m m o n l y u s e d p a r a m e t e r i s t h e r e l a t i v e e n e r g y p e n a l t y w h i c h i s g i v e n b y : A E E " E m E m max T T 1 ( 2 . 1 7 ) E E E max max W h e n t h e a c c e l e r a t i n g v o l t a g e p a r a m e t e r s a r e c o n s t a n t , t h i s q u a n t i t y i s g i v e n b y : A E V a c c • = 1 ( 2 . 1 8 ) E V 1 (1 - € ) W h e n t h e a c c e l e r a t i n g voltage p a r a m e t e r s a r e t i m e -v a r y i n g , i t c a n b e a p p r o x i m a t e d b y : / ' A E i V a c c ( t ) d t = 1 2 ( 2 . 1 9 ) E N T V 1 (1 - €) T h e v a l u e s o f t h e p a r a m e t e r s a p p l i c a b l e t o t h e T R I U M F c y c l o t r o n a r e p r e s e n t e d i n T a b l e I I I . 54 TABLE III PARAMETERS OF THE SIMPLIFIED MODEL OF ACCELERATION IN THE 520 MeV CYCLOTRON 00o = 27Tx 4.6 x 10° rad/s ION ROTATION FREQUENCY CO = 5 W = 2 7T x 2 3 x 1 0 o rad/s T = 2 7T / CO = 217.4 ns E i = 300 keV (fixed) E f = 500 MeV (variable) dE/dn = 2q V = 400 keV (maximum) acc N = E f - E. <dE/dn> = 270 keV (typical) = 1300 (minimum) = 1950 (typical) RF FREQUENCY ION ROTATION PERIOD INITIAL ION ENERGY FINAL ION ENERGY ENERGY GAIN PER TURN NO. OF TURNS REQUIRED FOR ACCELERATION TO 500 MeV t f = N T = 282 (is (minimum) ; ELAPSED TIME REQUIRED I FOR ACCELERATION TO = 424'fis (typical) 500 MeV 55 S t a t i c Tolerances The model can be used to estimate the s t a t i c tolerances on the amplitude and phase s t a b i l i t y of the f i r s t and t h i r d harmonic. The expression for beam energy i s expanded in terms of </>, the phase of the RF with respect to the ion beam, and 5, the phase of the t h i r d harmonic with respect to the fundamental. The s t a t i c tolerances are estimated by determining how much each parameter can vary before the energy d r i f t or dispersion exceeds the set bounds. These tolerances are summarized i n Table IV. The RF system frequency and magnetic f i e l d s t a b i l i t y tolerances w i l l be met with r e l a t i v e ease compared to the amplitude and phase s t a b i l i t y of the accelerating voltage. The short term s t a b i l i t y of the main magnetic f i e l d already exceeds the required tolerance and very stable frequency synthesizers are readily available from commercial suppliers. Orbit codes trace the trajectory of the ion beam step-by-step using a f i e l d map of the cyclotron (which, in the case of TRIUMF, was measured during the la s t magnetic f i e l d survey in 1974) and by ca l c u l a t i n g the v e l o c i t y of the beam at a p a r t i c u l a r instant. The t r a j e c t o r i e s presented in f i g . 2.10 were calculated using such a code which accounted for the phase history of the main magnet and some non-linear e f f e c t s . Laxdal [26] and others have used o r b i t tracing codes such as GOBLIN to, calculate the trajectory of the ion beam in TRIUMF and to v e r i f y the predictions of the 56 TABLE IV TOLERANCES REQUIRED FOR SEPARATED TURN ACCELERATION Parameter Normal Separated Turns Third Harmonic, £ 0 0 - 0 . 1 2 Trim C o i l s 35 54 55 AB/B + 1.0 x IO" 5 + 0.3 x IO' 6 + 1.4 x 1 0 " 6 Aw/w + 0.25 x IO" 5 + 0.15 x IO' 6 + 0.9 x I O - 6 Phase Acceptance + 4 0 C i + 0. .5° + 6 . 7 ° AV 1/V 1 + 0.4 x IO" 3 + 1.0 x IO' 4 + 0.8 x I O - 4 A V 3 / V 3 - - + 6.6 x I O - 4 A<5 - - + 0 . 1 2 ° T o t a l Energy Spread + 600 keV ± 5 0 keV + 25 keV AE/E @ 450 MeV + 1.33 x IO" 3 + 1 . 1 1 X IO" 4 + 0 . 5 6 x I O - 4 Notes: 1. S t a b i l i t y i s defined as the nominal peak to peak value of the AC component divided by the value of the DC component. 2. Phase acceptance i s measured i n fundamental RF degrees. 3. " 6 " i s measured i n t h i r d harmonic RF degrees. 57 s i n * 294- 0 298 0 i i i i I i 302- 0 306-0 31 0 0 R( in ) 294 0 298 0 302 0 306 0 310 0 R(in ) 00 The spread i n radius due to spread i n phase of ions with respect to the peak i n the accelerating potential waveform i s c l e a r l y v i s i b l e . The leading and t r a i l i n g edges of the beam packet no longer overlap the central portion of the preceding turn. F i g . 2.10 Orbit Simulation Showing Reduction of E f f e c t i v e Beam Width by Flat-topping the Accelerating Voltage The o'rbit code traces the path of test charges as they traverse o r b i t s i n the cyclotron magnetic f i e l d . The v e r t i c a l axis, s i n <p , i s the sine of the phase difference between the time the beam crosses the dee gap and the accelerating p o t e n t i a l i s maximum. The continuous lines trace a phase front for a p a r t i c u l a r test charge. The dots are spaced 20 turns apart. In each diagram, the test charges defining i n d i v i d u a l beam packets are connected for six successive turns near R = 300 inches (about 450 MeV). s i m p l i f i e d model. The s t a t i c tolerances presented i n Table I V are v a l i d only for very slow d r i f t s in the amplitude or phase of the accelerating voltage. The effects of amplitude and phase modulation of higher frequency on the e f f e c t i v e r a d i a l width of the ion beam were analyzed by Brackhaus [24]. Using (2.19), he developed expressions for the effect of modulation of each of the four parameters V , e, <f>, 8 that define the accelerating voltage waveform. These expressions are summarized for reference in Table V . 2.4 THE ACCELERATING VOLTAGE PROFILE Up to this point, the discussion has t a c i t l y assumed that the shape of the accelerating voltage waveform w i l l be the same at a l l r a d i i . In general, however, the r a t i o of the fundamental and the t h i r d harmonic accelerating voltage amplitudes w i l l not be constant along the dee gap since their voltage p r o f i l e s are not uniform. Fortunately, the correction that the t h i r d harmonic voltage makes to the energy dispersion within the beam packet i s a linear and therefore cumulative effect [27]. Since turn separation i s required only at the extraction radius, one need only make certain that the cumulative amplitude of the t h i r d harmonic voltage seen by the beam during acceleration i s in the correct r a t i o to the cumulative amplitude of the fundamental voltage seen by the beam. Although i t might be preferable for the fundamental and t h i r d harmonic accelerating p r o f i l e s to share a close resemblance, i t i s not esse n t i a l and minor 59 TABLE V EFFECT OF AMPLITUDE AND PHASE MODULATION OF THE ACCELERATING VOLTAGE ON THE ENERGY OF THE ION BEAM N V = V . ( c o s <|> - e cos(3<j> + 6 ) ) ; E , j E = 2q I V / 2qN (1 - e) V.. a c c 1 T m a x , a c c ^ 1 n=l m, to , and A> a r e t h e a m p l i t u d e , V . ( t ) = V , (1 + m cos ( o o t + 4) ) ) m m l i m m _ , f r e q u e n c y , and phase o f t h e m o d u l a t i n g s i g n a l E T E max [c o s 4> - E c o s 3<fr .. . m ( s i n(o) t. + <f ) - s i n <(> ) | -, l l i H — m r m m l 1 - e J L V f J e ( t ) = e (1 + m cos(w t + <*> ) ) m T m E T E max [c o s <(> - e c o s 3<j> L , m ( s i n ( o o t_ + <|> ) - s i n <j> ) \ z — - — m r m m l 1 - e J L V f J 6 ( t ) = 6 cos(n i t + <|> ) m m E T = 1 | c o s o> - e J Q ( 6 ) c o s 3<J> ^max _ 0 0 - ~j Z ( - D n s i n ( 3<j > - ( l - n)tt) J (6) (s in(n(u> t . + <)>)) - sin(n<f» ) ) V f n = l 2 — m f m m n J <Kt) = <|> + <|>, cos ( to t + <t> ) o i m m ^T - 1 J (<(>.,) c o s * - e J (3<jO c o s 3<(> — o 1 o o i o E 1 - e max Z f ( - l ) n + 1 ( s i n ( n ( c o t_ + 4> ) ) - sin(n<() ) )1 -— , I • m l m m I • t f n -1 J [( J ( 4 0 sin(<|> + ( l - n ) i r ) - e j (3<|>.) sin(3(f. + (l-n)ir))l n l o y n i o 2 " | 60 deviations are acceptable provided the tolerances on the l e f t - r i g h t and up-down symmetry between dees are respected [12].. In absolute terms, i t would be desirable for the composite accelerating voltage p r o f i l e to increase with increasing radius for two reasons. Since dR/dn for a given dE/dn decreases with increasing R, i t i s obviously desirable to give the ion beam as large a dE/dn as possible at large R to keep the separation between turns as large as possible. A second order e f f e c t , phase compression-phase expansion, also suggests that a r a d i a l l y increasing voltage p r o f i l e i s desi rable. F i r s t mentioned by Mueller and Mahrt [14] and generalized by Joho [17], the phase compression-phase expansion effect i s caused by the RF magnetic f i e l d that accompanies a non-uniform accelerating voltage p r o f i l e . This f i e l d compresses the bunch size of the c i r c u l a t i n g beam for a r a d i a l l y increasing voltage or expands i t for a r a d i a l l y decreasing voltage. This phase compression-phase expansion effect was f i r s t v e r i f i e d experimentally in the 590 MeV ring cyclotron at SIN [28]. This effect plays an ess e n t i a l role in the operation of both the IUCF main stage cyclotron and SIN's injector II cyclotron. The phase expansion-phase compression e f f e c t has not yet been studied in connection with the TRIUMF cyclotron, however. 61 The o r i g i n a l design for TRIUMF spec i f i e d a uniform accelerating voltage p r o f i l e because i t was believed t h i s would naturally develop given the regular shape of the RF cavity . Recent measurements have shown that the accelerating voltage in TRIUMF decreases toward the outer r a d i i (see Appendix B) which i s obviously not desirable. It i s currently planned to f l a t t e n the voltage p r o f i l e by making appropriate changes to the RF cavity's geometry. The factors described here suggest that the p o s s i b i l i t y of reversing the slope in the voltage p r o f i l e , rather than merely f l a t t e n i n g i t , should be investigated. References: [1] E.W. Blackmore, M.K. Craddock, G. Dutto, D.A. Hutcheon, C.J. Kost, R. L i l j e s t r a n d , G.H. Mackenzie, CA. M i l l e r , J.G. Rogers, and P.W. Schmor. "Improved Beam Quality at TRIUMF." IEEE Trans NS-26(3):3218-3220 (June 1979). [2] W.R. Rawnsley, G.H. Mackenzie, and C J . Oram. "The Production and Measurement of 150 ps Beam Pulses from the TRIUMF Cyclotron." Proc 10th Int Conf on Cycl ot rons and their Appl i cat i ons, pp 237-240 (1984). [3] D.J. Clark. "Beam Diagnostics and Instrumentation" IEEE Trans NS-13(4):15-23 (August 1966). [4] M. Olivo. "Beam Diagnostic Equipment for Cyclotrons." Proc 7th Int Conf on Cyclotrons and t h e i r Appl i cat i ons, pp 331-340 (1975). [5] E.W. Blackmore, D.A. Dohan, G.H. MacKenzie, and R. P o i r i e r . "Developments Toward Separated Turns at TRIUMF." Proc 9th Int Conf on Cyclotrons and t h e i r A p p l i c a t i o n s , pp 485-487 (1981). [6] R. Baartman, G. Dutto, P.W. Schmor. "The TRIUMF High E f f i c i e n c y Beam Bunching System." Proc 10th Int Conf Cyclotrons and t h e i r Applications, pp 158-160 ( 1984). 62 [7] M.K. Craddock, R.J. Louis, M. Reiser. "H- Ion Injection into the Central Region of the TRIUMF Cyclotron." Proc 5th Int C y c l o t r o n Conf, pp 666-669 (1969). [8] G. Dutto, C. Kost, G.H. Mackenzie. "Optimization of the Phase Acceptance of the TRIUMF Cyclotron." Proc 6th Int Cycl ot r on Conf, pp 340-350 (1972). [9] M.K. Craddock. "Effects of Third Harmonic on the RF Waveform." TRIUMF Design Note TRI-DN-72-15. (June 1972). [10] M.M. Gordon. "Improving the Energy Resolution and Duty Factor of Isochronous Cyclotrons." P a r t i c l e A c c e l e r a t o r s 2:203-209 (1971). [11] J.R. Richardson. "The Present Status of TRIUMF." Proc 6th Int C y c l o t r o n Conf, pp 126-140 (1972). [12] G. Mackenzie, C. Kost, G. Dutto. "Allowable Left-Right Asymmetry in Accelerating Voltage." TRIUMF Design Note TRI-DN-73-17 (A p r i l 1973). [13] T.A. Welton. "Sector-Focused Cyclotrons." Nucl Sci Ser Report No. 26, NAS-NRC-656, p 192 (Washington, D.C., 1959). [14] R.W. Mueller and R.W. Mahrt. "Phase Compression and Phase D i l a t i o n in the Isochronous Cyclotron." Nucl Inst Meth 86:241-244 (September 1970). [15] M.M. Gordon. "The Longitudinal Space Charge Eff e c t and Energy Resolution." Proc 5th Int Cyclotron Conf, pp 305-317 (1969). [16] S. Adam. "Calculation of Space Charge Ef f e c t s in Isochronous Cyclotrons." IEEE Trans NS-32(5):2507-2509 (October 1985). [17] W. Joho. "Application of the Phase Compression - Phase Expansion Eff e c t for Isochronous Storage Rings." P a r t i c l e A c c e l e r a t o r s 6:41-52 (1974). [18] W. Joho, S. Adam, B. Berkes, T. Blumer, M. Humbel, G. Irminger, P. Lanz, C. Markovits, A. Mezger, M. Olivo, L. Rezzonico, U. Schryber, P. Sigg. "Commissioning of the New High Intensity 72 MeV Injector II for the SIN Ring Cyclotron." IEEE Trans NS-32(5):2666-2668 (October 1985). [19] H.G. Blosser. "Future Cyclotrons." Proc 6th Int Cyclotron Conf, pp 16-32 (1972). 63 [20] M.K. Craddock, E.W. Blackmore, G. Dutto, C.J. Kost, G.H. MacKenzie, and P. Schmor. "Improvements to the Beam Properties of the TRIUMF Cyclotron." IEEE Trans NS-24(3):1615-1617 (June 1977). [21] G.H. MacKenzie, M. Zach, R.E. Laxdal, J.R. Richardson, M.K. Craddock,and G. Dutto. "Plans for the Extraction of Intense Beams of H- Ions from the TRIUMF Cyclotron." Proc 10th Int Conf on Cyclotrons and Their Appl i cat i ons, pp 233-236 (1984). [22] J.R. Richardson and M.K. Craddock. "Magnetic F i e l d Tolerances for a Six-Sector 500 MeV H- Cyclotron." TRIUMF Report TRI-67-2 (December 1968). [23] J.R. Richardson. "Energy Resolution in a 500 MeV H-Cyclotron." TRIUMF Report TRI-69-6 (June 1969). [24] K.H. Brackhaus. "The Generation and Control of 1.'5 Megawatts of RF Power for the TRIUMF Cyclotron." Ph.D. Dissertation, University of B r i t i s h Columbia (1975). [25] L. Durieu. "Re-examination of RF System Tolerances for Separated Turn Acceleration in TRIUMF." TRIUMF Internal Report (September 1983). [26] R.E. Laxdal. "Tolerances Associated with Alternate Extraction Schemes." TRIUMF Design Note TRI-DN-83-50 (December 1983). [27] M.M. Gordon. "Design Considerations for A Separated Turn Isochronous Cyclotron." Nucl Inst Methods 58:245-252 (1968). [28] S. Adam, J . Cherix, W. Joho, M. Olivo. "Determination of E f f e c t i v e Accelerating Voltages in Cyclotrons Using the Phase Compression-Phase Expansion E f f e c t . " IEEE Trans NS-26(2):2146-2149 ( A p r i l 1979). 6k CHAPTER 3 DEMONSTRATION OF A F L A T - T O P P E D ACCELERAT ING VOLTAGE IN THE RF SYSTEMS TEST F A C I L I T Y CAVITY 3.1 INTRODUCTION A conceptual design for a t h i r d harmonic RF system to f l a t - t o p the accelerating voltage in the TRIUMF cyclotron, based on the concept of exciting both the fundamental and th i r d harmonic accelerating modes in the same RF cavity, was presented by Erdman et a l [ l ] in 1969, shortly after the detailed design of the cyclotron and i t s support systems began. During the period from 1970 to 1972, a more detailed design of the cyclotron's fundamental and t h i r d harmonic RF system emerged. Several internal design notes [2] - [10], two graduate theses [11] [12], and two conference publications [13] [14] presented the res u l t s of t h i s work, which culminated in the f i r s t successful operation of the cyclotron 23 MHz RF system at f u l l power under vacuum in late 1974. Unfortunately, several problems appeared during the f i r s t several months of operation, including f a i l u r e of the cavity's automatic tuning actuators and "leakage" of 23 MHz RF into the supposedly f i e l d - f r e e region between and behind the RF cavity's dees or "hot arms", as described in Appendix B. Further design and development of the t h i r d harmonic RF system was postponed while these problems were 65 investigated and solutions were developed. In 1983, the concept of t h i r d harmonic flat-topping was resurrected for reasons of both opportunity and need: •Under the terms of the RF Resonator Replacement Program [15], major s t r u c t u r a l improvements w i l l be made to the cyclotron RF cavity in 1986/87. This w i l l provide an opportunity to make the changes to the RF cavity that are necessary to permit t h i r d harmonic flat-topped operation; and, •The plan to extract H- by e l e c t r o s t a t i c d e f l e c t i o n into a magnetic channel for inj e c t i o n by charge exchange into the proposed KAON factory post-accelerator (instead of extracting protons by stripping the electrons from the H-ions from the cyclotron as i s currently done) requires that the beam accelerate in separated turns at the extraction radius. As discussed in chapter two, flat-topping the accelerating voltage i s one of three ways of increasing the separation between adjacent turns while the ion beam i s being accelerated. The design of the modifications to the cyclotron RF system that are required begins with a description of the e x i s t i n g system and formulation of a set of design objectives for the upgrade program. For reference, these have been summarized and presented in the l a s t section of Appendix B. 66 The ideal vehicle for developing the modifications to the cyclotron RF cavity that are required by the RF Resonator Replacement Program [15] and the RF Third Harmonic Flat-topping Project [16], including new cavity tuning and coupling mechanisms, would be the cyclotron i t s e l f but since regular beam production began in 1975, access to the cyclotron has generally been r e s t r i c t e d to the month-long shutdowns for maintenance and modification that are scheduled approximately twice per year. Even then, residual radiation in the cyclotron vacuum tank l i m i t s access by individuals to less than an hour per day. The l i m i t i s set by the time required to accumulate the maximum permissible radiation dosage of 50 millirem per day or 300 millirem per shutdown. The walls of the vacuum tank are activated by the frac t i o n of the ion beam lost to electromagnetic stripping at higher energies, e s p e c i a l l y greater than about 450 MeV [17] [18], and to the fr a c t i o n removed by beam scrapers used to l i m i t the v e r t i c a l extent of the ion beam during acceleration at energies beyond about 70 MeV [17] [19]. Great care has been taken to l i m i t a c t i v a t i o n of the floor and c e i l i n g of the vacuum tank and other structures and to concentrate a c t i v a t i o n in the outer walls of the vacuum tank because they can be r e l a t i v e l y e a s i l y shielded to reduce exposure of personnel to radiation during maintenance periods. As a result of t h i s plan, the residual radiation l e v e l s in the cyclotron increase with distance from the 67 central region. Personnel working in the v i c i n i t y of the flux guides and number ten resonator segments accumulate the maximum radiation dosage far more quickly than personnel working in the central region of the machine (see f i g . 7 in [19]). Techniques for improving c h a r a c t e r i s t i c s of the beam, including new accelerating voltage detection and regulation schemes, can only be tested • using the cyclotron. Such a c t i v i t i e s normally take place during the twenty-four hour long beam/cyclotron development s h i f t s that are scheduled approximately once every two weeks during normal operation. Because of the residual radiation problem, the bulk of cyclotron RF system development must take place using r e p l i c a s of the cyclotron, e s p e c i a l l y during the i n i t i a l stages of a project. During the past three years, two replicas of the cyclotron RF system have been developed: a. a 1:10 scale model of the cyclotron vacuum tank and radio frequency cavity; and, b. a singly reentrant resonant cavity constructed from components i d e n t i c a l to those that make up the cyclotron radio frequency cavity which together with two high power RF transmitters, a vacuum system, a resonator water cooling system, and other support services i s referred to as the RF systems test f a c i l i t y . The 1:10 Scale Model The e f f e c t of changes in the geometry of the cyclotron 68 on the accelerating modes in the cyclotron radio frequency cavity and p a r a s i t i c modes in the beam gap are being studied using a radio frequency network analyzer to excite and measure the response of the 1 : 1 0 scale model of the cyclotron vacuum tank and radio frequency cavity (see section B . 4 ) . These investigations have been concerned with reducing the coupling of energy from the two accelerating modes into p a r a s i t i c modes in the beam gap and making the accelerating voltage p r o f i l e at fundamental and t h i r d harmonic more uniform by: a. modifying the shape of the cavity shorting plane (root) in the central region of the cyclotron; b. modifying the shape of the cyclotron centre post and associated structures; c. modifying the shape of the flux guides; and, d. covering the s l o t s formed by the gap between indi v i d u a l resonator segments and the upper and lower flux guides. The RF Systems Test F a c i l i t y The RF systems test f a c i l i t y provides an opportunity to investigate problems (at f u l l scale rather than reduced scale) that complement those being studied on the scale model of the cyclotron, including: a. design and development of cavity tuning mechanisms and a tuning control algorithm for the fundamental and t h i r d harmonic modes; and, b. design and development of coupling loops and 69 matching networks for the fundamental and t h i r d harmonic modes. Most importantly, however, the test f a c i l i t y permits the f e a s i b i l i t y of RF flat-topping at TRIUMF to be experimentally v e r i f i e d using a cavity with operating c h a r a c t e r i s t i c s and mechanical construction similar to those of the cyclotron RF cavity and under r e a l i s t i c operating conditions. Objectives that were met during the course of t h i s design study included: a. demonstration of flat-topping of the accelerating voltage at both signal l e v e l s (a few volts) and operational l e v e l s ^ ^ e e = k i l o v o l t s at 23 MHz) [20] [21]; b. testing and evaluation of prototype and production versions of radio frequency cavity hardware [20] [22]; and, c. testing and evaluation of a prototype version of the new radio frequency control system [21] (see also chapter four). 3.2 THE RF SYSTEMS TEST FACILITY 3.2.1 General Description The test f a c i l i t y i s b u i l t around a re-entrant resonant cavity formed by mounting two resonator segments and flux guides ( i d e n t i c a l to the components that make up the cyclotron RF cavity) within a large c y l i n d r i c a l tank capable -7 of sustaining a vacuum of 10 Torr. A photograph of the 70 test f a c i l i t y i s shown in f i g . 3.1. The vacuum chamber has been opened and the RF cavity has been r o l l e d out for modification and maintenance. A plan view of the f a c i l i t y i s presented in f i g . 3.2. Plan and side views of the RF test cavity are presented in f i g . 3.3., The dashed l i n e s to the right of the cavity outline represent the e l e c t r i c a l image of the opposite dee in the dee t i p grounding plane. A cross-section of the cavity through the dee gap is presented in f i g . 3.4, showing the dimensions of the test f a c i l i t y cavity in r e l a t i o n to the cyclotron RF c a v i t y . There are three p r i n c i p a l differences between the test f a c i l i t y cavity and the cyclotron RF c a v i t y : 1. The beam gap in the test f a c i l i t y does not support p a r a s i t i c modes coupled to the accelerating modes in the RF cavity l i k e the beam gap in the cyclotron does. One can excite the test f a c i l i t y RF cavity without concern for unexpected heating and sparking in the region between and behind the resonators; 2. The test f a c i l i t y cavity contains just over 1/40th of the volume that the cyclotron cavity does, so i t requires just over l/40th the power that the cyclotron cavity requires to achieve a 100 k i l o v o l t dee voltage approximately 40 kilowatts compared to approximately 1.2 megawatts; and, 3. The test f a c i l i t y cavity cannot be excited in the push-push mode because i t is singly rather than doubly re-71 7 2 RF CAVITY COOLING WATER VIEWING PORT 10 kV T R A N S M I T T E R S RF CONTROLS 69 MHz 23 MHz 10 kW 40 kW 0 C A M A C Serial Highway (VAX-11/730) F i g . 3.2 RF Systems Test F a c i l i t y - Plan View Test F a c i l i t y RF Cavity F i g . 3.3 RF Transmitters F i g . 3.5 RF Controls Rack F i g . 4.11 f, up f3 down -ROOT beam gap strongback ROOT flux guide resonator segment flux guide 324 f, down f3 up f, down f3down dee voltage ^ probe T TIP F i g . 3»3 RF Systems Test F a c i l i t y - RF Cavity A l l dimensions i n centimetres TEST FACILITY RF CAVITY lux guide central region 1 0 l 9 l 8 l 7 l 6 l 5 l 4 l 3 l 2 l l l l l 2 l 3 l 4 l 5 l 6 l 7 l 1 center CYCLOTRON RF CAVITY post beam aperture F i g . 3.4 Comparison Between the Test F a c i l i t y RF Cavity and the Cyclotron RF Cavity A l l dimensions i n centimetres entrant. This i s unfortunate for two reasons: The e f f e c t s of asymmetric misalignment of the dees (hot arms) on either side of the dee gap cannot be studied; and, A technique for estimating the equivalent capacitance of the short gap, based on the frequency difference between the push-push and push-pull modes, cannot be used. Our estimate of the short gap capacitance of the test f a c i l i t y RF cavity was made i n d i r e c t l y using equation (3.4). T e s t F a c i l i t y Support Equipment Two custom-built high-power transmitters are used to excite the test f a c i l i t y cavity during high-level t e s t s . The f i r s t i s based on a 4CX25 000 tetrode in the f i n a l stage and i s capable of d e l i v e r i n g in excess of 40 kilowatts of power into a 50 ohm load at 23 MHz. The second i s also based on a 4CX25 000 tetrode and is capable of d e l i v e r i n g in excess of 1C kilowatts into a 50 ohm load at 69 MHz. A block diagram showing the two transmitters i s presented in f i g . 3.5. F u n c t i o n and Operation Development of the test f a c i l i t y was begun in response to the needs of the RF Resonator Replacement Program. In t h i s role, one of the two standard resonator segments in the test f a c i l i t y i s replaced with a prototype of a new . . . -7 resonator segment. The test f a c i l i t y i s evacuated to 10 Torr by turbo-molecular and o i l d i f f u s i o n vacuum pumps and a l i q u i d nitrogen cryopanel. High pumping speeds tend to 76 23 MHZ SOURCE 50 S / S OHM PREAMP ATT'f l MAX-12 W 23 MHz Transmitter FUNDAMENTAL CONTROL PANEL 120 VAC 120 VAC n l THIRD HARMONIC CONTROL PANEL 69 MHz Transmitter DRIVER AMP 4-4000C CLASS "C" AIR COOLED B R I O S C R E E N P L A T E DUAL OUTPUT BIAS POWER SUPPLY = T O T T V D C - 8 3 0 V D C MULTI--OUTPUT POWER SUPPLY • 4 0 0 V D C •1800 vac VSWR METER PANEL •1000 V D C MULTI--OUTPUT POWER SUPPLY +750 VDC +8500 VDC +300 VDC DUAL OUTPUT BIAS POWER SUPPLY -500 VDC - I P S V D C 69 MHZ SOURCE S / S PREAMP AND SWR METER MAX 14 W BID" B I SCREEN PLATE] DRIVER AMP 4CX250B (2) CLASS "C" AIR COOLED FINAL AMP 4CX20. OOOA CLASS "C" AIR COOLED Q R I D S C R E E N P L A T E VSWR LINE SECT-ION R O 1 7 .» 10 kV CONTROL RELAY VSWR METER PANEL H . V . P . S 11 kV * 10 kV CONTROL RELAY 8RTP SCR FINAL AMP 4CX20. OOOA CLASS "C" AIR COOLED VSWR LINE SECT-ION 23 MHZ TO RF CAVITY 40 kW max FROM CONTROL PANEL 480 VAC 3W 3 PHASE FROM CONTROL PANEL 69 MHZ TO RF . CAVITY 10 kW max F i g . 3.5 RF Systems Test F a c i l i t y - RF Power Sources a l l e v i a t e the recurring problem of contamination of the vacuum by minute water leaks in the resonator cooling system. The cavity i s then driven w i t h , s u f f i c i e n t power at 23 MHz (approximately 30 kilowatts) to support a dee voltage in excess of 80 k i l o v o l t s . Because an automatic tuning mechanism i s not usually available during such tests, the system i s driven at the natural frequency of the cavity rather than at a fixed frequency, i . e . in a s e l f - e x c i t e d mode. The mechanical c h a r a c t e r i s t i c s of the prototype segment are evaluated under r e a l i s t i c operating conditions, including: a. the amplitude of cantilevered panel (hot arm) vibrations excited by fluctuations in the pressure of the resonator cooling water; b. deformation and sagging of the resonator support structure (strongback) caused by heating due to power dissipated by p a r a s i t i c modes in the cyclotron vacuum tank (and simulated in the test f a c i l i t y by heating c o i l s ) ; and, c. the general mechanical soundness of the prototype resonator segment. This procedure was used to v e r i f y the mechanical soundness of the prototype resonator segment that was i n s t a l l e d in the cyclotron in the spring of 1985 [22] as part of the f i r s t phase of the resonator replacement program. 78 As studies of the fundamental and t h i r d harmonic modes progressed on the 1:10 scale model of the cyclotron, the need to begin developing new cavity tuning, coupling, and control systems to support a flat-topped accelerating voltage in the cyclotron became apparent. The test f a c i l i t y was pressed into service as described in t h i s chapter. 3.2.2 Description of the Radio Frequency Cavity The test f a c i l i t y cavity i s a short gap re-entrant cavit y . Such c a v i t i e s and their c h a r a c t e r i s t i c s have been described by several authors including Slater [23], Ramo et a l [24], Liao [25], and Mavrogenes and Gallagher [26]. The transmission l i n e model provides a convenient method for determining the resonant frequencies of such a cavity, as well as the approximate f i e l d d i s t r i b u t i o n within the cavit y . To simplify the analysis of various tuning and coupling mechanisms, the test f a c i l i t y cavity's transmission-line equivalent c i r c u i t was used to generate an equivalent short gap coaxial resonator as suggested by f i g . 3.6. Parameters of the Transmission Line Model Three parameters define the cavity in the transmission l i n e model: a. the c h a r a c t e r i s t i c impedance of the transmission l i n e , Z Q; b. the length of the transmission l i n e , 1; and, c. the capacitance of the short gap, C • 79 R O O T T I P < 314 »• COAXIAL SHORT-GAP RESONATOR F i g . 3.6 A' Transmission Line Model of the Test F a c i l i t y RF C a v i t y z0 ~ 15 0 f 1 ~ 23.30 x 1 0 6 Hz C g ~ 1 ? * 5 p F f 3 ~ 6 9 - 9 1 x 1 0 6 Hz / ~ 314 cm A l l dimensions i n centimetres 80 Using the concept of transverse resonance, one obtains: j = j Z tan kl (3.1) o CO C g 1 j Z Q tan kl + = 0 (3.2) j " C g To solve for co, one rewrites equation (3.2) as: tan (col/c) = 1 (3.3) o g and solves the resul t i n g transcendental equation. This has been done graphically in f i g . 3.7. Although in the ideal case, the frequency of the TEM-like t h i r d harmonic push-pull mode is greater than three times the frequency of the TEM-l i k e fundamental push-pull mode (because the capacitive reactance of the short gap i s smaller at the higher frequency), in practice i t may also be lower due to various perturbations in the geometry of the structure. The percentage of e l e c t r i c f i e l d energy stored in the short gap of the cavity i s of pa r t i c u l a r i n t e r e s t . If i t i s s u f f i c i e n t l y small, the analysis of cavity coupling and tuning mechanisms can be further s i m p l i f i e d by ignoring i t and simply modelling the test f a c i l i t y cavity as a quarter-wave coaxial resonator instead. F i r s t , given the c h a r a c t e r i s t i c impedance and length of 81 F i g . 3.7 Solution of ( 3 .3 ) i the Eigenvalue Equation of the Test F a c i l i t y RF Cavity the transmission l i n e , and the resonant frequency of the cavity, the short gap capacitance can be estimated: 1 (3.4) v = . Z Q co tan(col/c) This y i e l d s a value for the short gap capacitance of approximately 17.5 picofarads. Next, given the unloaded q u a l i t y factor of the cavity (approximately 5800), the equivalent capacitance of the cavity measured with respect to the dee voltage can be est imated: C- = u (1/4 C • V2, ) (3.5) o equiv dee C 4 Q P equiv ^o (3.6) 2 CO V J dee Measurements show that i t takes approximately 40 kilowatts of power at 23 MHz to support a 100 k i l o v o l t dee voltage in the test f a c i l i t y cavity at 23.3 MHz. This gives an equivalent t o t a l capacitance of approximately 635 picofarads. Because the capacitance of the short gap (~ 17.5 pF) i s so much smaller than the t o t a l equivalent capacitance of the cavity (— 635 pF): 83 c g < 3 % of C equiv (3.7) i t was neglected in the s i m p l i f i e d model of che cavity used for analysis of tuning and coupling schemes. Char a c t e r i s t i c s of a Loop-Coupled Cavity The c h a r a c t e r i s t i c s of a loop-coupled RF cavity when i t is driven near resonance may be described using the simple equivalent c i r c u i t shown in f i g . 3.8. It i s assumed that the self-inductance of the coupling loop i s cancelled by a comparatively broadband matching network represented by the series capacitance C m. A measure of the difference between the driving frequency f and the cavity's resonant frequency f may be obtained by comparing the phase of the transmission l i n e current i 1 and the RF cavity root current i ~ : Assuming a constant forward drive power, the f r a c t i o n of the power re f l e c t e d back to the transmitter i s given by: (3.8) Q L ( f/ £o> 2 1 + j Q L ( f / f c ) (3.9) 84 AS ideal F i g . 3.8 An Equivalent C i r c u i t Model f o r Loop Coupling to the Test F a c i l i t y RF Cavity The r e l a t i v e dee voltage is given by: | T | 2 = | i - r | 2 = 1 + j Q L ( f / f Q ) - Q L ( f / f 0 ) 2 (3.10) 1 + j Q L ( f / f Q ) Of p a r t i c u l a r interest i s the r e l a t i v e amount of forward power required to develop a given dee voltage as a function of tuning error: These four relations (3.8, 3.9, 3.10, 3.11) are plotted in f i g . 3.9 over a small range of frequencies near t^. Each re l a t i o n i s shown for each of three values of loaded Q. Measurement Techniques Although good mathematical models are useful and necessary during the conceptual design stage, they are of limited usefulness i f they are not complemented by good measurement techniques during the prototype and development stages. A brief outline of experimental techniques for measuring the parameters of a resonant cavity, including i t s resonant frequencies and i t s quality and coupling factors, are outlined in Appendix C. The methods developed by Ginzton [27] and updated by Kajfez and Hwan [28] are p a r t i c u l a r l y useful. M / T | 2 = 1/h - r | 2 (3.11) 86 P H A S E ( d e g ) at) JO CQ > < tr1 (D CQ fB CD CQ ^ II O H-a> o p .—X •a H- c + p P H* v — ' CO H- O fD 3 '—X a. o o o • =S H j o 00 H* • ' c+ O tr fa -> <! a1 ' « o < o o H- O o • <<: « i— 1 s o o ' — 3 at? o • o o w PJ mete: rror 4 CQ RELATIVE V O L T A G E ( |T|) RELATIVE POWER (|r|2) ( i/|T|2) 3.3 MECHANISMS FOR TUNING THE RADIO FREQUENCY CAVITY 3.3.1 Introduction The cyclotron radio frequency cavity i s a large and mechanically complex structure. It requires two sets of tuning mechanisms: one for coarse tuning under manual control, the other for fine tuning under automatic cont r o l . At present, very coarse tuning i s provided from within the cyclotron vacuum tank by the strongback l e v e l l i n g arm adjusters and coarse tuning i s provided from outside the vacuum tank by a set of ground arm adjusters. They are used to s h i f t the resonant frequency of the cavity to within the range of the automatic tuning system. The automatic fine tuning system i s used to suppress d r i f t s in the resonant frequency of the cavity caused by thermal expansion and contraction of the cavity and variations in the pressure of the resonator cooling water. Compensation for Disturbances to the Resonant Frequency of the Cavity General As the temperature of the cavity or the pressure of the cooling water in the cantilevered hot arms fluctuates, the resonant frequency of the cavity changes. The effect i s esp e c i a l l y pronounced when the resonator cooling water i s f i r s t turned on, and when radio frequency power i s i n i t i a l l y applied to the cavity, causing i t to heat up and expand. The RF cavity presents an acceptable load impedance to the transmitter over a very narrow bandwidth for three reasons: 88 The transmitter cannot dissipate large amounts of re f l e c t e d power for extended periods of time without lowering the f i n a l tetrode or triode's l i f e t i m e ; The transmitter's phase and amplitude response may be degraded when large amounts of power are r e f l e c t e d back to i t [29]; and, most importantly, The transmitter may not be able to de l i v e r s u f f i c i e n t power to support the desired accelerating voltage when i t i s drivi n g the cavity off resonance. Experience has shown that most large and mechanically complex accelerator RF c a v i t i e s , such as TRIUMF's, require some form of automatic control mechanism to suppress d r i f t s in i t s resonant frequency. The accelerating c a v i t i e s used in the National Synchrotron Light Source [30] at Brookhaven National Laboratory are c l a s s i c examples of such c a v i t i e s . The NSLS cap a c i t i v e l y loaded X/4 c a v i t i e s are tuned by three separate tuning systems. Adjusting the temperature of the center electrode's cooling water causes the stem length to change, hence the gap capacitance and the cavity frequency. The back wall of the cavity (on which the center electrode i s mounted) can be deflected by a pair of hydraulic rams c a l l e d the crusher tuner which also adjusts the gap capacitance. The piston tuner that i s mounted in the c y l i n d r i c a l wall of the cavity i s used to displace the resonant mode's magnetic f i e l d . TRIUMF's RF System Following system start-up, the TRIUMF radio frequency 89 system i s operated in s e l f - e x c i t e d mode, i . e . the frequency of the RF drive signal i s made to follow the resonant frequency of the cavity u n t i l the RF cavity has achieved thermal equilibrium. This i s accomplished either by configuring the system as a Barkhausen o s c i l l a t o r (see chapter four, section 4.1) or by adjusting the frequency of the master o s c i l l a t o r in such a way that the cavity tuning error signal (see eqn (3. 8 ) ) , a measure of the difference between the natural frequency of the cavity and the driving frequency, i s forced to zero. Free from constraints imposed by the r e l a t i v e l y slow response of the mechanical cavity tuning actuators, one may operate the radio frequency system with n e g l i g i b l e tuning error. During normal cyclotron operation, the RF drive frequency i s fixed at the f i f t h harmonic of the ion rotation frequency. Even after the cavity has reached thermal equilibrium, variations and fluctuations in the temperature and pressure of the resonator cooling water cause small disturbances to the cavity's resonant frequency which must be corrected to permit operation with a fixed frequency drive s i g n a l . When the radio frequency system i s operated in driven mode, the RF control system must monitor the cavity tuning error signal and automatically re-tune the cavity as required. The high frequency components of the tuning error signal are rejected either by r e s t r i c t i n g the compensation bandwidth or, preferably, by low-pass f i l t e r i n g the tuning error s i g n a l . 90 Past Work A number of mechanisms for automatic fine tuning of the cyclotron radio frequency cavity have been proposed for TRIUMF. Early work was reported by Grundman, T i l l s o n , Bradley, Fredriksson, and Thomas [31] in their "Conceptual Design Study of RF Resonators for a 500 MeV H- Cyclotron" (1970) in which methods for tuning the cavity to the desired fundamental resonant frequency were f i r s t discussed. Prochazka [11], in his doctoral d i s s e r t a t i o n , "The Design of the RF System for the TRIUMF Cyclotron" (1972), was the f i r s t to consider methods for tuning the TRIUMF radio frequency cavity to resonance at the t h i r d harmonic of the fundamental driving frequency as well. The cyclotron radio frequency system was commissioned in late 1974. Pneumatically actuated bellows in the root of the resonators were used to tune the cavity i n i t i a l l y but had to be removed from service about a year later when they began to f a i l . In the existing automatic cavity tuning system, the resonant frequency of the cavity i s adjusted by varying the pressure of the resonator cooling water. Neither the o r i g i n a l system nor the existing system have provision for making the RF cavity harmonically resonant they can only tune the cavity for operation with either the fundamental or the t h i r d harmonic mode but not both simultaneously. The o r i g i n a l analog tuning c o n t r o l l e r , which was described by Brackhaus [12], i s s t i l l in service; i t s output section was modified to permit i t to drive the 91 resonator cooling water pressure control valve rather than the pneumatic tuning system. The existing system has performed in a s a t i s f a c t o r y way for over a decade but w i l l be replaced by a more robust tuning mechanism with a greater tuning range when new resonator segments are i n s t a l l e d in the cyclotron during phase two of the RF resonator replacement program. In 1983, with renewed interest in t h i r d harmonic f l a t -topping, new schemes for t h i r d harmonic cavity tuning were proposed for TRIUMF by Susini [32], Lanz [33], and Worsham [34]. These schemes are being evaluated on the 1:10 scale model and the RF systems test f a c i l i t y . 3.3.2 Selection C r i t e r i a for Cavity Tuning Schemes The c r i t e r i a for selecting a p a r t i c u l a r cavity tuning scheme over another divide into three categories: 1. The scheme must s h i f t the resonant frequency of the cavity over a s u f f i c i e n t range to compensate for the normal d r i f t s and disturbances present after the cavity has reached thermal equilibrium; 2. The tuning scheme must not s i g n f i c a n t l y a f f e c t the shape of the dee voltage p r o f i l e . It has been suggested that a 2 dB increase in the voltage standing wave r a t i o i s a reasonable working s p e c i f i c a t i o n although i d e a l l y i t w i l l be far l e s s . It is p a r t i c u l a r l y important that the dee voltage p r o f i l e does not change as the cavity i s tuned because this would introduce a disturbance to the r a d i a l position of the extracted turn (increasing i t s e f f e c t i v e r a d i a l width) that 92 would be very d i f f i c u l t to compensate f o r ; and, 3. The mechanism should be mechanically simple and r e l i a b l e given the high radiation environment and the r e s t r i c t e d access to the cyclotron. It should be easy to i n s t a l l and service, preferably using remote handling equipment. The t h i r d c r i t e r i o n i s p a r t i c u l a r l y d i f f i c u l t to s a t i s f y and has, thus far, held up the f i n a l s election of the fundamental and t h i r d harmonic tuning mechanisms that w i l l be incorporated into the new resonator design. Possible Tuning Schemes The tuning schemes suggested to date constitute a nearly complete l i s t of the reasonable options. They can be divided into four basic categories: 1. Perturbing the cavity with special hardware mounted inside the cavity: a. the pneumatic root tuning f o i l s that were o r i g i n a l l y i n s t a l l e d in the cyclotron; and, b. the tuning diaphragms and similar structures proposed by Prochazka [ 1 1 ] ; 2. Perturbing the cavity with special hardware mounted outside the c a v i t y : a. d e f l e c t i n g the dee l i n e r near the t i p , as can currently be done manually with sixty four "ground arm adjusters" mounted on either side of the dee gap on the top and bottom of the vacuum tank; and, b. d e f l e c t i n g the dee l i n e r near the t h i r d harmonic 93 magnetic or e l e c t r i c f i e l d strength minima, as proposed by Worsham [34]; 3. Perturbing the cavity by i n d i r e c t means: a. varying the pressure and/or temperature of the resonator cooling water, as i s currently done in the cyclotron; and, b. varying the temperature of the strongback l e v e l l i n g arm support with a heating c o i l , causing the t i p of the cantilevered panel to rock up and down, as suggested by Dohan [35]; 4. Detuning the main cavity by a closely coupled p a r a s i t i c cavity, i . e . the tuning stubs proposed by Prochazka [11], Susini [32], and Lanz [33]. Prochazka . [11] concluded that c a p a c i t i v e l y coupled tuning stubs were probably not suitable for tuning the fundamental mode of the TRIUMF RF cavity because the voltage p r o f i l e along the accelerating gap was highly disturbed near the location of a stub. Use of tuning stubs for tuning the t h i r d harmonic mode has been investigated on the 1:10 scale model of the TRIUMF RF cavity by Lanz [33] and Pacak [36]. Unlike the other cavity tuning schemes that have been proposed, a tuning stub can s h i f t the resonant frequency of the t h i r d harmonic mode without noticeably a f f e c t i n g the fundamental mode. Lanz and Pacak have confirmed that the tuning stub can adversely af f e c t the uniformity of the dee voltage p r o f i l e but the e f f e c t may be tolerable i f a s u f f i c i e n t number of stubs are d i s t r i b u t e d along the length 94 of the c a v i t y . Lanz suggested that four tuning stubs would keep the voltage v a r i a t i o n below 2 dB. Recent results (October 1985) obtained by Pacak [36] show l i t t l e e f f e c t on the voltage p r o f i l e and are generally very encouraging. Based on tests conducted using half scale models of the resonator segments, Prochazka [11] concluded that only tuning by dee-liner t i p d e f l e c t i o n or root tuning bellows s a t i s f i e d the sp e c i f i e d tolerances on: a. the variation of the voltage along the accelerating gap; and, b. the cavity's q u a l i t y factor. Although a tuning method based on ground arm t i p d e f l e c t i o n would normally be preferred because i t offers a larger tuning range, " f o r e n g i n e e r i n g r e a s o n s a n d b e c a u s e o f p o s s i b l e p r o b l e m s w i t h RF f l a t - t o p o p e r a t i o n , " he recommended that tuning using pneumatic bellows be adopted. Unfortunately, the pneumatic bellows f a i l e d after about a year of cyclotron operation and had to be taken out of service. Interest in tuning the cavity by d e f l e c t i n g the dee l i n e r at the t i p and at the t h i r d harmonic magnetic or e l e c t r i c f i e l d minima stems from: a. the mechanical s i m p l i c i t y of the scheme; b. the ease with which the tuning mechanism can be d i s t r i b u t e d along the cavity to prevent unwanted variations in the dee voltage p r o f i l e ; and, 95 c. the r e l i a b l e operating record that has been compiled by the stepping motor system that was i n s t a l l e d to deflect selected dee-liner t i p s by remote control from the RF console. The dee-liner d e f l e c t i o n scheme has been implemented on the RF systems test f a c i l i t y for evaluation. 3.3.3 Analysis of the Dee Liner Deflection Tuning Scheme The design of the dee-liner d e f l e c t i o n tuning scheme i s based on a simple application of perturbational methods that are used to calculate changes in the resonant frequency of a resonant cavity due to small changes in the material contained by the cavity or small deformations of the cavity wall, such as those described by Harrington [37]. The change in the resonant frequency of a cavity whose walls have been deformed can be estimated by: oo - co A w - Aw o m e . (3.12) co0 w where A w m and AW g are the time-average e l e c t r i c and magnetic energies contained in the volume removed by deformation and W i s the t o t a l energy stored in the o r i g i n a l cavity whose resonant frequency i s given by ccQ [37]. Inspection of the above r e l a t i o n shows that an inward defle c t i o n of the cavity wall w i l l raise the resonant 96 freguency of the cavity i f i t i s made at a point of large H and w i l l lower the resonant frequency i f i t i s made at a point, of large E . The dee-liner de f l e c t i o n scheme i s shown schematically in f i g . 3.10. To get the largest e f f e c t for a given d e f l e c t i o n , the def l e c t i o n points should be located at f i e l d maxima or minima. There are four "best" locations for tuning by cavity wall deformation. In the discussion that follows, a def l e c t i o n i s assumed to be inward unless otherwise noted: 1. Deflection of the root of the cavity w i l l cause both the fundamental and t h i r d harmonic frequencies to increase because the root i s a magnetic f i e l d maximum and e l e c t r i c f i e l d minimum for both modes; 2. Deflection of the dee-liner at the t i p w i l l cause both the fundamental and t h i r d harmonic frequencies to decrease; 3. Deflection of the dee l i n e r about one metre from the root w i l l cause the fundamental frequency to increase s l i g h t l y since the fundamental magnetic f i e l d energy density is s l i g h t l y larger than the e l e c t r i c f i e l d energy density. The t h i r d harmonic frequency w i l l decrease, however, because at t h i s point, the t h i r d harmonic e l e c t r i c f i e l d i s maximum strength and the magnetic f i e l d i s zero; and, 4. Deflection of the dee l i n e r about 1 metre from the t i p w i l l have pr e c i s e l y the opposite effect as above - the fundamental frequency w i l l decrease s l i g h t l y and the t h i r d harmonic frequency w i l l increase. 97 F i g . 3.10 A Perturbation Model fo r Cavity Tuning by Dee Liner Deflection If the displaced volume i s of small extent, one can approximate the displaced Ws by A T times the energy densities w at the position of the deformation. The t o t a l W can also be written as 7 times a space-average energy density w. Thus, following Harrington [37], the above equation can be re-written: co-co ( w - w ) A r C A T o m e = = ; (3.13) C0 q W T T where C depends only on the cavity geometry and the position of the perturbation. The parameter C was calculated for both the fundamental and t h i r d harmonic modes as a function of position along the axis of a s i m p l i f i e d model of the cavity for both a point-l i k e and a step-like d e f l e c t i o n . As noted e a r l i e r , the energy stored in the gap capacitance i s quite small compared to the e l e c t r i c f i e l d energy stored in the rest of the cavity (less than 5%) so, to simplify the c a l c u l a t i o n , i t was neglected. The c a l c u l a t i o n of the parameter C was made for a quarter-wave stub terminated by an ideal open c i r c u i t . The expressions for the cavity tuning factor as a function of position and displacement are presented in Table VI. The r e s u l t s are presented in f i g s . 3.11 through 3.13. Inspection of f i g . 3.11 shows the four optimum locations for tuning by dee-liner d e f l e c t i o n , i . e . those locations where a given d e f l e c t i o n results in the greatest frequency change. 99 TABLE VI EXPRESSIONS FOR CAVITY TUNING FACTOR AS A FUNCTION OF DEFLECTION POSITION POINT-LIKE DEFLECTION - FUNDAMENTAL MODE C (£) = 1 - ( a / b ) 2 c o s ( 2 B £ ) 1 2 ln(b/a) POINT-LIKE DEFLECTION - THIRD HARMONIC MODE C (£) = 1 - (a/b ) 2 cos(6t3£) 2 ln(b/a) STEP-LIKE DEFLECTION - FUNDAMENTAL MODE C ( £ , A £ ) = 1 - (a / b ) 2 c o s ( 2 g £ ) s i n ( B A £ ) 1 2 ln(b/a) gA£ STEP-LIKE DEFLECTION - THIRD HARMONIC MODE C ( £ , A £ ) = 1 - ( a / b ) 2 cos ( 6 B £ ) s i n ( 3 g A £ ) 2 ln(b/a) 3 g A £ 100 F i g . 3.11 Cavity Tuning Factor as a Function of Deflection P o s i t i o n Point-Like Deflection 2.0' O F i g . 3.12 Cavity Tuning Factor as a Function of Deflection P o s i t i o n -Step-Like Deflection (Fundamental Mode) F i g . 3 . 1 3 Cavity Tuning Factor as a Function of Deflection P o s i t i o n -Step-Like Deflection (Third Harmonic Mode) 3.3.4 Implementation of the Dee Liner Deflection Tuning Scheme Although the tuning scheme that w i l l be incorporated into the design of the new resonators has not yet been selected, i t i s f a i r l y certain that the RF cavity w i l l be tuned to the correct fundamental frequency either by: a. d e f l e c t i n g the dee-liner t i p ; or, b. deforming the segment root. The RF cavity w i l l then be tuned to the correct t h i r d harmonic frequency either by: a. d e f l e c t i n g the dee-liner at the t h i r d harmonic e l e c t r i c or magnetic f i e l d minima and readjusting the t i p or root as required; or, b. tuning a t h i r d harmonic p a r a s i t i c cavity/tuning stub(s). Root Tuning It i s not possible to deflect the dee-liner near the root for obvious mechanical reasons. If root tuning was adopted for the replacement resonator segments, a special root piece would have to be developed. Given the high surface current densities in the root of the cavity, the result i n g heating and mechanical stress on the mechanism, and the r e l i a b i l i t y problems that made i t necessary to remove the o r i g i n a l root tuning system from service, interest in t h i s option has been subdued. Tip Tuning When the cyclotron was constructed, provision was 104 included for manually adjusting the position of the dee-l i n e r t i p s of resonator segments two through nine for coarse tuning of the cavity's resonant frequency. The adjustment i s made by turning a ground arm adjustment mechanism that i s attached to the dee l i n e r through a vacuum bellows and the mount for the dee voltage probe. When Po i r i e r [38] found that the position of the dee l i n e r t i p s of the number eight resonator segments was c r i t i c a l for successful operation of the radio frequency system, the lower number eight resonator t i p adjusters (four in a l l ) were motorized to permit them to be adjusted from the RF console in the RF room. Their record of r e l i a b l e performance over the past several years has increased interest in using t h i s technique in the new automatic tuning system and led to i t s implementation for evaluation on the RF systems test f a c i l i t y . The geometry of the accelerating gap in the cyclotron is shown in f i g . 3.14. The position of the dee voltage probe mount i s also shown. The z - f o i l connecting the ground arm/dee-liner to the center probe housing allows the t i p to be deflected up to 1.5 centimetres with only reasonable e f f o r t . Such a wide range of movement permits one to compensate for misalignment of the cantilevered dee by maintaining a uniform dee to l i n e r spacing or to tune the cyclotron RF cavity. A similar scheme was i n s t a l l e d on the RF systems test 105 o ON Centre Probe Housing ^ Dee (Hot Arm) 4 RF CAVITY | 10 3 0 -314.1-to root Dee-Liner (Ground Arm) I „ I i * i w i\j t i < i ) 11 v \ i i n \ \ I I O M | | I I \>/)\\n w Vacuum Tank Capacitive j Voltage Probe F i g . 3.14 RF Cavity Geometry - Dee Tip Region c f . F i g . 2.1 A l l dimensions i n centimetres f a c i l i t y . The stepping motors that move the dee t i p s inwards and outwards can be controlled manually or automatically by a stepping motor c o n t r o l l e r developed at TRIUMF and based on the TRIMAC a u x i l i a r y processor [39] for CAMAC systems. Commands and data are passed between the l o c a l control processor and the stepping motor c o n t r o l l e r through a set of CAMAC addressable registers referred to as a CAMAC memory. Third Harmonic Tuning Ideally, one would deflect the middle of the dee-liner using a scheme similar to that used for t i p d e f l e c t i o n . This would involve cutting new vacuum ports in the cyclotron vacuum tank near the desired d e f l e c t i o n points. Inspection of f i g . A.3 in Appendix A shows that the s p i r a l sectors of the main magnet yoke block access to the mid parts of the cavity, e f f e c t i v e l y preventing one from adopting such a d i r e c t approach. An alternative scheme was developed which permits the access port to be located at a more convenient location. The middle of the dee-liner i s deflected by the wedge-roller i l l u s t r a t e d in f i g . 3.15. The 10:1 slope on the wedge causes the dee-liner to be pushed in 6 mm over the 60 mm that the wedge can t r a v e l . In practice, deflections of less than 2 mm were found to be adequate for obtaining the desired r a t i o of t h i r d harmonic frequency to fundamental. A spring i s used to restore the panel to i t s o r i g i n a l position 107 PULL P L A N V I E W o o 0 PULL ? SIDE VIEW ROLLER R F PANEL VACUUM TANK WALL F i g . 3 . 1 5 Mechanism f o r Dee-Liner D e f l e c t i o n as the r o l l e r i s released. The force required to deflect the panel was not i n s i g n i f i c a n t but was e a s i l y handled by the stepping motor used to p u l l the wedge. At the present time, there are no plans to build and test an inductively coupled t h i r d harmonic tuning stub for the test f a c i l i t y cavity. Tuning Range The tuning range of the prototype tuning scheme with the resonator cooling water pressure off and on i s presented in f i g s . 3.16 and 3.17. A diagonal l i n e indicates the desired 3:1 tuning r a t i o . The resonant frequencies were determined by measuring transmission through the cavity with an RF network analyzer. The network analyzer source and receiver were loosely 2 coupled to the cavity through a small (1cm ) loop in the root of the cavity and one of the capacitive dee voltage probes mounted in the t i p of the dee-liner. Maximum transmission occurs, of course, when the frequency of the network analyzer source coincides with the natural frequency of the cavit y . By measuring the transmission bandwidth for a given mode and applying the r e l a t i o n : fo Qn = (3.14) o Af 3 dB one may obtain the unloaded Q. 109 F i g . 3 . I 6 Test F a c i l i t y Cavity Timing Range -Low Water Pressure 110 F i g . 3.1? Test F a c i l i t y Cavity Tuning Range -High Water Pressure 111 S t a r t - u p and T u n i n g C o n t r o l By adjusting the resonator tuning elements while observing the network analy7er display, i t was possible to tune the cavity to resonance at both the desired fundamental frequency and i t s t h i r d harmonic while the cavity was at atmospheric pressure and driven with low power. Such a scheme i s impractical when the cavity i s evacuated and substantial amounts of power are applied to i t because of: a. the cavity's r e l a t i v e l y high q u a l i t y factor and problems associated with maintaining the desired accelerating voltage i f the RF power amplifiers are driven into a poorly matched load; and, b. resonant secondary electron emission, commonly referred to as "multipactoring", a discharge phenomena that sets a l o w e r l i m i t to: i . the radio frequency potential that can be maintained between two electrodes in a vacuum; i i . the rate at which the' resonator voltage must r i s e in order to successfully pass through the multipactor discharge range. These problems have been recognized for some time, of course. Riedel [40] reported that during the commissioning of the 37-inch cyclotron (1938), " i t soon became a p p a r e n t t h a t a h i g h e n e r g y s t o r a g e , h i g h Q d e e w h i c h f r e q u e n t l y s p a r k e d d i d n o t l o o k l i k e an a n t e n n a l o a d and d i f f e r e n t m e t h o d s had t o be f o u n d t o d r i v e i t . " Since then, 112 methods and procedures for safe and r e l i a b l e operation of cyclotron RF systems have been developed. Based on experience with the procedure currently used on the TRIUMF cyclotron, a special cavity start-up and tuning procedure was developed to f l a t - t o p the accelerating voltage in the RF test f a c i l i t y cavity, as described in section 3.5. 3.4 COUPLING POWER INTO THE RADIO FREQUENCY CAVITY 3.4.1 Introduction A general review of the analysis of resonant c a v i t i e s and cavity-coupling systems as microwave c i r c u i t elements was presented by Beringer [41]. Design formulas for coupling c i r c u i t s for accelerator RF c a v i t i e s were presented by Botha and Van der Merwe [42] and Pogue and Buskirk [43]. Power can be coupled to the accelerating modes in the radio frequency cavity through either magnetic or e l e c t r i c coupling. In practice, both methods have been shown to be p r a c t i c a l and r e l i a b l e . The choice i s dictated mainly by the available space. With e l e c t r i c coupling, the RF power tube should be close to the accelerating electrodes whereas magnetic coupling requires that the coupling loop be close to the short. Susini [47] considers magnetic coupling to be preferable for two reasons: the voltages encountered in the v i c i n i t y of the loop feedthrough are smaller than in the case of a coupling capacitor feedthrough and therefore place less stress on the vacuum seal and, the power amplifier can be assembled and tested independently of the RF c a v i t y . 113 Regarding the f i r s t point, experiences at TRIUMF during the commisioning of several high power RF systems over the past f i f t e e n y a r s have shown that mechanical f a i l u r e of the ceramic vacuum seal that separates the RF cavity from the rest of the transmission l i n e can cause a great deal of trouble. The 23 MHz power source i s loop-coupled to the cyclotron RF cavity for convenience. Not only did loop coupling permit the four 400 kilowatt amplifiers (see Appendix --B) to be tested independently of the rest of the RF system during construction but i t also minimizes the problems associated with a f a i l u r e of one of the power amplifiers during normal operation. The amplifier in question can be taken out of service and cyclotron operation restored while repairs are in progress by boosting the output of the remaining three am p l i f i e r s . If the RF power amplifiers were ca p a c i t i v e l y coupled to the RF cavity and therefore situated in the cyclotron vault, radiation problems would make i t d i f f i c u l t to implement th i s solution. The t h i r d harmonic and RF deflection systems have s u f f i c i e n t l y small power requirements (less than 100 kilowatts) that a transmitter based on a single RF power tube can e a s i l y supply the required power. While e l e c t r i c coupling i s a p o s s i b i l i t y in such cases, magnetic coupling permits RF power amplifiers to be tested and their performance v e r i f i e d independently of cyclotron operating schedules. This s i m p l i f i e s the repair and maintenance 114 problem and also permits one to reduce development time by using power amplifiers purchased from external suppliers. In the TRIUMF cyclotron, fundamental power is coupled into the RF cavity through a coupling loop mounted in the ground panel (dee l i n e r ) of resonator segment 3L3. The location "3L3" i d e n t i f i e s a p a r t i c u l a r resonator segment among the eighty that make up the RF cavity: a. 3 refers to the t h i r d quadrant - the south-west corner of the cyclotron; b. L refers to the Lower half of the cavity - the half mounted on the floor of the vacuum tank; c. 3 refers to the t h i r d resonator segment from the middle of the dee, i . e . the cyclotron center post and the central region, where the eight "number ten" resonator segments are those attached to the cavity flux guides. During construction of the cyclotron, provision was made for mounting a t h i r d harmonic coupling loop in the ground panel of resonator segment 4L3. Some aspects of the design of the fundamental and t h i r d harmonic coupling loop assembly were presented by Prochazka I 11] but they were mostly concerned with power loss in the coupling loop and the ceramic vacuum seal. In t h i s section, guidelines for the mechanical design of a high power coupling loop are reviewed and design c r i t e r i a for the optimum size and location of the 115 fundamental and t h i r d harmonic coupling loops for the RF cavity are considered. 3 . 4 . 2 D e s i g n C r i t e r i a - T h e C o u p l i n g L o o p The problem of designing a loop to c r i t i c a l l y couple a transmission l i n e to a high power radio frequency accelerating cavity i s a familiar one that i s e a s i l y solved. Given the resonant mode's f i e l d d i s t r i b u t i o n , the size of the loop that w i l l give c r i t i c a l coupling can be calculated by considering the mutual inductance between the loop and the resonant mode and the quality factor of the mode. The loop and the cavity form an autotransformer that reduces the shunt impedance of the cavity seen by the transmission l i n e from several hundred thousand ohms to approximately 50 ohms as required for c r i t i c a l coupling to a 50 ohm transmission l i n e . A b o i l e r plate model of the loop can then be constructed and i t s dimensions checked with the assistance of a network analyzer that displays the r e f l e c t i o n c o e f f i c i e n t on a polar display. Once the dimensions of the loop have been v e r i f i e d with the boiler plate model, a f i n a l version of the loop can be constructed which w i l l usually incorporate: a. a ceramic vacuum seal to p a r t i t i o n the high vacuum of the cavity from the a i r f i l l e d transmission l i n e ; and, 116 b. water cooling of the loop and portions of the inner conductor of the transmission l i n e that are exposed to vacuum. Because high currents are passed through the loop, the e l e c t r i c a l connections between the mechanical components that make up the loop must be excellent. The components are soldered together wherever possible. S i l v e r solder i s preferable to soft (tin-lead) solder because soft solder tends to sputter in environments combining high vacuum and high voltages. In a transmission l i n e , the current on the inner conductor i s uniformly d i s t r i b u t e d around i t s circumference. On the coupling loop, however, t h i s i s not the case. Just as in the two wire transmission l i n e , the bulk of the current i s concentrated on the inner surfaces of the loop. If the loop i s manufactured from c y l i n d r i c a l stock, t h i s can lead to intense l o c a l heating. The portions of the coupling loop that are perpendicular to the walls of the cavity are usually just extensions of the inner conductor of the transmission l i n e and are manufactured from c y l i n d r i c a l stock. The portion of the coupling loop that i s p a r a l l e l to the cavity wall i s usually a f l a t plate having a width comparable to the diameter of the inner conductor. Cooling water enters the 117 loop through the short and travels through a channel along the inside of the inner conductor and the top of the plate then on to the far end of the loop. It makes i t s return t r i p along b a s i c a l l y the same path. The cooling water outlet i s located next to the i n l e t . In practice, water cooling of the coupling loop i s generally not required when less than about ten thousand watts of power are coupled through the loop i f alte r n a t i v e heat evacuation paths are present. Experiences at TRIUMF and elsewhere have shown that the coupling loop i s often p a r t i c u l a r l y susceptible to multipactoring. This problem has been solved in SIN's t h i r d harmonic RF cavity [45] by sit u a t i n g the coupling loop in a ceramic cup which p a r t i t i o n s the vacuum in the cavity from the a i r - f i l l e d transmission l i n e and coupling loop assembly. The structure has proven to be quite r e l i a b l e and might be worth adapting for use at TRIUMF. 3 .4 .3 D e s i g n C r i t e r i a - Loop L o c a t i o n The optimum location for a c r i t i c a l l y coupled loop, i . e . the location that w i l l give the smallest loop, therefore the lowest Joule losses and the smallest loop self-inductance, i s in the region that has the highest magnetic flux density. In the TRIUMF radio frequency cavity, t h i s would place the coupling loop for fundamental at the root of the cavity where for a dee voltage of 100 k i l o v o l t s the maximum magnetic f i e l d strength i s approximately 2.5 kiloamperes/metre. There are two such 118 optimum locations for a t h i r d harmonic loop. One i s at the root of the cavity, where for a dee voltage of 11 k i l o v o l t s , the maximum magnetic f i e l d strength i s 0.25 kiloamperes/metre and the second i s one half of a wavelength closer to the t i p , where the magnetic f i e l d strength has a second maximum of 0.25 kiloamperes/metre. In general, the position of the coupling loop in a single mode cavity i s not c r i t i c a l . The selection of a loop mounting location depends mostly on mechanical convenience and ease of access. An additional selection c r i t e r i o n becomes important when one considers the case of a cavity that supports two modes and therefore requires two coupling loops. The addition of a second coupling loop makes the cavity a two-port system (see f i g . 3.18) and raises the p o s s i b i l i t y of power from one transmitter being coupled through the cavity and into the transmission l i n e supplying power from the second transmitter. This is c l e a r l y an undesirable s i t u a t i o n . Following Beringer [41], the transmission-loss function T(co) for the equivalent c i r c u i t in f i g . 3.18 i s given by: T(co) = (3.15) (1 + j31 + j32) + Qo (w/coo - (coo/co)2 If one port i s c r i t i c a l l y coupled and the cavity i s driven at resonance, then transmission through to the other port i s given by: 119 O g. 3-18 An Equivalent C i r c u i t Model for Transmission Through the Test F a c i l i t y Cavity F i g . 3.19 Transmission Through a C r i t i c a l l y Coupled RF Cavity as a Function of the Cavity Coupling Factor of the Second Port Equation ( 3 - 1 6 ) T<0 2)- 4 ^ / ( 2 + / 3 2 ) 2 (3.16) where (3 is the input/output coupling parameter given by: j8 = n 2 ZQ/R (3.17) Although the coupling loop matching networks provide some f i l t e r i n g action and therefore some i s o l a t i o n , i t i s not generally s u f f i c i e n t and additional transmission l i n e f i l t e r s may be required. An a l t e r n a t i v e method i s to physically i s o l a t e the undesired mode(s) by careful selection of the loop mounting locat i o n . The discussion that follows i s s i m p l i f i e d by assuming that the self-inductance of the loop i s cancelled by the matching network. The matching network i s represented by a single capacitor C m chosen so that i t s reactance cancels the inductive reactance of the loop at frequencies close to resonance, i . e . the matching network and the coupling loop inductance are series resonant at the frequency of i n t e r e s t . Although t h i s cannot occur simultaneously at both frequencies, i t does not detract from the essence of the discussion. N e t w o r k s f o r M a t c h i n g t h e C o u p l i n g L o o p s t o T r a n s m i s s i o n L i n e s In practice, a pi network, resonant l i n e section, or transmission l i n e stubs would be used to cancel the s e l f -inductance of the coupling loop. The resonant l i n e i s used 1 2 2 in most successful cyclotrons [46], including the TRIUMF fundamental RF system. A pi network or "matched l i n e feed" was o r i g i n a l l y chosen for TRIUMF, partly because of the long distance from the RF power source to the RF cavity and pa r t l y because i t i s the conventional system for trans f e r r i n g large amounts of RF power to complex loads where the p r o b a b i l i t y of frequency s h i f t s and discharges are small. The pi network must transform the complex impedance of the load into a resistance equal to the c h a r a c t e r i s t i c impedance of the transmission l i n e . Such a network has a r e l a t i v e l y small bandwidth, however, and would be d i f f i c u l t to keep tuned i f there are s h i f t s in the cavity's resonant frequency due to displacement of the dees or conditions leading to glow discharges or breakdown phenomena. The transmission l i n e has no voltage nodes where insulators can be placed without subjecting them to high e l e c t r i c f i e l d s . With a "resonant l i n e section", a portion of the transmission l i n e supports a standing wave. Supporting insulators are placed at voltage nodes and surge protectors are placed at the appropriate places to prevent flashover surges in the machine from t r a v e l l i n g back to the power amp l i f i e r s . This matching structure i s also not as sens i t i v e to matching conditions or to impedance changes produced by glow discharges. The test f a c i l i t y and cyclotron t h i r d harmonic RF systems operate with much less power than the fundamental does. The test f a c i l i t y fundamental matching 1 2 3 network i s a conventional pi network. The test f a c i l i t y and cyclotron t h i r d harmonic matching networks are based on quarter-wave transmission l i n e stubs. The two methods are s i m i l a r ; the choice i s a p r a c t i c a l matter: a. at 23 MHz, quarter-wave stubs would be unwieldy; and, b. at 69 MHz, lumped vacuum capacitors and a i r core inductors are very much less than i d e a l . For similar reasons, coaxial c a v i t i e s are used as anode tank c i r c u i t s for VHF power amplifiers while lumped components are pe r f e c t l y adequate at HF. The primary advantages of matching schemes l i k e a pi network, resonant l i n e , or tuning stubs over a single capacitor are: a. Cancellation of the loop self-inductance w i l l have a wider bandwidth than a single capacitor could provide; and, b. The c o e f f i c i e n t of coupling can be adjusted over a limited range. This i s required to compensate for changes in the cavity's quality factor, including changes due to heavy beam loading. I s o l a t i o n o f T h i r d H a r m o n i c M o d e f r o m F u n d a m e n t a l C o u p l i n g L o o p By mounting the fundamental coupling loop at a location one-quarter of a t h i r d harmonic wavelength from the root, where the t h i r d harmonic mode's magnetic f i e l d strength goes through a minimum, coupling between the t h i r d harmonic mode 124 and the fundamental transmission' l i n e and power amplifier i s minimized. The need for a transmission l i n e f i l t e r to is o l a t e the fundamental amplifier from the t h i r d harmonic mode i s , therefore, largely eliminated. The loop must be s l i g h t l y increased in area (by a factor of 1.15) compared to a c r i t i c a l l y - c o u p l e d loop mounted at the root of the cavity to compensate for the s l i g h t l y smaller (0.87 H m a x) magnetic f i e l d strength. Given the size of loop needed to c r i t i c a l l y couple the loop to the fundamental mode, the coupling factor between that loop and the t h i r d harmonic mode can be estimated. As expected, i t i s much greater than unity. Note that the quali t y factor of the t h i r d harmonic mode i s a factor root 3 larger than the fundamental qua l i t y factor. Given the same mutual inductance to both modes, a loop that i s c r i t i c a l l y coupled to the fundamental mode would be overcoupled to the t h i r d harmonic mode, i . e . the input impedance to the loop i s greater than 50 ohms. I s o l a t i o n of Fundamental Mode from the T h i r d Harmonic Co u p l i n g Loop Unfortunately, there i s no location to mount the t h i r d harmonic coupling loop that w i l l completely eliminate coupling between the fundamental mode and the t h i r d harmonic transmission l i n e . A f i r s t choice for the loop would be the second magnetic f i e l d maximum located one-half of a t h i r d harmonic wavelength from the resonator root. Given the same mutual inductance to both modes, a loop that is c r i t i c a l l y 125 coupled to the t h i r d harmonic mode would be undercoupled to the fundamental mode, i . e . the input impedance to the loop i s less than 50 ohms. At t h i s point, the fundamental magnetic f i e l d strength i s reduced to half of i t s maximum value. The mutual inductance between the loop and the fundamental mode i s therefore half the mutual inductance between the loop and the t h i r d harmonic mode so the undercoupling i s act u a l l y even weaker. One might consider, on the basis of mutual inductance, whether i t might be advantageous to locate the t h i r d harmonic coupling loop closer to the t i p than the forward magnetic f i e l d maximum. In fact, there i s a very s l i g h t advantage. As the loop i s moved closer toward the t i p , the ra t i o of t h i r d harmonic to fundamental mutual inductance, given a loop that i s c r i t i c a l l y -coupled to the t h i r d harmonic mode, i . e . the r a t i o cos <j> 0 < <f> < 7T/2 (3.18) cos (</>/3 + 7T/3) increases from 2 at the maximum to 3 in the l i m i t as <f> approaches the t i p . The loop area required to c r i t i c a l l y couple the loop to the t h i r d harmonic mode increases as secant of </>, however, and therefore approaches i n f i n i t y in the l i m i t as approaches the t i p . It i s d i f f i c u l t to j u s t i f y the larger loop area in l i g h t of the s l i g h t increase in i s o l a t i o n . The forward magnetic f i e l d maximum i s chosen as the optimum point at which to couple to the t h i r d harmonic mode. 126 In summary, the optimum locations and r e l a t i v e sizes of the coupling loops are presented in f i g . 3.20._ E f f e c t of Loop Location on the Accelerating Voltage P r o f i l e The considerations just discussed were concerned with maximizing the i s o l a t i o n between the coupling loops by careful selection of the loops' location with respect to the root. An additional consideration i s the e f f e c t of the loops' location along the accelerating gap on the accelerating voltage p r o f i l e . To support a uniform voltage p r o f i l e , the cyclotron RF cavity must be lo n g i t u d i n a l l y resonant everywhere, i . e . the di r e c t i o n of power flow from magnetic to e l e c t r i c energy in the RF cavity must always be s t r i c t l y perpendicular to the accelerating gap. To an observer looking down the accelerating gap, the cyclotron RF cavity must appear "cut-o f f " just as a rectangular waveguide does when the wavelength of the guided wave i s twice the long dimension of the waveguide cross-section, i . e . X c = 2a. If a longitudinal cross-section of the RF cavity i s detuned, a transverse component to the power flow appears. This a f f e c t s the voltage p r o f i l e along the accelerating gap as shown in f i g . B.8 of Appendix B. In section three of t h i s chapter, i t was noted that the presence of a tuning stub can severely .distort the accelerating voltage p r o f i l e in i t s v i c i n i t y for similar 127 [Flat-topped Operation] _ _ _ _ _ _ _ _ _ _ _ _ Fundamental Loop [Flat-topped Operation] Fundamental and Third Harmonic Loop [either mode excited separately] F i g . 3.20 Optimum Locations f o r Fundamental and Third Harmonic Coupling Loops reasons. To the fundamental mode, unfortunately, the t h i r d harmonic loop and transmission l i n e look l i k e an inductively coupled tuning stub and vice versa. This raises the p o s s i b i l i t y of severe d i s t o r t i o n in the accelerating voltage p r o f i l e at both frequencies. This p o s s i b i l i t y i s d i f f i c u l t to evaluate a n a l y t i c a l l y and cannot be checked experimentally using the RF systems test f a c i l i t y . It should be investigated using the 1:10 scale model of the RF cavity. It seems reasonable to mount the coupling loops d i r e c t l y behind the cyclotron centre post where minor disturbances to the voltage p r o f i l e are more ea s i l y tolerated than elsewhere along the accelerating gap. 3.5 DEMONSTRATION OF A FLAT-TOPPED ACCELERATING VOLTAGE A major objective of th i s study was the demonstration of a flat-topped accelerating voltage at operational power levels in the test f a c i l i t y cavity. This was accomplished in December 1984. It showed that: a. swayback deflection could be used to make the radio frequency cavity harmonically resonant with a r e l a t i v e l y small def l e c t i o n giving a reasonably large tuning range; and, b. the presence of the fundamental mode in the radio frequency cavity suppressed multipactoring when t h i r d harmonic power was applied, thus eliminating the need to pulse through multipactoring at t h i r d harmonic and greatly simplifying the t h i r d harmonic start-up and tuning procedure. 129 Tests of the system were conducted in a l o g i c a l progression: a. small signal in a i r ; b. intermediate power in a i r ; c. intermediate power in vacuum; and, d. high power in vacuum. The f i r s t two sets of tests were conducted in a i r so that the problems of multipactoring were avoided. They were intended to v e r i f y the tuning and coupling concepts in a forgiving environment. In the small signal t e s t s , only a few watts of power were coupled into the test cavity - the large kilowatt transmitters were not used. Satisfactory completion of the small signal tests led to excitation of the RF cavity with a few thousand watts of power in a i r . It was not possible to excite the cavity with high power in a i r due to the r e l a t i v e l y low breakdown strength of a i r at atmospheric pressure which, in practice, l i m i t s the maximum dee voltage to about 50 k i l o v o l t s or one-quarter power. The l a s t two sets of tests were conducted in vacuum (< 10 ^  Torr) to more c l o s e l y approximate the normal operating environment of the TRIUMF RF cavity. Multipactoring prevents operation with a fundamental dee voltage of between twenty v o l t s and about f i v e k i l o v o l t s , and makes i t d i f f i c u l t to excite the t h i r d harmonic mode by i t s e l f . Although the multipactoring threshold and the cavity q u a l i t y factor increase with frequency, the resonator time constant decreases with frequency, increasing the rate 130 compared to the fundamental mode. When the fundamental mode was excited in the cavity, however, t h i r d harmonic multipactoring was suppressed, as noted above. A l l operations, including: a. operation of power amplifiers; b. control of RF drive frequency; c. control of RF drive l e v e l ; d. control of cavity tuning; and, e. monitoring of RF detectors; were performed manually. A TRIMAC CAMAC-based microcomputer [39] (see section 4.2) was used to interface the tuning mechanism c o n t r o l l e r a to a standard video d i s p l a y t e r m i n a l to permit convenient c o n t r o l . During the second phase of testing, the TRIMAC program was extended to permit use of a prototype version of the new radio frequency control system in place of the r e l a t i v e l y crude set-up used i n i t i a l l y . 3.5.1 Automatic S t a r t - u p and C a v i t y Tuning Procedure An important objective of t h i s work was the development of an automatic start-up and cavity tuning procedure that could be implemented in RF control system software (see section 4.2) to drive and tune the cavity at f u l l power under vacuum. The new procedure i s based on the ex i s t i n g cyclotron start-up and tuning procedure that has evolved during the past decade. It i s described below. More detailed descriptions of surface conditioning, multipactoring, and 131 sparking are presented in the l a s t appendix. 1 . During the f i r s t step, the cavity surfaces are "conditioned" by pulsing the radio frequency drive at high power with a low duty cycle, often for several hours. The system operator varies the frequency of the radio frequency drive while he observes the shape of the pulse taken off a dee voltage probe (see f i g . 3.21). When the system operator sees that the dee voltage pulse has a clean shape, i . e . no ringing, and has reached a reasonable value, the driving frequency i s close to the resonant frequency of the cavity and the surfaces are reasonably free 1 from contamination. The RF drive i s switched from pulsed to continuous drive. 2. As soon as the RF Logic Unit starts d r i v i n g the cavity continuously, power dissipated in the cavity walls increases by as much as a factor of twenty-five. In response to the resulting r i s e in i t s temperature, the cavity structure changes i t s shape s l i g h t l y and i t s resonant frequency r e l a t i v e l y greatly, i . e . by several cavity bandwidths. In the exi s t i n g system, the RF Logic Unit immediately reconfigures the RF system as a Barkhausen o s c i l l a t o r - the input to the power amplifiers i s derived from a signal taken from the cavity that has been delayed by an appropriate phase s h i f t . In the new system, the frequency of the master o s c i l l a t o r i s adjusted to track the natural frequency of the cavity instead. While t h i s complicates the system software s l i g h t l y , i t s i m p l i f i e s the system hardware considerably as w i l l be explained in the next chapter. 1 3 2 V e r t i c a l : 5 kV/division Horizontal: 2 5 0 usec/division V e r t i c a l * 5 kV/division Horizontal: 1 2 5 usec/division (b) F i g . 3 . 2 1 Cavity Dee Voltage During Pulsed Operation (a) Negligible Tuning Error (b) Some Tuning Error 1 3 3 Vertical» 25 kV/division Horizontal: 5 n s / d i v i s i o n (a) V e r t i c a l : 50 kV/division Horizontal» 10 n s / d i v i s i o n (b) Demonstration of a Thir d Harmonic Flat-topped Accelerating Voltage i n the RF Systems Test F a c i l i t y (a) Accelerating Voltage Unregulated (b) Accelerating Voltage Regulated 13k 3. The radio frequency drive signal i s then ramped to f u l l power. Sparking occurs sporadically as the dee voltage i s increased - appropriate action i s taken to prevent damage to the system. The system i s kept in s e l f - e x c i t e d mode u n t i l the temperature of the cavity, hence i t s resonant frequency, has s t a b i l i z e d ; 4. Once the cavity has s t a b i l i z e d , the resonant frequency of the cavity i s adjusted (manually, in the case of the exi s t i n g system, although i t could be done automatically in the new system) u n t i l i t coincides with the desired operating frequency. Appropriate action i s also taken to minimize coupling of power to p a r a s i t i c modes in the beam gap. At thi s point, the system i s switched from s e l f -excited mode to driven mode and the automatic tuning system begins adjusting the cavity tuning mechanism (the pressure of the resonator cooling water) in response to the tuning error signal (the r e l a t i v e phase between the coupling loop current and the cavity magnetic f i e l d ) . The radio frequency system i s now f u l l y operational and control i s passed from the RF console to the cyclotron operators' console in the central control room and the process of in j e c t i n g beam and tuning the cyclotron begins. Normally, in the new system, the t h i r d harmonic mode would not be excited u n t i l the cyclotron i s operating r e l i a b l y with just the fundamental accelerating mode. The process of in j e c t i n g the ion beam, tuning the various magnet 1 3 5 trim c o i l s to bring the ion beam to extraction radius, and centering the ion beam orbits to prevent the excitation of betatron o s c i l l a t i o n s often takes several hours. Only after the ion beam has been successfully accelerated to maximum energy and extracted with the somewhat forgiving sinusoidal accelerating voltage would one normally attempt to a l t e r the shape of the accelerating voltage for "high performance" operation. At t h i s point, the cavity surfaces have already been conditioned and, in the RF systems test f a c i l i t y at least, the presence of the fundamental mode seems to i n h i b i t multipactoring when the t h i r d harmonic mode i s excited. Therefore, i t i s expected that the start-up procedure for t h i r d harmonic w i l l be far less d i f f i c u l t than i t was for the fundamental mode. The system operator w i l l enjoy the luxury of increasing the t h i r d harmonic dee voltage to any arbi t r a r y l e v e l at an arb i t r a r y rate, in stark contrast to the case at fundamental where the dee voltage must be stepped from almost zero to almost f u l l voltage to prevent a multipactor discharge from loading down the cavity. 5. The f i r s t step in st a r t i n g up the t h i r d harmonic radio frequency system i s to determine the frequency of the cavity's t h i r d harmonic resonance. This could be accomplished in one of two ways: The t h i r d harmonic mode could be driven at a low level (to minimize the refl e c t e d power) by an RF drive signal 1 3 6 whose frequency i s exactly three times the fundamental. The RF system operator could then tune the cavity's t h i r d harmonic frequency downward from the highest possible frequency ( i . e . push the dee-liner inwards) u n t i l a dramatic r i s e in the t h i r d harmonic dee voltage i s observed, indicating that the cavity i s very close to the desired resonance and the output from the tuning error detectors is now v a l i d . Note, however, that the fundamental tuning loop would have to compensate for detuning of the fundamental resonant frequency i f the dee-liner d e f l e c t i o n scheme was used to .tune the t h i r d harmonic. A l l the stepping motor movement associated with t h i s slows the search process down considerably ; A l t e r n a t i v e l y , a variable frequency o s c i l l a t o r operating near the t h i r d harmonic of the fundamental could be temporarily used as the t h i r d harmonic RF system's master o s c i l l a t o r . Rather- than tune the cavity u n t i l a dramatic r i s e in the t h i r d harmonic dee voltage i s observed, one would sweep the o s c i l l a t o r frequency u n t i l a dramatic r i s e in the t h i r d harmonic dee voltage i s observed. Knowing the present t h i r d harmonic mode resonant frequency, one could calculate the frequency s h i f t required to make the cavity harmonically resonant. Once the cavity was c o r r e c t l y tuned, the VFO would be switched out and the output of the frequency t r i p l e r switched i n . In most respects, sweeping the cavity's t h i r d harmonic resonance across i t s range i s the preferred method for 137 locating the t h i r d harmonic resonance. The end result of the procedure i s a co r r e c t l y tuned cavity and i t doesn't require that a special variable frequency o s c i l l a t o r be included in the system. This would simplify both the system software and hardware. The variable frequency o s c i l l a t o r technique does offer some advantages, however. The VFO can be swept faster than the cavity can be tuned and i t permits the t h i r d harmonic resonance to be located without a f f e c t i n g the fundamental tuning loop. Both methods have been successfully demonstrated on the RF systems test f a c i l i t y . The VFO technique w i l l doubtless prove to be very useful during i n i t i a l operation of the th i r d harmonic RF system but sweeping the cavity resonant frequency instead w i l l make the new radio frequency control system much simpler. A t h i r d option e x i s t s , however. I f , instead of being driven from a frequency t r i p l e r , the t h i r d harmonic system is driven from i t s own frequency synthesizer that has been phase locked to the fundamental frequency synthesizer, the VFO technique becomes far simpler to implement than i t would be i f the t h i r d harmonic system was driven by a fundamental frequency t r i p l e r . , The prototype radio frequency control system described in the next chapter incorporates provision for evaluating both schemes. 1 3 8 6. Once the cavity i s tuned to the desired frequencies, the cavity tuning system must keep the cavity harmonically resonant. Here the advantages of stub tuning of the t h i r d harmonic mode become most apparent - the fundamental and t h i r d harmonic tuning control loops would be v i r t u a l l y independent. The dee-liner de f l e c t i o n tuning scheme i s a l i t t l e more complicated. An expanded version of the cavity tuning diagram presented in f i g . 3«17 is shown in f i g . 3.23. The grid structure shows l i n e s of constant wedge and t i p displacements. The desired resonant frequencies have been chosen, in t h i s example, to be 23.3 MHz and 69.9 MHz. The deadband regions, i . e . regions in which the r e f l e c t e d power i s below acceptable l i m i t s , are indicated by the dotted l i n e s . The function of the automatic tuning system i s to place the resonant frequencies of the f i r s t and t h i r d harmonic in the central rectangular region using a bang-bang control algorithm. The current tuning status i s determined by measuring the root-loop phase of both modes. If the root-loop phase of either the fundamental or t h i r d harmonic mode has exceeded the bounds of the outer rectangle, then a tuning algorithm brings them both back to within the inner rectangle. 7. The tuning c o n t r o l l e r must be able to trap a number of p o t e n t i a l l y dangerous events: The tuning loop must be disabled whenever the dee voltage i s so low, i . e . when a spark occurs, that the output 139 I I ' i i I L 21 9 .5 0.5 9 21 REFLECTED POWER (Assuming 40 KW Input) I I i l « I L 2 5 4 3 7 7 102 138 155 9 0 ROOT LOOP PHASE (Degrees) F i g . 3.23 Test F a c i l i t y RF Cavity - Tuning Chart 140 of the tuning phase detectors i s meaningless. One way of dealing with t h i s problem i s to use a comparator to generate a logic signal to indicate i f the dee voltage i s above the multipactoring threshold and i f the tuning error detector output can be considered to be v a l i d . The cavity tuning loop would be disabled when the signal was f a l s e . Usually, in such a case, the system c o n t r o l l e r would recycle the system through the start-up procedure; and, Disturbances to the resonant frequency of the cavity a r i s i n g from external disturbances to the temperature or pressure of the resonator cooling water may occur that the tuning c o n t r o l l e r cannot adequately suppress and may actually make worse. In such a case, top p r i o r i t y would be directed to keeping the fundamental mode active in order to keep the cavity thermally stable. By keeping the structure of the cavity at standard operating temperature at a l l times, one minimizes the time required for the cavity to reach thermal equilibrium, a prerequisite for successful operation of the RF system in driven mode. In case of serious trouble, one would f i r s t stop beam i n j e c t i o n and cut the t h i r d harmonic RF drive. With only fundamental mode to be concerned with, the tuning c o n t r o l l e r should be able to "ri d e " the disturbances out then bring the t h i r d harmonic mode back on as described above. If the disturbances make the cavity tuning loop very unstable, i t may prove necessary to switch the system back to se l f excited mode. 141 The advantages of keeping the cavity near operating temperature are well known. At Fermilab, a "keep-alive" system ensures that the temperature of each Tevatron accelerating cavity i s maintained at operating temperature (and therefore in tune) at a l l times, including when the high-level RF i s momentarily turned off [47], Unhappily, such a "keep-alive" system has not been implemented at TRIUMF. N. Carlson [48] has suggested a modification to the resonator cooling system that w i l l keep the resonator from cooling down i f the RF drive i s removed. By automatically dive r t i n g the flow of cooling water from the heat exchangers and back into the cavity should the RF drive be momentarily l o s t , the cavity w i l l not cool down nearly as fast as i t does with the cooling system in i t s present form. This would reduce the time required to bring the RF system back to normal operating mode. 3.6 THE RF SYSTEM UPGRADE Phase one of the resonator replacement program has been concerned with the design of a new cantilevered panel with greatly improved mechanical c h a r a c t e r i s t i c s compared to the exi s t i n g panel. Once studies of the dee voltage p r o f i l e and p a r a s i t i c modes on the cyclotron scale-model are complete, the design of a new root and dee l i n e r which incorporate mechanisms for tuning the fundamental and t h i r d harmonic modes and coupling loops to excite the modes, such as those described in t h i s chapter, can begin. This w i l l be part of phase two of the resonator replacement program which, 142 according to current plans, w i l l include the manufacture and i n s t a l l a t i o n of eighty new resonator segments in 1986/87. The RF systems test f a c i l i t y w i l l be used to demonstrate the r e l i a b i l i t y of the replacement segments, p a r t i c u l a r l y the new tuning mechanisms, prior to the i r manufacture in quantity and i n s t a l l a t i o n in the cyclotron. Design and development of a new radio frequency control system i s p r o c e e d i n g in p a r a l l e l with the design and development of the new radio frequency cavit y . Operation of the system under manual control i s possible, as was documented in t h i s chapter, but successful cyclotron operation demands that the resonant frequency of the RF cavity and the amplitude and phase of the accelerating potential be cl o s e l y and automatically regulated. The next chapter i s concerned with the i n i t i a l development of the new radio frequency control system. The prototype radio frequency control system w i l l be thoroughly tested and debugged on the test f a c i l i t y before i t i s tested with the cyclotron radio frequency system. References: [1] K.L. Erdman, A. Prochazka, O.K. Fredriksson, R. Thomas, and W.A. Grundman. "A Square-Wave RF System Design for TRIUMF." Proc 5th Int Cyclotron Conf, pp 105-110 (1969). [2] A. Prochazka, K.L. Erdman, M. Zach. "RF Square Wave." TRIUMF Design Note TRI-DN-70-33 (5 March 1970). 1 4 3 3] M. Zach. "Error Signal for Resonator Fine Tuning." TRIUMF Design Note TRI-DN-70-43 (2 June 1970). 4] A. Prochazka. "Tuning the Cavity for fo3/fo1 = 3.0000." TRIUMF Design Note TRI-DN-71-7 (8 March 1971). 5] A. Prochazka, K. Erdman. "Rate of Rise of Resonator Voltage." TRIUMF Design Note TRI-DN-71-15 (27 A p r i l 1971 ). 6] A. Prochazka. "Detuning of the Resonator." TRIUMF Design Note TR-DN-71-34 (September 1971). 7] A. Prochazka, K.L. Erdman. "Estimate of Beam Loading." TRIUMF Design Note TRI-DN-71-39 (21 October 1971). 8] A. Prochazka. "Some Considerations With Regard to RF Flat-top Operation I." TRIUMF Design Note TRI-DN-72-22 (18 October 1972). 9] A. Prochazka. "Some Considerations with Regard to RF Flat-Top Operation I I . " TRIUMF Design Note TRI-DN-72-31 (30 November 1972). 10] R. P o i r i e r . "Determining the Resonator Frequency." TRIUMF Design Note TRI-DN-73-8 (21 February 1973). 11] A. Prochazka. "The Design of the RF System for the TRIUMF Cyclotron.". PhD Dissertation, University of B r i t i s h Columbia (1972). 12] K.H. Brackhaus. "The Generation and Control of 1.5 Megawatts of RF Power for the TRIUMF Cyclotron." PhD Dissertation, University of B r i t i s h Columbia (1975). 13] K.L. Erdman, R. P o i r i e r , O.K. Fredriksson, J.F. Weldon, W.A. Grundman. "TRIUMF Amplifier and Resonator System." Proc 6th Int Cyclotron Conf. pp 451-458 ( 1 972). 14] K.L. Erdman, K.H. Brackhaus, R.H.M. Gummer. "Some Aspects of the Control and S t a b i l i z a t i o n of the RF Accelerating Voltage in the TRIUMF Cyclotron." Proc 6th Int Cyclotron Conf. pp 451-458 (1972). 15] TRIUMF F a c i l i t y Development Plan. "RF Resonator Replacement Program." (12 July 1984). 16] TRIUMF F a c i l i t y Development Plan. "RF Third Harmonic Flattopping." (12 July 1984). 17] M.K. Craddock, E.W. Blackmore, G. Dutto, C.J. Kost, G.H. Mackenzie, and P. Schmor. "Improvements to the Beam Properties of the TRIUMF Cyclotron." IEEE Trans NS-24(3)1615-1617 (June 1977). \ 144 [18] R. Baartman, J. Beveridge, E.W. Blackmore, M.K. Craddock, D. Dohan, J . Doornbos, G. Dutto, K.L. Erdman, C.J. Kost, R. Laxdal, J.A. MacDonald, G.H. Mackenzie, P.W. Schmor, J.S. Vincent. "Beam Developments at TRIUMF." IEEE Trans NS-28(4):2879-2881 (August 1981). [19] E.W. Blackmore, P. Bosman, R. Burge, G. Dutto, D. G i l l , G.H. Mackenzie, and P.W. Cchmor. "Achievement and Control of the 100uA Beam at TRIUMF." IEEE Trans NS-26(2):2320-2323 (A p r i l 1979). [20] T. Enegren, L. Durieu, G. Dutto, D. Michelson, R. P o i r i e r , and R.E. Worsham. "Recent Prototype Studies and Measurements Toward Third Harmonic Flat-topping at TRIUMF." 1984 European Conference on Progress in Cyclotrons: Aachen, West Germany. (November 1984). [21] T. Enegren, L. Durieu, D. Michelson, and R.E. Worsham. "Development of Flat-topped RF Voltage for TRIUMF." IEEE Trans NS-32(5):2936-2938 (October 1985). [22] G. Stanford, R. Worsham, K. Fong, and S. Hutton. "A New and Improved RF Resonator Segment for the TRIUMF Cyclotron." IEEE Trans 32(5):2942-2944 (October 1985). [23] J.C. Slater. "Microwave E l e c t r o n i c s . " Princeton:D. Van Nostrand, p 81 (1950). [24] S. Ramo, J.R. Whinnery, T. Van Duzer. "Fields and Waves in Communication E l e c t r o n i c s . " New York:John Wiley, pp 558-561 (1967). [25] S.Y. Liao. "Microwave Devices and C i r c u i t s . " Englewood C l i f f s : P r e n t i c e - H a l l , Inc. pp 137-139 (1980). [26] G. Mavrogenes and W.J. Gallagher. "Coaxial Cavities with Beam Interaction." IEEE Trans NS-32(5):2778-2780 262-265 (October 1985). [27] E.L. Ginzton. "Microwave Measurements." New York: McGraw-Hill Book Co. Inc., pp 391-461 (1957). [28] D. Kajfez and E.J. Hwan. "Q-Factor Measurement with Network Analyzer." IEEE Trans MTT-32:666-670 (July 1984). [29] D.R. Vaughan, G.E. Mols, D.W. Reid, J.M. Potter. "A High-Power, Solid-State RF Source for Accelerator C a v i t i e s . " IEEE Trans 32(5):2857-2859 (October 1985). [30] R.B. McKenzie-Wilson. "Tuning System for Capacitively Loaded Quarter-Wavelength Accelerating Cavity." IEEE Trans NS-32(5):2786-2787 (October 1985). 145 [31] W.A. Grundman, L.J.P. T i l l s o n , W.J.F. Bradley, O.K. Fredriksson, R.E. Thomas. "Conceptual Design Study of RF Resonators for a 500 MeV H- Cyclotron." TRIUMF Report TRI-70-2 (May 1970). [32] A. Susini, CERN. Private communication (1983). [33] P. Lanz, SIN. Private communication (1983). [34] R.E. Worsham., TRIUMF. Private communication (1983). [35] D.A. Dohan, TRIUMF. Private communication (1984). [36] J.V. Pacak, TRIUMF. Private communication (1985). [37] R.F. Harrington. "Time-Harmonic Electromagnetic F i e l d s . " New York: McGraw-Hill Book Co., pp 317-446 (1961). [38] R. P o i r i e r . "RF Impedance of the Accelerator Beam Gap and Its Significance to the TRIUMF RF System." IEEE Trans NS-26(3):3947-3950 (1979). [39] D.P. Gurd, D.R. Heywood, J.V. Cresswell. "Developments in the TRIUMF Control System." Proc 9th Int Conf on Cyclotrons and their Applications, pp 565-569 ( 1981 ) . [40] J. Riedel. "R.F. Systems." IEEE Trans NS-26(2):2133-2136 (April 1979). [41] R. Beringer. "Resonant Cav i t i e s as Microwave C i r c u i t Elements," in P r i n c i p l e s of Microwave C i r c u i t s , C.G. Montgomery, R.H. Dicke, and E.M. P u r c e l l , ed. New York: McGraw-Hill Book Co. pp 207-239 (1948). [42] A.H. Botha and F.S. Van der Merwe. "Design of C i r c u i t s for Coupling Power Amplifiers to Resonators." IEEE Trans NS-26(3):3962-3964 (1979). [43] E.W. Pogue and F.R. Buskirk. "Cavity-Coupling Investigation for the Phermex 50 MHz RF Accelerator." IEEE Trans' NS~32(5):2852-2853 (October 1985). [44] A. Susini, CERN. Private communication (1983). [45] B. Bischof. "The RF System of the Flatt o p Acceleration Structure in the SIN 590 MeV Ring Cyclotron." IEEE Trans NS-32(2):2186-2189 ( A p r i l 1979). [46] E.W. Vogt and J . J . Burgerjon, ed. "TRIUMF Proposal and Cost Estimate." University of B r i t i s h Columbia, p 63 (1966). 146 [47] Q. Kerns, C. Kerns, H. M i l l e r , S. Tawzer, J . Reid, R. Webber, D. Wildman. "Fermilab Tevatron High Level RF Accelerator Systems." IEEE Trans NS-32(5):2809-2811 (October 1985). [48] N.L. Carlson, TRIUMF. Private communication (1985). '14? CHAPTER 4 CONCEPTUAL DESIGN AND INITIAL DEVELOPMENT OF THE NEW RADIO FREQUENCY CONTROL SYSTEM 4.1 INTRODUCTION The primary function of TRIUMF's radio frequency control system i s to ensure that the cyclotron's accelerating voltage is s u f f i c i e n t l y stable in amplitude, phase, and frequency to meet operational requirements. The problems of: a. disturbances to the amplitude and phase of the accelerating voltage; b. sparking and multipactoring; and, c. d r i f t s in the resonant frequency of the accelerator RF cavity; are dealt with, by the use of a sequential c o n t r o l l e r and several feedback control loops, to permit both safe and r e l i a b l e operation of the cyclotron radio frequency system and production of an external ion beam meeting operational requi rements. An extensive system of safety interlocks are required to prevent damage to the RF system in case of f a i l u r e s within the RF power amplifiers or various support services. 1 4 8 Such f a i l u r e s might include loss of vacuum in the cyclotron tank or a f a i l u r e in one of the cyclotron's water cooling systems. Many of the RF system interlocks are implemented in software by the cyclotron's CAMAC-based central control system rather than through the RF control system. This s i m p l i f i e s the design of the RF control system considerably. The software safety system, which also supports an extensive error-logging f a c i l i t y , i s backed up by a comprehensive set of hard-wired interlocks. The radio frequency system i s supervised from an operator's console in the RF power source room (located in the service annex d i r e c t l y under the central control room and immediately next to the cyclotron vault) during system start-up. The radio frequency system can begin normal operation a few hours after successful start-up, i . e . when the system i s operating with a continuous RF drive signal at operational accelerating voltage l e v e l s and the resonant frequency of the RF cavity has s t a b i l i z e d . Control of certain functions, including RF drive frequency, cavity tuning mode (driven or s e l f - e x c i t e d ) , and the r e l a t i v e amplitude and phase of the t h i r d harmonic accelerating voltage but notably no t the fundamental accelerating voltage amplitude, can then be transferred from the RF console to the operators' console in the central control room. During normal operation, accelerating voltage regulation, cavity tuning, and spark detection and recovery are performed automatically with l i t t l e need for operator intervention. 149 The existing cyclotron RF control system was constructed in the early 1970's based on experience with an e a r l i e r control system b u i l t for the TRIUMF Central Region Model (CRM) [1], The exis t i n g system was described in conference publications by Erdman et a l [2] and Gummer [3], and in Brackhaus's thesis [4]. A si m p l i f i e d block diagram of the existing RF control system i s shown in f i g . 4.1. Compared to the cyclotron's radio frequency cavity, the radio frequency control system at TRIUMF has seen r e l a t i v e l y l i t t l e attention since i t was commissioned. This i s mainly due to i t s r e l i a b l e performance record, the r e l a t i v e l y modest accelerating voltage amplitude and phase s t a b i l i t y required by the cyclotron in i t s present modes of operation (see Table IV), and urgent p r i o r i t i e s elsewhere in the RF system, as noted in Appendix B. The exi s t i n g 23 MHz (fundamental) RF control system i s almost f i f t e e n years old and, technologically, i t i s beginning to show i t s age. Although i t has performed r e l i a b l y during the past decade, operational experience has suggested that improvements to many aspects of i t s design could s i g n i f i c a n t l y improve i t s performance. Of p a r t i c u l a r interest i s the replacement of the hard-wired f i n i t e state machine with a control program running on a dedicated control processor and the development of a better feedback compensation network and better RF detection c i r c u i t r y . 150 remote display _dc _drlye 2o_eJectro-jmjumatlC tuning. .system Dual Frequency Meter local display I MHz clock 2305 MHz c Logic Unit local manual control RF on/off self-excited/driven regulation mode pulsing mode remote control Schomondl ND30M Frequency Synthesizer local manual control ^ remote control |_ Tuning Controller . _lL . . J ._ 3 LLc P h. det. limiter I ph. det. C H limiter] limiter I phase reference RF signal paths analogue signals digital links resonotor current signal coupling loop current signal RF feedbock signol for self-excitation RF Unit 1-8 MW Transmitter }  tuning system resonator — I PA Screen Supply — Screen Modulator resonator voltage detector ^ c ^ e j ^ e l j ^ r M O T a _ t o r _ v o l t a g e _ 10 V r e f . i 4 feedback amplifier screen set-point control -Resonator Voltage Reference Unit local manual control L _ If*. s c r e e n _ supj>ly_ s e t - p o i r r t _ c o n t r o l j [_ J PJ\ j » c j M n _ m o d u l o t o r _ d r i v e _ s i g n a l J remote control ± Resonotor Voltmeter local display remote display F i g . 4.1 T R I U M F RF S y s t e m - RF S y s t e m C o n t r o l l e r (1973 t o p r e s e n t ) A second set of RF control c i r c u i t s , operating in p a r a l l e l to the f i r s t , w i l l be required to support the 69 MHz (t h i r d harmonic) RF system that w i l l probably become operational in late 1987 following completion of modifications to the cyclotron RF cavity that are planned for 1986/87. For ease of i n s t a l l a t i o n and maintenance, i t w i l l be i d e n t i c a l to the new (23 MHz) fundamental RF control system currently being designed, except for some frequency dependent elements such as RF f i l t e r s and phase s h i f t e r s . It was decided (1983) to replace rather than modify the exis t i n g 23 MHz control system because i t was believed that the problems associated with attempting to modify the exis t i n g RF control system outweighed any potential advantages associated with retaining i t . For example, Durieu's [5] attempts to increase the gain and extend the bandwidth of the e x i s t i n g feedback compensation amplifiers were p a r t i a l l y successful but caused " u n e x p e c t e d a n d undesirable e f f e c t s o n o t h e r a s p e c t s o f R F c o n t r o l s y s t e m o p e r a t i o n , " p a r t i c u l a r l y system start-up, and i t was necessary to return the system to i t s o r i g i n a l configuration. In addition, there have been s i g n i f i c a n t advances in analog and d i g i t a l e lectronics since the ex i s t i n g RF control system was designed (about 1970) of which microprocessor technology and high q u a l i t y monolithic operational amplifiers are obvious examples. A system incorporating improved components and packaging would be " e a s i e r a n d f a s t e r t o s e r v i c e , m a i n t a i n , a n d i m p r o v e " [6]. 152 The current re-design e f f o r t began in mid-1983 with i n i t i a l studies by Durieu [7] during a sixteen-month stay at TRIUMF. His interest was "mainly to assess the p o s s i b i l i t y of a c h i e v i n g (an a c c e l e r a t i n g voltage that i s s u f f i c i e n t l y s t a b l e to permit s i n g l e turn e x t r a c t i o n ) and how." He designed a compensation network for the accelerating voltage amplitude and phase control loops and supervised the design and development of the prototype version of the new control system that i s described in section 4.3 of thi s chapter. Sigg [6], during a short v i s i t to TRIUMF in August 1983, made a number of suggestions concerning the design of the new radio frequency control system based on his experience with the SIN cyclotron's RF control system. His comments were p a r t i c u l a r l y concerned with selection and evaluation of electronic components and cables for the system. The clear d i v i s i o n between RF control functions and RF safety functions has s i m p l i f i e d the control system upgrade. The e x i s t i n g f a c i l i t y for automatic f a u l t protection and equipment monitoring that i s provided by hard-wired safety interlocks, the cyclotron central safety system, and the cyclotron central control system's CAMAC and analog multiplexer (MUX) systems should not be substantially affected by the i n s t a l l a t i o n of a new RF control system. However, two safety-related functions are b u i l t into both the e x i s t i n g and new RF control systems to improve the 1 5 3 r e l i a b i l i t y of the cyclotron: 1. Detection and recovery from sparking and multipactoring are performed automatically. By eliminating the need for operator intervention each time a spark or other discharge occurs, normal operation can be restored in a few seconds rather than several minutes; and, 2. Under certain conditions, the RF cavity tuning c o n t r o l l e r can become i n e f f e c t i v e , either because the cavity tuning mechanisms are forced to an upper or lower tuning l i m i t , or because t*he cavity's resonant frequency may (under certain conditions) d r i f t faster than the c o n t r o l l e r can respond. If the cavity tuning error signal exceeds a certain threshold for a spec i f i e d time i n t e r v a l , the cavity tuning mode i s switched from driven to s e l f - e x c i t e d mode and must be manually reset by an operator. By keeping the RF cavity at operating temperature, the time required to restore normal operation is reduced. Chapter O u t l i n e The new RF control system w i l l be described in four pa r t s: 1. In the f i r s t section (4.1), the basic organization of the new RF control system i s described. The objectives for, and the constraints on, the design of the new system are outlined. They were suggested by experience with the exis t i n g TRIUMF cyclotron RF control system, the RF systems test f a c i l i t y , and the SIN ring cyclotron RF control system; 2. The second section (4.2) describes the four modes of RF 154 system operation: pulsed mode, s e l f - e x c i t e d mode, driven mode (fundamental only), and driven mode (flat-topped), and the organization of the control program that w i l l supervise the system. C r i t e r i a for selecting the new l o c a l control processor which w i l l run the control program are discussed; 3. The t h i r d section (4.3) describes the prototype version of the new RF control system that was used to control the RF systems test f a c i l i t y during the RF flat-topping tests described in chapter three and modifications and enhancements to the design of the prototype that were suggested during those tests; and, 4. The fourth section (4.4) describes some of the problems associated with the design of a radio frequency detection system for the new control system and the current state of i t s development. 4.1.1 Organization of the New Control System A t y p i c a l accelerator RF system consists of an RF source/master o s c i l l a t o r which drives an RF power amplifier which in turn excites the RF cavity in which the accelerating voltage i s developed. The accelerating voltage (usually a few hundred k i l o v o l t s ) i s often developed across the short gap of a foreshortened coaxial cavity. By modulating the amplitude and phase of the RF drive signal, the amplitude and phase of the accelerating voltage can be controlled, as suggested by f i g . 4.2. RF Detection In order to accurately control or regulate the 155 RF SOURCE COARSE MODULATORS RF POWER AMPLIFIER PHASE DETECTOR METER T l — - 0 R F C A V I T Y T U N I N G A C T U A T O R • B E A M METER METER 0-RF DETECTORS REF J V O L T A G E P R O B E F i g . h.2 A S i m p l e R a d i o F r e q u e n c y C o n t r o l S y s t e m accelerating voltage, i t i s necessary to have accurate measures of i t s amplitude and phase. It i s surp r i s i n g l y d i f f i c u l t to measure the accelerating voltage accurately, however. An approximate measurement of the voltage (to within a few percent) can be made with a capacitive probe i n s t a l l e d near the accelerating gap. The res u l t i n g capacitive divider steps the voltage down from several hundred thousand vo l t s to the order of several v o l t s . It is passed over low-loss/phase-stabilized coaxial cable to RF amplitude and phase detectors. Problems with mechanical vibration of the dee and temperature fluctuations in the probe, cable, and within the amplitude and phase detector c i r c u i t s introduce perturbations in the output and, in the case of TRIUMF, w i l l make i t d i f f i c u l t to measure the accelerating voltage to the desired accuracy, as w i l l be discussed in section 4.4. An oscilloscope i s perhaps the most useful form of amplitude and phase detector during RF system development. As noted in chapter three, the resonant frequency of the RF cavity varies with changes in i t s temperature and other e f f e c t s which may subtly a l t e r i t s geometry. Some mechanism for tuning the cavity i s required. Smaller RF c a v i t i e s , such as those used for chopping and bunching the ion beam in TRIUMF's inj e c t i o n l i n e or even the RF separator cavity i n s t a l l e d in TRIUMF's beam l i n e M9, are normally tuned once (to achieve the highest accelerating voltage for a given RF power input) and can then l e f t for days or even 157 weeks u n t i l minor retuning i s again required. Large and mechanically complex accelerator RF c a v i t i e s , p a r t i c u l a r l y those with high q u a l i t y factors, must be retuned more often, perhaps every few minutes. This requires a better measure of the cavity tuning error (the difference between the RF drive frequency and the resonant frequency of the cavity) than the r e l a t i v e amplitude of the accelerating voltage since i t i s desirable to also have some indication of whether the cavity i s tuned above or below the driving frequency. For small tuning errors, the difference in phase between the current in the coupling loop (and the transmission l i n e feeding i t ) and the RF magnetic f i e l d in the cavity (and the surface currents they induce in the cavity walls) is approximately proportional to the signed (rather than absolute) difference between the driving and resonant frequencies, as was described in chapter three. The Need for an Automatic RF Control System The manually controlled (open-loop) system shown in f i g . 4.2 i s adequate during testing and development of system components but during normal operation i t i s desirable to automate the more r e p e t i t i v e control functions and provide mechanisms for protecting the system (and personnel) from damage in the case of equipment f a i l u r e or other problems. Cyclotron radio frequency control systems are generally much simpler than other accelerator RF control systems. 1 5 8 Unlike synchronous accelerators, they operate at a fixed frequency and unlike most linear accelerators, they operate at r e l a t i v e l y low frequencies, i . e . tens of megahertz rather than hundreds or thousands of megahertz. In the case of TRIUMF, however, these advantages are s l i g h t l y offset by the size of the RF cavity (and i t s consequent mechanical complexity and large power requirements) and the extremely tight s p e c i f i c a t i o n s on amplitude and phase regulation that must be met i f the major design objective of the control system upgrade, acceleration of the,ion beam in separated turns, i s to be achieved. Functions of the RF Control System The new TRIUMF RF control system w i l l perform two major functions: F i r s t , i t w i l l control the sequence of events required -to bring the accelerating voltage up to f u l l value and i t w i l l keep the cavity tuned to the d r i v i n g frequency as required for safe and r e l i a b l e operation with either a fundamental or flat-topped accelerating voltage. This i s a sequential or programmed control task [9] because s t r i c t l y "on/off" functions are performed in response to s t r i c t l y " t r u e / f a l s e " inputs and the current "state" of the process. Note that t h i s d e f i n i t i o n of sequential control can include so-called "bang-bang" process regulators [9]. The sequential control algorithm i s implemented in software running on a dedicated " l o c a l control processor". The sequential control algorithm requires only modest bandwidth 159 compared to accelerating voltage regulation; and, Second, i t w i l l c losely regulate the amplitude and phase of the accelerating voltage as required to accelerate the ion beam with turn separation and energy resolution that meet operational requirements. The regulation of the amplitude and phase of the cyclotron accelerating voltage requires a feedback control loop with r e l a t i v e l y high bandwidth (several tens of kilohertz) and very high precision (equivalent to greater than 16 b i t s ) and cannot be e f f i c i e n t l y implemented using d i g i t a l techniques. The four accelerating voltage c o n t r o l l e r s are based on d i g i t a l l y supervised discrete analog c o n t r o l l e r s [10], as suggested by f i g 4.3. Inspection of the accelerating voltage s t a b i l i t y tolerances l i s t e d in Table IV of chapter two suggests that fundamental RF voltage s t a b i l i t y and r e l a t i v e phase s t a b i l i t y between fundamental and t h i r d harmonic w i l l be the most d i f f i c u l t s p e c i f i c a t i o n s for the drive c o n t r o l l e r to s a t i s f y . In most cyclotrons, disturbances to the accelerating voltage are generally thermal or mechanical in o r i g i n and consequently require a c o n t r o l l e r with only a moderate bandwidth. The major problem that l i m i t s the effectiveness of accelerating voltage regulation in such cases arises from the limited resolution of the RF voltage detectors. Unfortunately, an additional high frequency perturbation introduced by pulsing the ion beam complicates the design of the accelerating voltage c o n t r o l l e r for TRIUMF. 160 COARSE MODULATORS FINE MODULATORS RF SOURCE i 1 • • 1 • RF POWER AMPLIFIER ON LOCAL CONTROL PROCESSOR RF CONTROL SYSTEM SUPERVISOR ANALOG ANALOG COMPENSATION COMPENSATION REF. REF* REF' RF DETECTORS 1. System Start-Up 2 . Cavity Tuning 3. Analog Controller Supervisor 4. CCS Interface PHASE DETECTOR CONTROLLER RF CAVITY T U N I N S A C T U A T O R • B E A M REF V O L T A G E P R O B E F i g . 4.3 Incorporation of Feedback Control Loops into a Simple Radio Frequency Control System The Accelerating Voltage Control Loop In TRIUMF, the ion beam injected into the cyclotron is pulsed at 1 kilohertz with a variable duty cycle to simplify the design of certain diagnostic probes. Unfortunately, t h i s periodic (and r e l a t i v e l y severe) v a r i a t i o n in the loading on the RF cavity greatly perturbs the accelerating voltage. At maximum beam current (200 uA) and constant RF power input (1.2 megawatts), beam loading (100 kilowatts) accounts for an almost 4 percent reduction in the accelerating voltage compared to the unloaded case. The r e l a t i v e l y high frequency (compared to the thermal and mechanical problems that otherwise perturb the beam) of the perturbation introduced by beam pulsing makes regulation of the accelerating voltage very d i f f i c u l t but, unfortunately, beam pulsing must be retained u n t i l these es s e n t i a l beam diagnostic devices can be replaced. There are three methods which could be used to reduce the e f f e c t of the perturbation. F i r s t , as noted in section 2.3, perturbations with frequency components that are exact multiples of the reciprocal of the tim e - o f - f l i g h t have a minimal ef f e c t on the f i n a l beam energy. The pulser frequency could be increased from 1 kHz to between 3 and 4 kilo h e r t z , depending on the accelerating voltage and the number of turns required to reach extraction. Second, since the disturbance i s both periodic and predictable, some sort of feed-forward compensation could be introduced into the control loop. The compensation signal could be introduced 1 6 2 from a manually set pulse generator, a signal derived from the pulser i t s e l f , or using a "ripple memory" of the type described by Vader and Schreuder [11]. Third, the gain and bandwidth of the control loop compensation network could be made s u f f i c i e n t l y large but the li m i t a t i o n s imposed by the s t a b i l i t y c r i t e r i o n are quite severe. Durieu's [4] proposed compensation scheme for both the amplitude and phase regulation feedback control loops is shown in f i g . 4.4. It i s based on a proportional double integral c o n t r o l l e r which he predicts w i l l s t a b i l i z e the accelerating voltage s u f f i c i e n t l y to meet the sp e c i f i c a t i o n s for separated turn acceleration, assuming the RF detectors meet similar tolerances. The compensation amplifiers introduce two zeros into the loop. The f i r s t zero, located at ~ 1 / t 1 on. the real axis, i s designed to cancel the pole at ~ l / t which accounts for energy storage in the cavity. The second zero, at - 1 / t 2 , i s a s t a b i l i z i n g zero which i s -st required in a type two control system. The term e 3 accounts for higher order poles and the f i n i t e transport delay in the control loop. Although Durieu considered implementing the compensation network using d i r e c t d i g i t a l c o n trol, he decided to use d i g i t a l l y supervised discrete analog c o n t r o l l e r s for reasons of economy. Controller Tuning To meet the s t a b i l i t y s p e c i f i c a t i o n s that have been imposed on the accelerating potential by separated turn 1 6 3 error signal r •-reference value r(+> measured value F i g . 4.4 s t a b i l i z i n g z e r o K ^ U s - t p ( l + s t 2 ) Compensation Network and RF Modulator K. K 2 e- s t3 G l 1 Accelerating Voltage RF Amplifier & RF Cavity + RF Amplitude or Phase Detector + P2 4—• m Output of the Dee Voltage Probe S i m p l i f i e d M o d e l o f t h e New RF C o n t r o l Loop e 3 a c c o u n t s f o r h i g h e r o r d e r p o l e s and f i n i t e t r a n s p o r t d e l a y i n t h e l o o p t h e p o l e a t - l / t j . a c c o u n t s f o r e n e r g y s t o r a g e i n t h e RF c a v i t y The RF control loop i s designed to suppress Pl-type perturbations i n the amplitude and phase of the accelerating voltage. Such disturbances, are caused by changes in the beam loading, small d r i f t s i n the resonant frequency of the cavity, or small variations i n the gain of the RF power amplifiers. Errors (P2) introduced into the measurement chain cannot be compensated for by the control loop and', i n fact, cause the control loop to introduce perturbations into the amplitude and phase of the accelerating voltage. The error signal (e) may be a r t i f i c i a l l y perturbed (PO) at various stages i n the compensation network i n order to provide feed-forward compensation of c e r t a i n known or predictable Pl or P2 type disturbances. acceleration despite' the disturbance introduced by the 1 kilohertz beam pulser, the analog control loops require the highest possible loop gain and bandwidth that are consistent with stable operation. If the poles, zeros, gain, and transport delay in the process ( i . e . the power amplifiers and the radio frequency cavity) are known exactly, then the poles, zeros, and gain of the compensation network that would result in the best s t a b i l i t y and steady state error could be determined a n a l y t i c a l l y . The e f f o r t required to obtain the exact frequency response of the process i s seldom j u s t i f i e d [10]. Instead, the control-loop dynamics are usually determined by analyzing the response of the system to a disturbance such as a step change in the reference setting (set point) or the process load. The step change introduced by beam pulsing in TRIUMF i s suitable for t h i s purpose. The gain and location of the dominant pole or zero for optimum response can be calculated for tuning proportional (P), proportional-integral (PI), and proportional-integral derivative (PID) c o n t r o l l e r s . Techniques such as the closed-loop Ziegler-Nichols procedure [12] are commonly used for t h i s purpose. Other methods include the Cohen-Coon procedure and the 3C tuning method [13]. Unfortunately, a l i t e r a t u r e search f a i l e d to locate a technique for tuning proportional-double integral c o n t r o l l e r s . Although the c o n t r o l l e r could be tuned by t r i a l and error, development of a tuning procedure would be useful and raises the p o s s i b i l i t y of making the new 165 control system automatically adaptive to changes in beam loading. A l t e r n a t i v e M o d u l a t i o n a n d D e t e c t i o n S c h e m e s It i s possible that other problems, p a r t i c u l a r l y those associated with measurement of the accelerating voltage, may l i m i t the achievable amplitude s t a b i l i t y . If t h i s i s the case, a l t e r n a t i v e modulation and detection schemes could be used: The 23 MHz preamplifiers could be driven into saturation and the amplitude controlled by screen modulating the Intermediate Power Amplifier (IPA) instead of amplitude modulating the RF signal that drives the fundamental amplifier chain. This was o r i g i n a l l y proposed for TRIUMF by Erdman et a l [2]. Provision for i t s implementation in the o r i g i n a l control system i s i l l u s t r a t e d in f i g . 4.1; A d i f f e r e n t i a l analyzing s l i t , as described by von Rossen, Euler, and Hinterberger [15], could be used to measure fluctuations in the energy gained per turn and therefore to generate an error signal for fine control of the amplitude of the accelerating voltage. This p o s s i b i l i t y i s b r i e f l y described in section 4.4. R e g u l a t i n g t h e O p e r a t i n g P o i n t o f t h e F i n e M o d u l a t o r s Of p a r t i c u l a r interest i s the use of two amplitude modulators and two phase modulators in cascade for RF drive signal control as shown in f i g . 4.3. The f i r s t two "coarse" modulators are controlled by the sequence c o n t r o l l e r or l o c a l control processor. The l a s t two " f i n e " modulators are 166 driven by the output of the analog compensation amplifiers to which a fixed bias has been added to set the modulator's operating point. This arrangement i s used in the prototype control system for two reasons: 1. The transfer c h a r a c t e r i s t i c of a t y p i c a l amplitude or phase modulator i s highly non-linear so the d i f f e r e n t i a l gain of the modulator varies with the operating point. One can adjust the coarse modulator so that the average value of the c o n t r o l l e r compensation signal d r i v i n g the fine modulator remains constant. For example, i f the average value of the compensated error signal d r i v i n g the fine modulator increases, then the l o c a l control processor should increase the signal d r i v i n g the corresponding coarse modulator u n t i l the average value of the compensated error signal drops back to near zero. This allows the system to maintain: a. a reasonably fixed operating point for the fine modulator; b. a reasonably constant d i f f e r e n t i a l gain for the fine modulator; and, c. predictable control loop behaviour over a wide operating range; 2. During system start-up, i t is desirable to operate without accelerating voltage regulation. The dual modulator arrangement presented here permits the l o c a l control processor to disable the analog c o n t r o l l e r s , either by driv i n g them into saturation or physically replacing their 167 control signal with a fixed bias, and c o n t r o l l i n g the amplitude and phase of the RF drive signal s t r i c t l y from the coarse modulators. As w i l l be described in section 4.2, t h i s greatly s i m p l i f i e s the procedure for closing the control loops once normal operation has been established. Burge [16] has suggested that i t i s preferable to design the modulator driver so that the combined driver-modulator transfer function i s l i n e a r ; he i s currently attempting to design a suitable modulator d r i v e r . The Local Control Processor The heart of the new system i s the l o c a l control processor which runs the sequential control program that supervises the RF system. It performs four basic functions: 1. It cycles the RF system through the sequence of events required to drive the accelerating voltage from zero to f u l l value; 2. It keeps the accelerator RF cavity tuned to the RF drive frequency, or, more pre c i s e l y , prevents the cavity tuning error from exceeding certain bounds ( i . e . i t implements a bang-bang [9] cavity tuning control algorithm); 3. It supervises the analog c o n t r o l l e r s . At the simplest l e v e l , i t merely sets the reference inputs and recycles the RF system through a "warm" start-up routine i f a spark i s detected but more sophisticated functions can be added as additional performance i s required. The coarse modulators can be used to s t a b i l i z e the operating point of the fine modulators, as discussed above, or the l o c a l control . 168 RF IN 23 M H z C O N T R O L L E R RF OUT RF IN R F O U T ON C E N T R A L C O N T R O L S Y S T E M ( C A M A C ) R E M C O N S Y S T E M Status. Command. ADC. DAC. Communications D I G I T A L I N T E R F A C E BUS L O C A L C O N T R O L PROCESSOR RF C O N S O L E T U N I N G M E C H A N I S M C O N T R O L L E R ( T R I M A C ) F i g . 4.5 The Local Control Processor and i t s D i g i t a l Interface Bus processor could be programmed to "tune" the c o n t r o l l e r , i . e . to adjust the response of the compensation amplifiers to achieve the best regulation (adaptive c o n t r o l ) , as described above; 4. A fourth function, which was not investigated in t h i s study, i s inter f a c i n g the RF control system to the cyclotron central control system and to the RF console. Certain aspects of t h i s important function are discussed in section 4.2, however. Organization of the New RF Control System The conceptual design of the new RF control system i s shown in f i g . 4.6 . Its resemblance to the RF system in figure 4.3 i s obvious although i t emphasizes d i f f e r e n t aspects of the RF system and i s , of course, more s p e c i f i c to the case of TRIUMF. The block diagram shows the 23 MHz and 69 MHz RF drive signals o r i g i n a t i n g from a common master o s c i l l a t o r . The 69 MHz signal i s obtained by passing the 23 MHz signal through a frequency t r i p l e r . The signals pass through amplitude and phase modulators then to the RF power ampl i f i e r s . Details such as the use of coarse and fine modulators in cascade are omitted for c l a r i t y . The diagram emphasizes the need to use DC blocks to prevent problems with 60 Hertz ground loops. Attention to such seemingly minor d e t a i l s i s very important given the harsh nature of the cyclotron environment and the degree to which the system i s expected to regulate the amplitude and phase of the accelerating voltage. 170 2 3 M H z R E F E R E N C E f 1 R F M A S T E R O S C I L L A T O R f l f 1 r * freq x 3 TY'tnoduTafor" P O W E R A M P L I F I E R f i it- f i fa modulator P O W E R A M P L I F I E R T / L C U R R E N T P R O B E r ^3 J?3 fa A N A L O G C O M P E N S A T I O N A N A L O G C O M P E N S A T I O N D A C S A D C R A D I O F R E Q U E N C Y S Y S T E M I C O N T R O L L A D D E R A N D ! I L O C A L C O N T R O L P R O C E S S O R S A F E T Y I N T E R L O C K S I I ! • I N T E R F A C E C Y C L O T R O N C E N T R A L C O N T R O L S Y S T E M R O O T C U R R E N T P R O B E fa P T / L C U R R E N T P R O B E r F I L T E R S G D E T E C T O R S l i F I L T E R S S D E T E C T O R S fa F I L T E R S S D E T E C T O R S Q i F I L T E R S G D E T E C T O R S 03 i z. , CYCLOTRON RF CAVITY D E E V O L T A G E P R O B E S R F C A V I T Y R O O T C U R R E N T F U N D A M E N T A L T R A N S M I S S I O N L I N E C U R R E N T R F C A V I T Y R O O T C U R R E N T T H I R D H A R M O N I C T R A N S M I S S I O N L I N E C U R R E N T F i g . 4.6 General Configuration and Outline of the New RF Control System _ - R F S I G N A L P A T H - C O N T R O L ( L F ) S I G N A L P A T H - I S O L A T I O N ri - D C B L O C K I N G O F R F S I G N A L 1:1 P A T H S - D I G I T A L S T A T U S L I N E S Q > - D I G I T A L D A T A P A T H Provision for Operating the System as a Feedback O s c i l l a t o r The major difference between f i g . 4.6 and the generic control system shown in f i g . 4.3 i s the inclusion of provision for operating the RF system as a feedback o s c i l l a t o r . When the system i s in s e l f - e x c i t e d mode, the frequency of operation i s determined by the resonant frequency of the RF cavity not by the frequency of the master o s c i l l a t o r . When the resonant frequency of the cavity i s changing rapidly, 'as i t does during system s t a r t -up, i t i s often more convenient to switch the system into s e l f - e x c i t e d mode than i t is to ph y s i c a l l y tune the RF cavi ty. Self-excited mode is the t r a d i t i o n a l method for operating cyclotron RF systems and the reasons i t was b u i l t into the existing existing control system are mainly h i s t o r i c a l [17], Configuring the RF system as a feedback o s c i l l a t o r complicates the design of the RF system hardware in three ways: a. It makes i t necessary to include a new RF path for the s e l f - e x c i t e d feedback si g n a l ; b. A g l i t c h free method of switching between s e l f -excited and driven mode must be devised; and, c. It complicates the design of the control program. An alt e r n a t i v e technique (which eliminates the hardware complications) i s to force the frequency of the master o s c i l l a t o r to track the resonant frequency of the RF cavity 172 F i g . 4.7 New RF Control System - Provision f o r Operation i n Self-Excited Mode using the cavity tuning error s i g n a l . The same control algorithm that tunes the cavity when the system i s in driven mode (fundamental) can also tune the master o s c i l l a t o r when the system i s in this a l t e r n a t i v e " s e l f - e x c i t e d " mode. Although both the ex i s t i n g RF control system and the prototype RF control system incorporate the hardware necessary for operating the RF system as a feedback o s c i l l a t o r , i t i s not required. The alte r n a t i v e technique should be implemented in i t s place although one caveat i s associated with i t s implementation. Some high purity frequency synthesizers, such as the Hewlett-Packard HP 8656A, sometimes drop th e i r output momentarily when switching their output frequency. The drop out i s disastrous so far as RF system operation i s concerned. No such problems were observed when the Rockland model 5600 frequency synthesizer was used to successfully drive both the RF test f a c i l i t y and the cyclotron RF system. In a similar vein, i t has been suggested that the frequency t r i p l e r that drives the frequency t r i p l e r that drives the 69 MHz RF modulator chain should be replaced by a 69 MHz master o s c i l l a t o r which would be phase locked to the 23 MHz master o s c i l l a t o r instead of using a frequency t r i p l e r , as i l l u s t r a t e d . This would permit one to also set the t h i r d harmonic drive frequency independently of the fundamental drive frequency as might be required during system start-up. This p o s s i b i l i t y was discussed in chapter 174 three. System Components It i s convenient to divide the new RF control system into fi v e d i s t i n c t sub-systems: 1. The accelerating voltage and cavity tuning error measurement sub-system (usually referred to as the RF detectors); 2. The master o s c i l l a t o r ( s ) which generate(s) the RF signals which drive the 23 MHz and 69 MHz power amplifiers and the two RF modulator chains which modify the amplitude and phase of the 23 MHz and 69 MHz RF drive signals in response to control signals from the analog c o n t r o l l e r s and the l o c a l control processor; 3.. The four analog c o n t r o l l e r s which regulate the amplitude and phase of the cyclotron accelerating voltage (each c o n t r o l l e r modulates the amplitude or phase of the appropriate RF drive signal in response to the output of the RF detector sub-system); 4. The cavity tuning mechanism c o n t r o l l e r ; and, 5. The l o c a l control processor and i t s peripherals (including a video display terminal and a VMEbus [18] [19] or CAMAC [20] type systems crate with both analog and d i g i t a l interface ports) which: a. step the RF system through system sta r t up and spark recovery sequences; b. keep the RF cavity tuned to the d r i v i n g frequency; c. supervise the analog c o n t r o l l e r s ; and, 175 d. manage communications between the various c o n t r o l l e r s , the RF system operators (through a video display terminal), and the cyclotron centra 1, control system (CCS) (through branch 5 of the CCS CAMAC p a r a l l e l highway). R e s p o n s i b i l i t i e s It should be noted that the replacement of the exis t i n g RF control system i s a major task so r e s p o n s i b i l i t y for i t s completion i s divided among several groups at TRIUMF: The s p e c i f i c a t i o n and conceptual design of the new system was the r e s p o n s i b i l i t y of the RF Systems Group with assistance from the Cyclotron Development and Controls Hardware Groups and v i s i t o r s to TRIUMF including L. Durieu of CERN (SPS Division) and P.K. Sigg of SIN; The f i n a l version of the RF detectors and analog RF co n t r o l l e r s w i l l be designed and b u i l t by the TRIUMF RF Systems Group; The tuning mechanism stepping motor c o n t r o l l e r and associated interfaces w i l l be supplied by the TRIUMF Electron i c s Development Group; The l o c a l control processor, i t s peripherals, and software development tools w i l l be selected by the RF Systems and Cyclotron Development Groups in consultation with the Controls Hardware Group and the TRIUMF Electronics Shop; RF Control System software w i l l be designed and written in house by the RF Systems and Cyclotron Development Groups 176 in consultation with the Controls Software Group; and, Responsibility for systems integration rests with the RF Systems Group. 4.2 SEQUENTIAL CONTROL OF THE RF SYSTEM The four modes of RF system operation: pulsed mode, se l f - e x c i t e d mode, driven mode (fundamental), and driven mode (flat-topped), and certain aspects of the conceptual design of the control program that w i l l supervise the RF system during both system start up and normal operation are described in t h i s section. C r i t e r i a for selecting the microcomputer ( l o c a l control processor) to run the control program are discussed. The control program i n i t i a t e s the sequence of events required to bring the accelerating voltage up to f u l l value and ensures that the cavity tuning error does not exceed set bounds, as suggested by the annotated flow chart shown in f i g . 4.8 and the timing diagrams in f i g s . 4.9(a) and (b). Because the primary function of the sequence c o n t r o l l e r i s to ensure that the RF system operates in a safe and r e l i a b l e manner, i t s role may be considered to be complementary to that of the analog (or process) c o n t r o l l e r s that s t a b i l i z e the amplitude and phase of the accelerating voltage in order to produce an extracted beam with excellent c h a r a c t e r i s t i c s . The programmer's model of the prototype RF control system i s presented in f i g . 4.10. It describes the command and status registers on the CAMAC d i g i t a l interface bus 177 RF system on -condition RF c a v i t y surfaces -pulse to CW at ~ 10 kV -control RF drive frequency f l -close f l voltage regulation loops at ~ 75 kV -control RF cavity frequency f l -search for RF cavity frequency f3 -turn t h i r d harmonic RF system on System I n i t i a l i z a t i o n 1 Pulsed Mode < . .... . Self-Excited Mode Driven Mode (Fundamental) Spark Detect and Recovery Spark Detect and Recovery - successful i n j e c t i o n acceleration, and extraction of the ion beam Driven Mode (Flat-topped) Spark Detect and Recovery MAJOR FUNCTIONS - DRIVEN MODE / NORMAL OPERATION - control RF cavity £1 and £3 frequencies - detect sparking and recover to normal operation - regulate accelerating voltage amplitude and phase - control f l and f3 modulator operating points - read CCS command registers / set CCS status registers F i g . 4.8 New RF Control System - Operating Modes 178 RF F R E Q U E N C Y D E E V O L T A G E RF Pulsing T = 25 milliseconds t = 1 millisecond DRIVEN MODE 'cold •manually set cavity tune to desired frequency hot 1 1 1 1 • search for resonant frequency •condition cavity surfaces f\ i 1 ; ! •track rapid changes t&i/ i - n resonant frequency / * during cavity warm up L : L L 1 multipactoring region 1 1^  I T t , 1 — m. F i g . 4 .9(a) RF System Operation During Start-Up RF FREQUENCY •RF frequency i s adjusted as required to track f i f t h harmonic of ion rotation frequency CO o DEE VOLTAGE •spark detection and recovery 1. detect spark either by level sense or dV/dt sense 2. remove RF drive for several tens of milliseconds reapply RF drive multipactoring region F i g . 4.9(b) RF System Operation i n Driven Mode Analog In Analog Out P a r a l l e l I/O 23 MHz Amplitude Control Detected Signal Error View 1 Error View 3 Phase Control Detected Signal Error View 1 Error View 3 Cavity Tuning Tuning Error View Forward Power Reflected Power (corresponding set for 69 MHz RF system also) 23 MHz Amplitude Control Open Loop Closed Loop Ref• Closed Loop Aux. Ref. Phase Control Open Loop 69 MHz Amplitude Control Open Loop Closed Loop Ref. Closed Loop Aux. Ref. Phase Control Open Loop Closed Loop Ref. Closed Loop Aux. Ref. GPIB D i g i t a l Voltmeter Network Analyzer Spectrum Analyzer Input Latch* Spark Detector (23 MHz & 69 MHz) RF Voltage Presence Detector (23 MHz & 69 MHz) Output Latch* d r i v e n / s e l f - e x c i t e d r f on/rf o f f closed loop/open loop local/remote spark detector reset *(corresponding set for 69 MHz RF system RF system also) F i g . k.10 - Programmer's Model of the Prototype RF Control System shown in f i g . 4.13 of section 4.3. 4.2.1 Pulsed Mode and Spark Detection and Recovery. It would be very convenient i f i t were possible to raise the cyclotron dee voltage to several tens of k i l o v o l t s at an a r b i t r a r i l y slow rate. Most of the problems presently associated with RF cavity tuning (and RF system operation generally) would simply vanish and the complexity of the RF system control program would in turn be greatly reduced. Unhappily, this i s not the case. Sparking and multipactoring in the TRIUMF RF cavity during system start up and the rapid thermal expansion (and detuning) of the cavity associated with the requirement that the RF drive power be pulsed on to a r e l a t i v e l y high l e v e l so that the dee voltage can successfully pass through the multipactor discharge region make i t necessary to adopt special procedures for system start up and for spark detection and recovery, as described in sections 3.5 (Demonstration of a Flat-topped Accelerating Voltage) and D.3 (Excitation of an Accelerator RF Cavity Under Vacuum). The following section describes a control program algorithm for operating the new RF control system in pulsed mode during system start-up. PULSED MODE RF Source: Master O s c i l l a t o r Master O s c i l l a t o r Frequency: Manually Controlled by RF System Operator 182 Cavity Tuning (Fundamental): Fixed Cavity Tuning (Third Harmonic): Fixed Fundamental Amplitude Control: Manually Controlled by RF System Operator (via Coarse Modulator) Control Loop Saturated; Reference set to > 90 kV RF Drive Pulsed by Toggling "RF_Enable" b i t Fundamental Phase Control: Disabled Third Harmonic Amplitude Control: Disabled Third Harmonic Phase Control: Di sabled Supervisory Control Loops: None VARIABLE PARAMETERS: PARAMETER DEFAULT Pulse Period 25 milliseconds Pulse Duty Factor 1 millisecond Pulse Amplitude 0 FLAGS: Threshold (Status) - indicates fundamental dee voltage i s above a preset threshold Pulse_to_CW (Command) ALGORITHM: 1. Disable a l l analog c o n t r o l l e r s ; 2. Set 23 MHz amplitude reference high to saturate control loop; (fine modulator -> minimum attenuation) 1 8 3 3. Set 23 MHz drive signal to minimum; (coarse modulator -> maximum attenuation) 4. Begin pulsing routine a. set pulse amplitude to according to current input from operator b. pulse on for 1 millisecond c. i f (Threshold and Pulse to CW) then go to SELF-EXCITED MODE else pulse off for 24 milliseconds d. go to "a" The RF system i s run in pulsed mode during system s t a r t -up to avoid problems caused by driv i n g the amplifiers into a a highly variable and often poorly matched load. The system i s operated in a continuously pulsed mode u n t i l the cavity surfaces have been conditioned and the cavity i s capable of sustaining a dee voltage pulse with s u f f i c i e n t amplitude, i . e . > 10 kV. The operator then adjusts the amplitude and frequency of the RF pulse u n t i l the dee voltage i s about 10 kV and the waveform has a clean shape. This indicates that the cavity i s being driven at i t s resonant frequency at a voltage well above the multipactoring threshold. Oscillographs showing the corresponding RF pulse waveforms were presented in f i g . When the operator sets the "pulse_to_CW" f l a g , the routine pulses the RF drive on then checks the "threshold" 3.21 . 184 f l a g , i . e . i t ensures that the dee voltage i s above the multipactoring threshold and no e l e c t r i c a l discharges (sparks/arcs) are taking place. If so, the system switches to s e l f - e x c i t e d mode. If not, the system continues pulsing u n t i l the condition i s met. 4.2.2 Self-Excited Mode The frequency of the RF drive signal should not d i f f e r by more than a few hundred Hertz from the resonant frequency of the RF cav i t y . If the cavity tuning error i s too large, the RF amplifiers may be unable to delive r s u f f i c i e n t power to keep the accelerating voltage above the m u l t i p a c t o r i n g threshold. Experience has shown that the resonant frequency of the cavity w i l l d r i f t as a function of: a. the pressure of the cooling water in the cantilevered hot arms (the Bourdon e f f e c t ) ; b. the temperature of the cooling water in the cantilevered hot arms; c. the temperature of the structure of the radio frequency cavity i t s e l f . Some form of feedback control i s necessary to keep the cavity's resonant frequency close to the driv i n g frequency. After the RF drive signal i s switched from pulsed to CW, the dee voltage i s ramped from the turn on value of about 10 kV to the operating value of between 85 and 100 kV. During t h i s period, the power dissipated in the RF cavity by the accelerating mode increases by a factor of between 50 and 185 1 0 0 . U n t i l the RF cavity reaches thermal equilibrium, the resonant frequency of the cavity may change too quickly or too much for the mechanical automatic tuning system to compensate. Although the frequency of the RF drive signal must remain fixed at the f i f t h harmonic of the ion rotation frequency during normal operation, there is no such r e s t r i c t i o n during system start-up. While the dee voltage is ramped to operating value, the RF system i s run in s e l f -excited mode - the RF frequency i s made to track the resonant frequency of the RF cavity instead of vice versa. There are two methods of accomplishing t h i s : a. configure the RF cavity/power amplifier/modulator chain as a feedback (Barkhausen) o s c i l l a t o r ; b. force the frequency of the master o s c i l l a t o r to follow the the resonant frequency of the cavity. Provision for investigating both options was b u i l t into the prototype of the new RF control system. S E L F - E X C I T E D MODE (Option I - Feedback O s c i l l a t o r ) RF Source : Dee Voltage Probe Master O s c i l l a t o r Frequency : Fixed (not used) Cavity Tuning (Fundamental) : Fixed Cavity Tuning (Third Harmonic) : Fixed Fundamental Amplitude Control : Manually controlled by RF System Operator (via Coarse Modulator) 186 Control Loop Saturated Reference set to > 100 kV RF Drive Enabled Fundamental Phase Control: Tuning Controller via Coarse Modulator Fine Modulator Control Loop Disabled Third Harmonic Amplitude Control: Disabled Third Harmonic Phase Control: Disabled PARAMETERS: Pulse Period 25 milliseconds Pulse Duty Factor 1 millisecond Pulse Amplitude variable Fundamental Amplitude variable Ma x_Tun i n g_Err o r to be determined Min_Tuning_Error to be determined FLAGS: Threshold (Status) Pulse_to_CW (Command) ALGORITHM: In s e l f -excited mode ( I ) , the RF system i s configured as a feedback o s c i l l a t o r as described in section 4 . 1 . The frequency of operation i s determined by the resonant frequency of the RF cavity and maintained by s a t i s f y i n g Barkhausen's c r i t e r i o n - the loop gain must be equal to one and the net phase s h i f t around the loop, which can be adjusted using the coarse phase modulator, must be equal to zero degrees [20]. During "pulsed" or "driven" operation, the power amplifier i s driven by a signal from the master o s c i l l a t o r 187 (frequency synthesizer) but during s e l f - e x c i t e d (I) operation, the power amplifier i s driven by the signal developed across the dee voltage probe. Both signals are presented to the RF modulator chain - they pass through separate RF l i m i t i n g amplifiers (so they have similar amplitudes) and coarse amplitude modulators. They are combined, then pass through common coarse phase, fine phase, and fine phase modulators, as shown in f i g . 4.7. The mode of operation i s selected by toggling a command b i t . When the driven mode i s enabled, the se l f excited mode is disabled and vice versa. Section 4.3 contains a more complete description of the modulator chain hardware. During system i n i t i a l i z a t i o n , the 23 MHz coarse phase s h i f t e r and 23. MHz se l f - e x c i t e d coarse amplitude modulator are set to pre-determined values for operation in s e l f -excited mode ( I ) . This ensures that the Barkhausen c r i t e r i o n (open-loop gain and phase-shift) for feedback o s c i l l a t i o n w i l l be met at an RF frequency of about 23 MHz and a peak dee voltage of about 5 kV. If a spark i s detected while the system i s in s e l f - e x c i t e d mode, the RF drive i s turned off for 24 milliseconds then pulsed back on with the "Pulse_to_CW" flag set. Although simple in concept, the Barkhausen o s c i l l a t o r option makes the control system hardware unnecessarily complicated. A l t e r n a t i v e l y , the cavity tuning c o n t r o l l e r can be used to adjust the frequency of the master o s c i l l a t o r 188 rather than the RF cavity during system start-up, as discussed in section 3.5 and section 4.1. The corresponding control program sequence i s l i s t e d below. SELF-EXCITED MODE (Option II - Variable Frequency Source) RF Source: Master O s c i l l a t o r Master O s c i l l a t o r Frequency: Set by Tuning Controller Cavity Tuning (Fundamental): Fixed Cavity Tuning (Third Harmonic): Fixed Fundamental Amplitude Control: RF System Operator (via Coarse Modulator) Control Loop Saturated RF Drive Enabled Fundamental Phase Control: Disabled Third Harmonic Amplitude Control: Disabled Third Harmonic Phase Control: Disabled PARAMETERS: Pulse Period 25 milliseconds Pulse Duty Factor 1 millisecond Pulse Amplitude variable Fundamental Amplitude variable FLAGS: Fundamental_Threshold (Status) ALGORITHM: (following pulsed mode...) 1. Wait 0.5 seconds 2. IF (.not.Threshold) THEN go back to pulse_to_CW mode 189 IF (tuning_error > max_error) THEN adjust master o s c i l l a t o r frequency u n t i l (tuning_error < min_error) IF (spark_detected) THEN turn off RF for 24 milliseconds and go back to pulse_to_CW mode set fundamental amplitude according to current input from operator Go To 3 4.2.3 Closing the Amplitude Feedback Control Loop It i s easier to operate the RF system in pulsed or s e l f -excited mode i f the analog control loops are disabled, i . e . the amplitude and phase of the RF drive signals are not dependent on the amplitude and phase of the accelerating voltage. One way of doing this i s to physically switch the input of the fine modulator driver from the c o n t r o l l e r output to a fixed bias. To "close the loop", one would simply switch back. Depending on the i n i t i a l state of the feedback capacitors in the compensation amplifiers, however, transients having rather large amplitudes might be generated during the switch over. Such transients in the accelerating voltage phase would have l i t t l e e f fect on the operation of the RF system but similar transients in the 1 9 0 accelerating voltage amplitude could be very d i f f i c u l t to deal with, p a r t i c u l a r i f they caused the accelerating voltage to dip beneath the multipactoring threshold. In any event, a clean t r a n s i t i o n i s d e f i n i t e l y preferable. Fortunately, there i s an easy way of ensuring that the 23 MHz (or 69 MHz) amplitude c o n t r o l l e r makes a smooth t r a n s i t i o n from open to closed loop control during the process of ramping the dee voltage from about 10 kV to operational values. The error amplifier used in the amplitude c o n t r o l l e r s i s designed to saturate i f the e f f e c t i v e difference between the reference and actual dee voltages i s greater than about 10 kV. If the reference i s set more than 10 kV higher than the actual dee voltage, the fine amplitude c o n t r o l l e r i s turned completely on and small changes in the dee voltage do not affec t i t . Thus only changes in the attenuation introduced by the coarse amplitude modulator, which i s under manual control, a f f e c t the amplitude of the RF drive si g n a l . By setting the 23 MHz amplitude reference signal to the equivalent of 90 kV or greater, the co n t r o l l e r operates in es s e n t i a l l y open loop mode u n t i l the accelerating voltage has reached operational values. Once the desired dee voltage has been reached, the reference voltage i s then lowered u n t i l the error amplifier comes out of saturation. A smooth t r a n s i t i o n i s thus made from open loop to closed loop c o n t r o l . 191 The fine amplitude and phase modulators are operated at a fixed bias with the c o n t r o l l e r output superimposed on i t . The transfer c h a r a c t e r i s t i c s of the modulators are highly non-linear, however, so i t i s desirable to keep the operating point fixed so the d i f f e r e n t i a l open loop gain remains predictable. One way of doing t h i s i s to keep the analog c o n t r o l l e r output near zero. Once.the RF control system i s ac t i v e l y c o n t r o l l i n g the amplitude and phase of the accelerating voltage, a r e l a t i v e l y slow supervisory control loop adjusts the r e l a t i v e attenuation of the two sets (amplitude and phase) of fine and coarse modulators in such a way that the output of the compensation amplifiers, as measured at test point Error_View 3, stays near zero. ALGORITHM: 1. Operator notes that cyclotron dee voltage has been successfully ramped to desired value and requests that the accelerating voltage amplitude control loop be closed. Error_View 3 should be > 0. 2. DO UNTIL 23 MHz amplitude reference is equal to desired value: a. Decrement 23 MHz amplitude reference b. IF (Error_View 3 < 0) THEN increase coarse attenuation 3. Accelerating voltage i s now being a c t i v e l y regulated. Begin operation of the r e l a t i v e l y slow supervisory control loop 192 designed to keep the mean value of Error_View 3 near zero and thus ensure that the fine modulators are driven near the desired operating point. The existing RF control system closes the accelerating voltage regulation loop immediately upon .entering s e l f -excited mode since the RF system must apparently be pulsed on at f u l l power in order for the dee voltage to successfully pass through multipactoring threshold. This requirement that the amplitude control loop be closed at that time can be waived by resetting the coarse amplitude modulator to the CW amplitude l e v e l immediately upon entering s e l f - e x c i t e d mode. Closing the accelerating voltage phase control loop w i l l be much easier, as noted above. Merely toggling the Phase Control Enabled/Disabled command b i t w i l l probably be s u f f i c i e n t . 4.2.4 Driven Mode In order to successfully accelerate the ion beam, the RF system frequency must be fixed at fi v e times the ion rotation frequency. Once the resonant frequency of the RF cavity has s t a b i l i z e d , the RF system operators can switch to driven mode. The master o s c i l l a t o r frequency i s fixed (except for small adjustments to match i t to the f i f t h harmonic of the ion rotation frequency) and the tuning c o n t r o l l e r begins to drive the cavity tuning error signal to 193 zero by suitably adjusting the tuning actuators - the resonator cooling water pressure control valve in the case of the exis t i n g RF system and the ground panel t i p adjustor stepping motor in the case of the RF systems test f a c i l i t y . The cyclotron's new fundamental mode tuning mechanism has not yet been selected, as noted in chapter three. DRIVEN MODE RF Source: Master O s c i l l a t o r Master O s c i l l a t o r Frequency: Fixed - Set by Operators Cavity Tuning (Fundamental): Set by Tuning Controller Cavity Tuning (Third Harmonic): Fixed Fundamental Amplitude Control: RF System Operator (via Control Reference) Control Loop Active RF Drive Enabled Fundamental Phase Control: Control Loop Active Third Harmonic Amplitude Control: Disabled Third Harmonic Phase Control: Disabled PARAMETERS: Pulse Period 25 milliseconds Pulse Duty Factor 1 millisecond Pulse Amplitude variable Fundamental Amplitude variable Max_Tuning_Error to be determined Min_Tuning_Error to be determined FLAGS: Fundamental_Threshold (Status) Pulse to CW (Command) 194 TRANSITION ALGORITHM: Once the cavity has been conditioned, the resonant frequency of the cavity has s t a b i l i z e d , and the dee voltage amplitude and phase regulation loops have been closed, one can consider switching to fixed frequency operation, i . e . driven mode. Operation i s almost exactly as in s e l f - e x c i t e d mode with active regulation of the accelerating voltage before except that the cavity tuning error i s cancelled by tuning the cavity instead of adjusting the coarse phase s h i f t e r or the master o s c i l l a t o r frequency. 1. Under manual control, set the resonant frequency of the cavity to the desired frequency. 2. Adjust the amplitude and phase modulator operating points as required. 3. Tune the radio frequency cavity: IF (tuning_error > max_error) THEN adjust cavity tuning actuator u n t i l (tuning_error < min_error) 4. Was a Spark Detected? IF (spark_detected/RF disabled) THEN wait 5 - 1 0 milliseconds, then re-enable RF drive 5. Go to 2 4 . 2 . 5 F l a t - t o p p e d Mode Operation with the t h i r d harmonic i s similar to the above since the fundamental and t h i r d harmonic control 195 systems are, by design, almost i d e n t i c a l . The major differences were noted in chapter three and are b r i e f l y summarized here: When the fundamental mode i s present in the RF cavity, the t h i r d harmonic mode's multipactoring threshold i s es s e n t i a l l y zero. The t h i r d harmonic RF drive power can be ramped to operational levels at an a r b i t r a r i l y slow rate which eliminates the need to pulse the RF drive during start-up. This, in turn, eliminates most of the d i f f i c u l t i e s associated with the rapid thermal expansion and detuning of the RF cavity that accompanies start-up of the fundamental RF system. It is not absolutely necessary that one be able to run the t h i r d harmonic RF system in either pulsed or se l f - e x c i t e d mode but since the fundamental and th i r d harmonic control systems are v i r t u a l l y i d e n t i c a l , the appropriate . fundamental RF functions could be adapted for use on the t h i r d harmonic system with only minimal e f f o r t and may prove useful when the cyclotron's t h i r d harmonic RF system i s being commissioned. The search for the t h i r d harmonic resonant frequency of the cavity and the automatic tuning procedure would be much as described in chapter three. The technique of saturating the amplitude control loop u n t i l the dee voltage reached operational levels would probably be used, as would the procedures for spark detection and recovery since i t was found that they s i m p l i f i e d operation of the RF systems test f a c i l i t y in flat-topped mode. 1 9 6 4.2 . 6 The RF Console and the Local Control Processor Most important RF system components can be manually controlled from the console including the position of the t i p s of the number eight resonator segments, the variable capacitors in the transmission l i n e matching section, the main transmitters and their power supplies, the temperature and pressure of the resonator cooling water hence the resonant frequency of the RF cavity, and the amplitude, phase, and frequency of the RF signals that drive the fundamental and t h i r d harmonic transmitters. The RF console i s usually unmanned-except during system start-up or when the RF system requires special attention. During normal operation, r e s p o n s i b i l i t y for setting selected RF system operating parameters such as RF drive frequency, system mode (driven or self-excited) and r e l a t i v e amplitude and phase of the t h i r d harmonic accelerating voltage (when the t h i r d harmonic RF system i s commissioned), and for monitoring the RF system i s passed to the cyclotron central control system and the cyclotron operators. When the RF system becomes "unreliable", as i t does from time to time, or during system testing, the task of restoring r e l i a b l e operation i s directed from the RF console. E f f i c i e n t access to operating data in real time would speed up i d e n t i f i c a t i o n and correction of problems. A large set of conventional panel meters and the central control system's REMCON (remote console) system 197 enable RF system operators to monitor the operation of most RF system components from a central location. These components include the RF transmitters and. their DC power supplies, the RF safety system, the RF accelerating voltage and cavity tuning control system, and the large set of parameters available from REMCON, such as the temperature of the resonator strongbacks in the RF cavity. Unfortunately, accessing them i s often rather awkward due to the rather primitive nature of the REMCON system. Also, when the RF safety system t r i p s , the lack of appropriate indicators often makes i t d i f f i c u l t to identif y the pa r t i c u l a r interlock that f a i l e d . Improvements in t h i s area should greatly decrease the down time attributed to RF problems. The Cyclotron Development Group i s developing new information displays for the RF console using data acquired from the central control system. Real-time graphic displays of cyclotron parameters such as: a. dee voltages at various locations along the accelerating gap; or, b. resonator temperatures at various points in the RF cavity and software generated mimic panels of various components of the RF system; are planned. There has been some question whether the computer that supervises the RF control system should also interrogate the central control system and generate the RF console display. 198 Such a consideration dictates the type of computer, display, and software development tools that one would specify for the new control system. Thus, u n t i l the design of the new RF console becomes somewhat more concrete, the selection of a l o c a l control processor and the design of software to run the new control system must remain somewhat abstract. Nevertheless, most of the control system functions that w i l l be implemented in software (and which are described in section 4.2) have been successfully tested on the RF systems test f a c i l i t y using a TRIMAC 8085-based CAMAC microcomputer running under the CP/M operating system. The programming environment i s s u f f i c i e n t l y unfriendly, however, that further software development has been postponed u n t i l a more powerful system is selected to serve as the l o c a l control processor in the f i n a l version of the new control system. Once the control program has been properly defined, the task of selecting the l o c a l control processor can begin. Although the convenience of the l o c a l control processor's programming environment, including support of s i t e standard graphics terminals, w i l l be an important selection c r i t e r i a , the acceptance of the processor as a s i t e standard by the TRIUMF Technology and Administration D i v i s i o n , which w i l l ensure that l o c a l software and hardware support i s available, w i l l probably have the largest influence on the f i n a l choice. 1 9 9 The present s i t e standard for l o c a l control i s the TRIMAC 8085-based CAMAC a u x i l i a r y processor developed at TRIUMF. Advanced versions of the TRIMAC support 32K of EPROM and 4K of RAM. An S-100/CP/M 2.2 development system is available which can be used to develop programs for stand alone applications . or to run large applications d i r e c t l y from the operating system. The TRIMAC has been used as a lo c a l control processor during the i n i t i a l development of the prototype radio frequency control system but the complexity of the Fortran-80 support l i b r a r i e s and the long compile and link time made program development extremely inconvenient. Experience with the prototype control program "RF_Host" tends to suggest that a microcomputer with c a p a b i l i t i e s similar to the IBM personal computer would s a t i s f y the major requirements and would eliminate the need for a special d i g i t a l interface bus. , Manufacturers such as Tecmar, Inc. provide a l l the analog and d i g i t a l input and output available in the prototype CAMAC crate, with the exception of the IEEE 488 interface, on a single PC expansion board. 200 4.3 THE PROTOTYPE RADIO FREQUENCY CONTROL SYSTEM In t h i s section, a brief description of the prototype version of the new RF control system i s presented. Bischof [21], Sigg [22], Meisner, Edwards, F i t z g e r a l d , and Kerns [23], Brown, Ciapala, Hansen, Peschardt, and Sladden [24], and Howard [25], among others, have described the technical problems (and solutions) encountered in the implementation of somewhat similar accelerator RF control systems. 4.3.1 Physical Description The components of the prototype RF control system are i l l u s t r a t e d in f i g . 4..11 as they are mounted in a 19 inch relay rack at the test f a c i l i t y : a. the 23 MHz frequency synthesizer (Rockland Model 5600) that drives both the 23 MHz RF modulators and the frequency t r i p l e r that drives the 69 MHz RF modulators; b. the 23 and 69 MHz analog control crates (see f i g . 4.12) that house the RF detectors, analog c o n t r o l l e r s , and RF modulators; c. the CAMAC crate (see f i g . 4.13) that houses the ADC's, DAC's and d i g i t a l I/O ports that interface the analog control crates and the cavity tuning motor c o n t r o l l e r to the l o c a l control processor (VAX-11/730); and, d. the TRIUMF Model 0650/2 stepping motor drivers and power supply that are used to tune the RF cavity. 201 1.75 IN - DVM HP 18 INCH PANEL FREQUENCY SYNTHESIZER, ROCKLAND MODEL 5600 REFLECTED POWER PROTECTION UNIT DVM HP _ 23 MHZ ANALOG CONTROL CRATE 69 MHZ ANALOG CONTROL CRATE PROTOTYPE RF CONTROL SYSTEM CAMAC CRATE KINETIC SYSTEMS 1500[ SHA5: 0650/2 MOTOR DRIVER 0650/2 MOTOR DRIVER 0650/2 MOTOR DRIVER 0650/2 POWER SUPPLY FRONT VIEW FI OUTLET STRIP Bl F2 HPIL /GPIB INTERFACE B2 F3 B3 F4 B4 F5 B5 F6 B6 F7 B7 F8 BB F9 B9 F10 BIO F l l B l l F12 B12 F13 B13 F14 B14 F15 B15 F16 B16 F17 B17 F16 B16 F19 B19 F20 B20 F21 B21 F22 B22 F23 B23 F24 B24 F25 B25 F26 B26 F27 B27 F2B B28 F29 B29 F30 B30 F31 B31 F32 B32 F33 OUTLET STRIP B33 F34 B34 F35 OUTLET STRIP B35 F36 B36 F37 B37 F3B B3B F39 B39 F40 B40 BACK VIEW Fig. - 4 . 1 1 RF Systems Test F a c i l i t y - RF Controls Rack 202 rcrc" A M P CT: AMP S9 MHZ MODULATOR SELF EXCITED I/P @ DRIVEN I/P @ OUTPUT TOST TUNING DETECTOR AMPL T T U " AMP AMP BB MHZ MODULATOR SELF EXCITED I/P © DRIVEN I/P @ OUTPUT TO DELAY © 68MHZ OUT © 23HHZ OUT © REF INPUT (Q © FROM DELAY o HILTEH G UtTECTOR TO DELAY © 88MHZ OUT © 23MHZ OUT IN LINE © 23 (g) ea © CAVITY ROOT REF © ® OUTPUT CTJHBT 23 A © 23 P @ 89 A © 89 P TRANS-MISSION o 0 a d BB MHZ TUNING DETECTOR O +1SV O *SV O -1SV O +1SV o. REF INPUT P C © FROM DELAY TRANS-MISSION LINE CAVITY ROOT ® OUTPUT a o O +15V O *SV o -lov O +15V 3 0 (a) Front Panel 23 MHz CRATE 117 VAC DIGITAL I/O AMPLITUDE CONTROL PHASE CONTROL ERROR VIEW -4-I 2 3 ® ® ® AUX INPUT i 4 ® ® RF IN AMP IN VAR SPARK RF OUT OUT ENV ® ® ( ERROR VIEW f ® ® AUX INPUT "4 •3 i a s ® ® ® ® ® ® RF IN AMP IN VAR SPARK DET OUT OUT PHASE ® ® ® ® ® 69 MHz CRATE 117 VAC DIGITAL I/O AMPLITUDE CONTROL PHASE CONTROL ERROR VIEW i 4-AUX INPUT -4 ~3 I B 3 ® ® ® ® ® ® RF IN AMP IN VAR OUT SPARK RF OUT ENV ® ® ® ® ® AUX INPUT 1 — 4 — 1 — 4 3 ® ® ® ® ® ® RF AMP VAR SPARK DET IN IN OUT OUT PHASE ® ® ® © ® (b) Rear Panel F i g . 4.12 , Prototype RF Control System - Analog Signal Processing Crates 203 CRATE NO.: SHA5 RF SYSTEMS TEST FACILITY CAMAC CRATE ASSIGNMENTS CTS LOGICAL NAME FUNCTION AND DESCRIPTION MODEL NO. _RFCRATE TYPE L-2 CRATE CONTROLLER INTERFACE TO VAX-11/730 VIA CAMAC SERIAL HIGHWAY. KS 3952 _RFGPIB GPIB INTERFACE INTERFACE TO DIGITAL VOLTMETER. AND NETWORK. SPECTRUM. S FFT ANALYZERS. KS 3388 RFSLAVE TRIMAC MICROCOMPUTER (MOTOR CONTROL) TR 0544 RFMASTER TRIMAC MICROCOMPUTER (MOTOR CONTROL) TR 0544 RFHOST TRIMAC MICROCOMPUTER (LOCAL CONSOLE) TR 0544 RFDISPLAY TRIMAC MICROCOMPUTER ( „ 0SS^ S T I C) TR 0544 — — RFCOMM COMMUNICATIONS INTERFACE KS 3340 RFMEMORY CAMAC MEMORY (128 WORDS X 24 BITS) TR 2401 RFIG0R3 INPUT GATE/OUTPUT REGISTER (4 BITS) TR 0576/1 RFIG0R2 INPUT GATE/OUTPUT REGISTER (4 BITS) TR 0576/1 RFIGOR1 INPUT GATE/OUTPUT REGISTER(4 BITS) TR 0576/1 RFINPUT DUAL 24 BIT INTERRUPT GATE TR 0519 RFFREQ DUAL 24 BIT OUTPUT REGISTER TR 0454 RF0UT2 DUAL 24 BIT OUTPUT REGISTER TR 0454 RF0UT1 DUAL 24 BIT OUTPUT REGISTER TR 0454 RFDAC3 DAC - 8 CHANNELS 10 BITS (MODIFIED RFDAC2 DAC - 8 CHANNELS 10 BITS (MODIFIED KS 3110 RFDAC1 DAC - 8 CHANNELS 10 BITS (MODIFIED KS 3110 RFADC ADC - 32 CHANNELS 12 BITS JO ADC-32 RFDATAWAY SYSTEM MONITOR AND DATAWAY DISPLAY KS 3291 RFSWITCH 24 BIT MANUAL INPUT GATE KS 3461 25 24 23 22 21 20 19 18 . 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 CRATE SLOT NO. LOCATION FRONT VIEW F i g . 4.13 Prototype RF Control System - CAMAC Interface 204 The r e f l e c t e d power protection unit i s a standard hardwired safety interlock which cuts the RF drive to the power amplifiers i f the r e f l e c t e d power exceeds a preset threshold. A similar c i r c u i t was described by Bowick [26]. A block diagram of the test f a c i l i t y ' s safety system (not including the power amplifier control ladders) i s shown in f i g . 4.14. D i g i t a l voltmeters (Hewlett-Packard model 3468B) are used to monitor the output of the 23 and 69 MHz amplitude detectors.' The two DVM's are mounted d i r e c t l y above the analog control crates. The RF cavity tuning c o n t r o l l e r i s shown in f i g . 4.15. The CAMAC-based stepping motor c o n t r o l l e r system was developed by the E l e c t r o n i c s Development Group for general purpose use at TRIUMF. The stepping motor system was modified and i n s t a l l e d on the RF systems test f a c i l i t y by B. Evans and D. Morris. 4.3.2 The RF Source and Modulator Sub-Systems The RF signals that drive the power amplifier are generated by the RF source and modulator sub-system. It consists of three major components: a. the RF source; b. the RF modulators; c. the RF modulator d r i v e r s . The RF Source The output of the RF source must have low phase noise and high frequency s t a b i l i t y . Although the f i n a l control 205 POWER AMPLIFIER O ON RF SOURCE AMPLITUDE MODULATOR MATCHING SECTION S TRANSMISSION LINE FILTER X X CAMAC ADC RFADC 13 BITS —S TO 5 V X X "DIRECTIONAL COUPLER TUNING ERROR Forward Power Reflected Power ROOT GROUND ARM HOT ARM TIP DEE GAP DEE VOLTAGE PROBE THRESHOLD REFLECTED POWER DETECTOR CAMAC INTERRUPT GATE RFINPUT TTL COMPATIBLE SPARK DETECTOR AMPLITUDE DETECTOR THRESHOLD RF PRESENCE DETECTOR . F i g . 4.14 RF Systems Test F a c i l i t y - Safety System o .Tuning Error 0-1 69 MHz 23 MHz Root Current 23 MHz Loop Current 69 MHz Root Current 69 MHz Loop Current TUNING DETECTOR MODULE TUNING DETECTOR MODULE Wedge Tuner (1 of 2) Wedge Tuner Stepping Motor 1 Tip Tuner Stepping Motor (1 Of 2) CAMAC I/O Ports RFIG0R1 RFIG0R2 TRIMAC MICROCOMPUTER (Motor Controller) I CAMAC ADC RFADC -3 TO S VDC 12 BITS RFMASTER RFSLAVE CAMAC Memory RFMEMORY I I I to Tip Tuner Stepping Motor Power Supplies to Wedge Tuner Stepping Motor Power Supply Spare F i g . 4.15 RF Systems Test F a c i l i t y - Cavity Tuning Controller Notes: 1. The RFMASTER processor provides a high l e v e l interface between the user and the RFSLAVE processors that d i r e c t l y control the position, v e l o c i t y , and acceleration of the individual stepping motors. 2. Commands, acknowledgements, and data are passed between the user, RFMASTER, and RFSLAVE through a set of registers referred to as a CAMAC Memory. system design w i l l probably use separate phase-locked frequency synthesizers for generating 23 and 69 MHz drive signals to permit more f l e x i b l e operation of the RF system, the prototype system included a frequency t r i p l e r to generate the 69 MHz drive s i g n a l . The prototype RF source i s shown in f i g . 4.16. A Rockland model 5600 was used because they were available on s i t e . Models from other vendors are probably just as suitable although some synthesizers, notably the Hewlett-Packard model 8656, are designed to momentarily drop their output when a new frequency i s selected u n t i l the signal s t a b i l i z e s , as noted in section 4.1, and are obviously not suitable. The frequency t r i p l e r and RF buffers are based on MECL li n e receivers type MC 10115. The design was based on considerations presented by Blood [27] and Motorola Semiconductor [28]. A l l modulator input signals, including the signal derived from the dee voltage probe that i s used in s e l f - e x c i t e d mode, are passed through ECL Schmidt triggers (e.g. 10115 A in f i g . 4.16) to give them a common amplitude and thus simplify the problem of setting the coarse amplitude modulators. The RF Modulator Chain The 23 MHz RF modulator sub-system i s shown in f i g . 4.17. It consists of the RF modulator chain and the RF modulator d r i v e r s . After both the master o s c i l l a t o r and dee voltage probe signals pass through RF l i m i t e r s , they pass through th e i r respective coarse modulators, then are 208 ROCKLAND FREQUENCY SYNTHESIZER MODEL 5600 +3 to 13 dBm 23 MHZ +3 dBm IP-luF 2 CAMAC WORDS 24 BITS EACH CAMAC DUAL OUTPUT REGISTER RFFREQ FREQUENCY CONTROL 23 MHZ REFERENCE (TO PHASE DETECTOR) TO 23 MHZ MODULATOR D TO ee MHZ MODULATOR 10115 M J L l ^ O.OluF ^ R8 560 F i g . 4.16 Prototype RF Control System - RF Source RF MODULATOR MODULE SELF EXCITED (1) r AMPLITUDE MODULATOR I (COARSE) MINI-CIRCUITS SRA-1 RF COMBINER AMPLITUDE MODULATOR II (COARSE) MINI-CIRCUITS SRA-1 MERRIHAC 113A PHASE MODULATOR (COARSE) MERRIMAC PSES-4-23 PHASE MODULATOR (FINE) MERRIMAC PSES-4-23 AMPLITUDE MODULATOR (FINE) MINI-CIRCUITS SRA-1 RF AMPLIFIER 2N9109 COMMON—BASE TRW CA2B30 HYBRID ro O AMPLITUDE MODULATOR DRIVER (COARSE) RF CONTROL AMPLIFIER MODULES AMPLITUDE AND PHASE PHASE MODULATOR DRIVER (COARSE) PHASE MODULATOR DRIVER (FINE) AMPLITUDE MODULATOR DRIVER (FINE) RF MODE SELECTORS FIXED BIAS CAMAC DIGITAL TO ANALOG CONVERTER RFDAC1 10 BITS o - s VDC COMPENSATED ERROR SIGNAL (RF PHASE) -<•) 0 T AL 1 FIXED BIAS *C+) COMPENSATED J ERROR SIGNAL 1 (RF AMPLITUDE) CAMAC DUAL OUTPUT REGISTER RF0UT1 RF ON/OFF FROM SPARK DETECTOR F i g . 4.17 New RF Control System - RF Modulator Chain combined. The combined signals pass through a coarse phase modulator, a fine phase modulator, a fine amplitude modulator, and f i n a l l y an RF amplifier that boosts the RF drive signal to the l e v e l required by the input to the power amplifier chain. The amplitude and phase modulators used in the prototype control system are commercially manufactured units. The units used in the f i n a l control system design w i l l be chosen on the basis of their s t a t i c transfer c h a r a c t e r i s t i c s and frequency response and their s t a b i l i t y with changes in temperature. The components used in the prototype RF control system modulator chain were chosen because they were e a s i l y procured. The three p r i n c i p a l components, the PSES-4 electronic phase s h i f t e r [30], the SRA-1 balanced mixer/amplitude modulator [31], and the RF amplifier/buffer (based on a common base 2N5109 followed by a TRW CA 2830 thin f i l m hybrid RF gain block [32] are shown in f i g . 4.18. Details regarding their p r i n c i p l e s of operation are described in the references. The amplitude and phase transfer c h a r a c t e r i s t i c s of the SRA-1 and PSES-4 modulators are plotted in f i g s . 4.19 and 4.20. Note that the SRA-1 i s a current c o n t r o l l e d device and the PSES-4 i s a voltage controlled device. Although the insertion phase of the SRA-1 i s quite constant over a large range of control current, the insertion loss of the PSES-4 211 SIGNAL INPUT 3 4 QUADRATURE HYBRID RFC RFC ' ^ 7 SIGNAL OUTPUT CONTROL VOLTAGE PSES-4 ELECTRONIC PHASE SHIFTER MERRIMAC SIGNAL INPUT SIGNAL OUTPUT SRA-1 BALANCED MIXER/AMPLITUDE MODULATOR MINI-CIRCUITS LABORATORIES Vcc . +15 VDC INPUT 0.0luF 1 Rl v W -39 R3 12K 2N5109 E C LI 1.0 uH C3 -T B.8uF-r O.luF C5 R2> R4 _L 1K< 47K Ql C2 47pF O.luF CA 2830 BROADBAND AMPLIFIER 31 50 OHM 9 ^ OUTPUT (TO POWER AMPLIFIERS) RF AMPLIFIER/BUFFER F i g . 4.18 Components of the RF Modulator Chain 212 F i g . 4.19 Amplitude and Phase Transfer C h a r a c t e r i s t i c s SRA-1 Balanced Mixer/Amplitude Modulator F i g . 4.20 Amplitude and Phase Transfer C h a r a c t e r i s t i c s -PSES-4 E l e c t r o n i c Phase S h i f t e r varies markedly over a large range of control voltage. For this reason i t was decided to imbed the RF amplitude control loop within the RF phase control loop. The modulator drivers are shown in f i g s . 4.21 and 4.22. As described e a r l i e r , the fine modulators are driven by the compensated error signal superimposed on a fixed bias while the coarse modulators are set d i r e c t l y by the l o c a l control processor. The coarse amplitude modulator driver must also provide a mechanism for smoothly switching the control current from the driven to s e l f - e x c i t e d modulator or vice versa. When node A2 i s held high, Q1 i s switched off and a l l the control current from Q1 flows through the Q3 which drives the modulator. When node A2 i s held low, Q1 i s turned on and diverts most of the control current from Q3. C1, the 2.7 uF capacitor, slows (and smoothes) the tr a n s i t i o n from a few microseconds to several milliseconds. 4.3.3 Analog Controllers The difference between the measured value of the RF amplitude or phase to be con t r o l l e d and the required set point i s generated by a d i f f e r e n t i a l gain stage shown in f i g . 4.23. The stage consists of three c i r c u i t s : a. the measured signal input buffer; b. the reference signal input buffer; c. the difference a m p l i f i e r . The measured signal input buffer i s an op amp based voltage follower. Its main purpose i s to provide some 215 + 12V + 12V AMPLITUDE MODULATOR DRIVER (FINE) I—1 ON -MODE SELECT AO CLOSED LOOP / OPEN LOOP Al RF ON / RF OFF A2 DRIVEN / SELF-EXCITED A2 Al AO CAMAC DUAL OUTPUT REGISTER RFOUT1 RF MODE SELECT CAMAC RFDAC 0 - sv 10 BITS Rl 15K R 2 ~ l 0 1 6K8-p 270pF R3 >3K3 Mnt COMPENSATED ERROR VOLTAGE R4 5K6 +5V DG509A. • a • 2 1 0 1N914 -w-Dl L AO.....^ . J RF MODE SELECT R5 £3K3 AMPLITUDE MODULATOR (FINE) MINI-CIRCUITS SRA-1 AMPnTul^TOuUATOR'TO _ L _ _ l AMPLITUDE MODULATOR (FINE) MINI-' T AMPLITUDE MODULATOR (FINE) ClrtCDTTS] SRA-1 DRIVEN IN SELF-EXCITED IN T RF MODE SELECT E'2N3906 RB 3K3 R9 3K3 +5V + 12V F i g . 4.21 Prototype RF Control System - RF Amplitude Modulator Drivers + 12V PHASE MODULATOR DRIVER (FINE) + 12V -MODE SELECTOR AO CLOSED LOOP / OPEN LOOP Al RF ON / RF OFF A2 DRIVEN / SELF EXCITED A2 Al AO CAMAC DUAL OUTPUT REGISTER RF0UT1 RF MODE SELECT CAMAC RFDAC o - sv 10 BITS RIN 15K R4 10K RF OUT R 2 ~ l CI 10K-j^ 270pF R3 IK 'int DG509A. • 3 • 2 1 6 PHASE MODULATOR (FINE) RF MODE SELECT HERRIMAC PSES-4 COMPENSATED ERROR VOLTAGE pm"S"E"M0mT0"R""D'rir7^ "TC-0A"Rs"Fj" RF MODE SELECT RF IN RF OUT PHASE MODULATOR (COARSE) MERRIMAC PSES-4 RF IN + 12V i g . ko22 Prototype RF Control System - RF Phase Modulator Drivers VARIATION INPUT from RF Detector LM318 + R5 -VW-10K ro 00 REFERENCE, 5AA_ (COARSE) ^ 22K ' REFERENCE. A , (FINE) ERROR VIEW 2 ERROR VIEW 3 COMPENSATION AMPLIFIER 1 -POLE FOR INTEGRAL CONTROL -ZERO FOR LOOP STABILITY -COARSE LOOP GAIN CONTROL COMPENSATION AMPLIFIER 2 -POLE FOR INTEGRAL CONTROL -ZERO FOR RESONATOR POLE CANCELLATION -FINE LOOP GAIN CONTROL OUTPUT to RF Modulator F i g . 4.23 Prototype RF Control System - Error Signal Generation and Compensation i s o l a t i o n between the RF detector and the analog c o n t r o l l e r . The buffer passes a re p l i c a of the measured signal to the difference amplifier and to external monitoring instruments. Such instruments, e.g. an oscilloscope or a high precision/low d r i f t voltmeter (HP 3468B or equivalent), should have a very high input impedance. R3 (2k2) merely provides some extra protection against the p o s s i b i l i t y of unexpected loading of the buffer output. The two reference signals, coarse and fine, are generated by 0 — 5 v o l t / 10 b i t CAMAC d i g i t a l to analog converters and are summed by the r e s i s t o r network consisting of R1 (22k), R2 (10M), and R4(22k). Further processing by an inverting summing amplifier y i e l d s a DC transfer funct ion: V r o f = 1 .06 x (2.27 x V + 0.1 x V,. ) (4.1) ref coarse fine The difference or error signal i s generated by an inverting summing amplifier. Its transfer function is given by: V e r r " 2 2 x ( V i n " V r e f ) ( 4 ' 2 ) Assuming a normal detector scaling of 1 volt per 10 k i l o v o l t s for amplitude and 27 m i l l i v o l t s per degree for phase detectors, the scaling for the reference inputs i s approximately as follows: 2 1 9 Amplitude Coarse 118 volts/step Fine .26 volts/step Phase Coarse 0.44 degrees/step Fine 0.001 degrees per step Setting the Reference Signals Setting the reference signals to a desired value may prove somewhat challenging since the sum of the coarse and fine reference signals w i l l not necessarily behave monotonically. The problem i s e a s i l y resolved, however: In the case of amplitude control, the detected signal i s measured by a high resolution d i g i t a l voltmeter connected to the v a r i a t i o n output. The reference signals w i l l be adjusted u n t i l the desired measurement i s obtained; In the case of phase control, the fundamental phase i s normally fixed and the t h i r d harmonic phase with respect to the fundamental must be made on the basis of beam measurements. Inspection of the reference signal buffer shows that the reference signal i s low pass f i l t e r e d before being compared to the measured si g n a l . This i s done to prevent problems with 60 Hertz interference introduced by either pick-up or imperfect grounding. It also tends to reduce the random noise which Durieu warned "can be quite high from DAC 220 c i r c u i t r y " [33]. The f i l t e r i s of t h i r d order with a l l real poles. A step change in reference input i s passed to the output with less than 50 parts per m i l l i o n error after less than 300 milliseconds. This delay i s of l i t t l e consequence in normal cyclotron operation since the set points are changed so infrequently and a r e l a t i v e l y long s e t t l i n g time i s quite acceptable. It does mean, however, that one cannot use the reference inputs: a. to pulse the RF drive on and off during cavity surface conditioning and pulsing through multipactoring; b. to introduce feed forward compensation for various disturbances. The delay should also be noted by RF control system programmers - i t i s quite long compared to delays that would normally be generated by the control program! The output of the d i f f e r e n t i a l amplifier is passed to the input'to the f i r s t compensation ampl i f i e r . The error signal may be monitored through a test point l a b e l l e d ERRORVIEW_1. Compensation Amplifiers The compensation amplifiers implement the compensation scheme designed by Durieu and presented in f i g . 4.4. A si m p l i f i e d diagram of the compensation amplifier chain i s presented in f i g . 4.24 (a). The amplifier chain consists of 221 ERROR SIGNAL IN Rnl V W -Rfl C f l r V W -Rn2 - V \ A A -R f 2 c f 2 i - v W )h OUTPUT STAGE 1 STAGE 2 F i g . 4.24(a) Simp l i f i e d Diagram of Analog Controller - Compensation Amplifiers two similar stages in cascade. Each stage introduces (a) selectable gain, (b) a pole at the o r i g i n , and (c) a zero at a selectable location. The gain of a single such stage i s given by: G(s) = [R f + l/sC]/R n (4.3) (1 + sR fC)/sR nC (4.4) K/s (1 + s/wc) (4.5) where: K = 1/RnC (4.6) w = c 1/RfC (4.7) The gain for two such stages in cascade i s simply given by: G c(s) = K c / s 2 (1 + s/w c 1) (1 + s/w c 2) (4.8) where: K c = (l/R n lC,) ( l / R n 2 C 2 ) (4.9) wc1 = 1/< R f i c i ^ (4.10) w c 2 = 1/(R f 2C 2) (4.11) The compensation amplifiers used in the prototype analog c o n t r o l l e r are shown in f i g . 4.24(b). The gain and response corresponding to a given setting are given in Table VII. Where appropriate, the resistance of the 223 AUX2 332K V W --t^- Error Signal ERROR VIEW 1 DG508 ANALOG SWITCH GAIN (COARSE) A2 Al AO 2K87 V W - i BK81 V W -14K7 T / W -31K6 T W ^ 88K1 TAAA-147K V W -316K VW— 681K •VW—' 4K7 r r V W -8K8 hvw-10K hvw-15K hAAA-22K -vW— 33K |VW— 47K hvw— 68K LVW— A2 Al AO DG508 ANALOG SWITCH POLE A ZERO 1 33nF ITT Zero Select LM31B f l O O p F IK TTT Gain Select AUX3 ERROR VIEW 2 5K1 332K V W— 17K8 -\AAA—i 19K6 V W — 21K5 V W — 23K7 V W — 26K1 • V W — 28K7 V W — 31K6 V W — 34K8 A2 Al AO VW— 1 0G508 ANALOG SWITCH GAIN (FINE) rn Gain Select 3K16 rVW— 3K83 4K64 V V \ A -5K62 V W — 6K81 V W — 8K25 V W — 10K0 V W — 12K1 SAAA-OG508 ANALOG SWITCH POLE B ZERO 2 A2 Al AO Hf— 33nF m Zero Select LM318 100pF p. IK ERROR VIEW 3 5K1 To Fine Modulator Driver F i g . 4.24(b) Analog Controller - Compensation Amplifiers TABLE V I I GAIN AND RESPONSE SETTINGS -PROTOTYPE ANALOG COMPENSATION AMPL IF IERS F i r s t Compensation Stage S t a b i l i z i n g Zero Address Gain Factor Time Constant Zero Frequency (usee) (Hz) 0 1 .0 226 4425 1 2.16 156 6410 2 4.64 110 9090 3 10.0 74 13510 4 21 .6 50.8 19700 5 46.4 34.3 29150 6 100.0 23.7 42190 7 216.0 16.8 59520 Second Compensation Stage Resonator Pole C a n c e l l a t i o n Address Gain Factor Time Constant Q(23.06) Q(69.18) (usee) MHz MHz 0 1.0 85.1 6150 18450 1 1.10 70.7 5110 15340 2 1 .21 58.8 4250 12750 3 1 .33 49.0 3540 10630 4 1 .47 40. 1 2960 8870 5 1 .62 34.2 2470 7420 6 1 .78 28.6 2070 6220 7 1 .96 24. 1 1740 5230 Note: The t o t a l gain (end to end) at 1 rad/s i s programmable from 40.9 x 10 6 to 16.6 g x 10 or, in logarithmic terms, from 152.2 dB to 204.4 dB. This corresponds to a 5 dB in t e r v a l approximately every six steps. 225 analog switch was taken into account when selecting feedback and input r e s i s t o r s . Provision i s made for observing the error signal at various stages in the compensation network (Error_View 1, 2, 3) and introducing feed-forward compensation (Aux 2, 3). The desired compensating and s t a b i l i z i n g zeros are chosen by selecting an appropriate feedback r e s i s t o r . By switching the feedback r e s i s t o r instead of the capacitor, the gain and zero settings are made independent of each other. Problems associated with transients that would normally accompany the process of switching charged and discharged capacitors in and out of the feedback loop are s i m i l a r l y avoided. The re s i s t o r values are chosen so that the time constants of the zeros form a geometric progression with seven i n t e r v a l s . The f i r s t stage gives the coarse gain selection in seven geometric steps. The second stage gives the fine gain selection, also given in seven geometric steps. The voltage gain of the combined compensation stages (at 1 rad/s) may be selected from the range of 40.9 x 10^ to approximately 1.66 x 10 1^ in 63 geometric steps. The gain and response of the compensation amplifiers can be set either by the l o c a l control processor or by shorting the appropriate pins of a connector mounted on the front panel of each of the four compensation amplifier modules. 226 4.4 T H E R F D E T E C T I O N S U B - S Y S T E M In i t s most basic form, the RF detection system i s designed to present the following low frequency / low l e v e l signals to the RF control system: DC voltages proportional to the difference between the fundamental and t h i r d harmonic resonant frequencies of the cyclotron RF cavity and the frequencies of the fundamental and t h i r d harmonic driving signals; DC voltages proportional to the r e l a t i v e amplitude and phase of the fundamental (23 MHz) and t h i r d harmonic (69 MHz) components of the accelerating voltage; and, D i g i t a l status b i t s to indicate i f the accelerating voltage exceeds a preset threshold, perhaps 5 kV, and i f a spark or multipactor discharge has been detected and the RF drive signal has been automatically disabled. In p r i n c i p l e , i f the accelerating voltage amplitude and phase feedback control loops have s u f f i c i e n t gain and bandwidth, the radio frequency control system should be able to suppress v i r t u a l l y a l l disturbances to the cyclotron's accelerating voltage and e a s i l y meet the amplitude and phase s t a b i l i t y requirements for acceleration in separated turns and single turn extraction. In practice, however, the a b i l i t y of the radio frequency control system to regulate the amplitude and phase of the accelerating voltage, hence the energy gained by the ion beam during each turn, i s l i m i t e d by the maximum control loop gain and bandwidth that are allowed by s t a b i l i t y considerations, as mentioned in 227 section 4.1, and errors and i n s t a b i l i t i e s in the output of the radio frequency amplitude and phase detectors, as discussed in this section. Simply stated, the RF control system can only regulate a given variable as well as i t can measure i t . To the feedback control loops, errors and i n s t a b i l i t i e s introduced by the radio frequency detectors are indistinguishable from errors and i n s t a b i l i t i e s in the controlled variable, i . e . the amplitude or phase of the accelerating voltage. This means that in the process of suppressing real disturbances such as the ef f e c t s of fluctuations in the resonant frequency of ttte cavity, the gain of the power amplifiers, or the degree of beam loading, the feedback control loops can a c t u a l l y introduce spurious disturbances into the accelerating voltage. The RF detectors are, therefore, c r i t i c a l components of the RF control system. 4.4.1 Measurement of the Accelerating F i e l d by Its Ef f e c t on the Ion Beam As mentioned in chapter two, i t i s very d i f f i c u l t to measure the exact value of the RF voltage encountered by the ion beam during acceleration. The best measures of the s t a b i l i t y and form of the accelerating voltage are, by d e f i n i t i o n , the mean energy gain per turn and the energy dispersion within the beam but in practice i t i s very d i f f i c u l t to measure these quantities so other less precise (but more convenient) measures such as a capacitive voltage d i v i d e r s mounted near the dee gap are usually employed. 228 One measure of the s t a b i l i t y of the accelerating voltage i s the s t a b i l i t y of the ra d i a l position of a given turn, as suggested by f i g . 2.6. A s l i g h t increase in the accelerating voltage w i l l cause the turn positions to s h i f t towards the outside of the cyclotron while a sl i g h t decrease w i l l cause the position to s h i f t inwards. One can obtain an error signal for accelerating voltage regulation by using an intercepting s l i t as a d i f f e r e n t i a l current probe. When a turn i s centered in the s l i t , the current in each of the t a i l s intercepted by the s l i t s i s equal. Small fluctuations in the accelerating voltage, hence r a d i a l position of the beam, w i l l cause one current to increase at the expense of the other. The difference between the two currents has been shown [15] to be a good measure of these fluctuations. When the cyclotron was being designed and commissioned, Richardson and Craddock [34] and Erdman [35] expressed interest in measuring the r a d i a l position of the ion beam for fine control of the accelerating voltage amplitude. It may prove necessary to incorporate such beam-related information into the new accelerating voltage amplitude regulation loops to achieve the s t a b i l i t y required to accelerate the beam in separated turns at the extraction radius. Although the development of instrumentation for measuring such parameters would be involved and time consuming, recent work by von Rossen, Euler, and Hinterberger [15] which was s p e c i f i c a l l y oriented towards 229 p h y s i c a l l y large cyclotrons such as TRIUMF has yielded encouraging r e s u l t s . It was hoped to investigate the use of d i f f e r e n t i a l current s l i t s for accelerating voltage regulation using the e x i s t i n g s l i t s H1, H2, H3, and H4 as part of t h i s design study. These s l i t s are not p e r f e c t l y configured but would have been useful for f i r s t tests using e x i s t i n g hardware in the cyclotron vacuum tank. Unfortunately, problems with RF pick-up encountered early in the development of the s l i t s ( c i r c a 1974) led to the decision to simply short them to ground inside the vacuum tank rather than pass the intercepted current signals through a coaxial cable to the service annex as was o r i g i n a l l y planned. Similar problems would undoubtedly be encountered during an attempt to implement a d i f f e r e n t i a l current s l i t s p e c i f i c a l l y designed to generate an error signal for accelerating voltage regulation. Once the probes are i n s t a l l e d in the vacuum tank they are r e l a t i v e l y inaccessible and must therefore be very r e l i a b l e . Unfortunately, the vacuum tank environment i s also extremely harsh - the probes are exposed to high voltages, hard vacuum, radiation, and sparking. Materials used in the probes construction must be c a r e f u l l y chosen to prevent both probe f a i l u r e or contamination of the tank vacuum due to outgassing. These problems are compounded by radio frequency interference from the main RF system which contaminates the desired error s i g n a l . No attempt was made to incorporate measurements of beam 230 properties into the existing RF control system and no s p e c i f i c plans to incorporate such measurements into the new control system have yet been made, although some interest in such techniques has been expressed in the past. Dohan [36] has expressed reservations concerning the ultimate effectiveness of d i f f e r e n t i a l current regulation at TRIUMF. In any event, the p r i o r i t y of such work i s now quite low. 4.4.2 Organization and Topology of the New RF Detection Sub-System The radio frequency detectors present a measure of the variables to be controlled or regulated to the appropriate control loops. The sub-system consists of (a) the set of capacitive dividers used to sample the dee voltage, hence a good estimate of the accelerating voltage and (b) the two pairs of inductive loops used to sample the transmission l i n e and cavity surface currents so that their phase difference can be measured and the difference between the cavity's resonant frequency and the dr i v i n g frequency determined. An equivalent c i r c u i t of the RF detectors i s shown in f i g . 4.25. A number of factors l i m i t the accuracy of the RF detectors, p a r t i c u l a r l y the measurement of the accelerating voltage, as suggested by f i g s . 4.26 and 4.27 and must be considered during the design and development of the detector subsystem: The effects of mechanical vibration or displacement of the dee on the r e l a t i o n s h i p between (i) the accelerating 2 3 1 -vw— - v W — ro ro T _L — r T T T RF POWER SOURCE TRANSMISSION LINE FILTER (as required) MATCHING SECTION - V W — r i R LOSSESH DEE GAP—l TRANSMISSION RF CAVITY LINE CURRENT ROOT CURRENT TUNING ERROR DEE VOLTAGE - V W — F i g . 4.25 Equivalent C i r c u i t - RF Tuning Error ;and Accelerating Voltage Detection ro >o0 HOT ARM (b) (c) CAPACITIVE PROBE 3 . KAPTON DIELECTRIC —1 ION BEAM 00 I TIP VIBRATION ' 5 Hz | f DEE CENTERLINE (d) (•) _ J -(f) EMI FROM OTHER SOURCES IS PREVENTED BY USING CABLE WITH A SOLID SHIELD Hf-DC BLOCKS PREVENT GROUND LOOPS RF SHIELD 50 OHM RF DETECTORS F i g . 4.26 Sources of Error i n Measurement of the Accelerating Voltage DEE GEOMETRY DEPENDENT ERRORS variations i n the relationship between: (a) V^ and V, at a given radius acc dee to (b) V at different r a d i i acc (c) C,. and C , t i p probe TEMPERATURE DEPENDENT ERRORS (d) varia t i o n s i n probe capacitance ( f i g . 4.27) (e) variations i n cable attenuation (f) , variati o n s i n cable i n s e r t i o n phase (g) vari a t i o n s i n detector transfer function DIELECTRIC CONSTANT VS. FREQUENCY (Type H Film 25 /nm (1 mil)) 3.6 3.5 £ 3 4 | 33 S 3.2 o gE 3.1 o U J 3.0 5 2.9 2.8 2.7 233K ( - 4 0 ° C ) 296K (23°C) 308K (35°C) 373K ( 1 0 0 ° C ) 473K (200°C) coot/ /icnor*\ 10" 105 106 107 3.6 3.4 3.2 3.0 2.8 DIELECTRIC CONSTANT VS. TEMPERATURE (Type H Film—25 /xm (1 mil)) lertz— > > hertz io»( 173K 273K 373K 473K 573K (-100°C) (0°C) (100°C) (200°C) (300°C) TEMPERATURE FREQUENCY HZ F i g . 4.27 Variation of the D i e l e c t r i c Constant of Kapton Polyimide Film with Frequency and Temperature voltage and dee voltage at a given radius, ( i i ) the accelerating voltages at d i f f e r e n t r a d i i , and ( i i i ) the r a t i o between the t i p capacitance and the probe capacitance, hence the output of the dee voltage probe; The e f f e c t s of variations in the temperature of the RF cavity on the d i e l e c t r i c constant of the Kapton insulator (see f i g . 4.26) hence the capacitance of the dee voltage probe; The e f f e c t s of variations in the temperature of the coaxial cable leading from the probe to the RF detectors on the attenuation and e l e c t r i c length (insertion phase) of the cable; The e f f e c t s of variations, in the temperature of the RF detectors on their transfer c h a r a c t e r i s t i c s ; and, The need to design RF f i l t e r s to separate the fundamental and t h i r d harmonic components of the dee voltage and cavity root current so they may be measured separately. As discussed in Appendix B, reduction of the amplitude of the 5 Hz mechanical vibrations of the dee by at least an order of magnitude i s a major design objective of the RF Resonator Replacement Program. Inspection of f i g . 2.1 shows that the dee gap i s formed by four mechanically independent sets of hot arms. Considering that the hot arms on either side of the centre post are also mechanically iso l a t e d from one another, one i s l e f t to compensate for the e f f e c t s of eight uncorrelated mechanical vibrations on the accelerating voltage. Although i t has been found that s i g n i f i c a n t l y 235 better regulation can be achieved by summing the output of two RF detectors sampling the dee voltage on diagonally opposing octants (e.g. Upper Quadrant 1 and Lower Quadrant 3) than by using the output of a single detector sampling at a given single point, s i g n i f i c a n t 5 Hertz perturbations in the dee voltage remain, as shown in f i g . 4.28. A low frequency spectrum analyzer (Nicolet Model 660B) was used to compare the perturbation spectra of the accelerating voltage va r i a t i o n seen by a given dee voltage probe and the accelerating voltage v a r i a t i o n inferred from measurements of the ion beam's time-of-f1ight. Despite the high gain of the accelerating voltage control loop at such low frequencies, the disturbance at 5 Hz i s s i g n i f i c a n t which suggests that the 5 Hz vibrat i o n i s corrupting the measurement through the mechanisms discussed e a r l i e r ( f i g . 4.26). A description of the development of a new dee voltage probe to replace the design commissioned with the cyclotron was presented by Hohbach [37], The p r i n c i p a l difference between i t and the design described by Brackhaus [4] i s the use of Kapton polyimide f i l m [38] as the capacitor's d i e l e c t r i c . From a mechanical standpoint, Kapton i s c l e a r l y an excellent choice since i t i s highly resistant to extremes of temperature, hard vacuum, and radiation. Unfortunately, i t s d i e l e c t r i c constant (see f i g . 4.27) has a rather large negative temperature c o e f f i c i e n t over the range of temperatures usually present in the cyclotron. Calculations have suggested that the v a r i a t i o n may be s u f f i c i e n t l y large 236 F i g . 4.28 Comparison of Time-of-Flight and Dee Voltage Perturbation Spectra CO VDEE o - 10OKV I O - 20KV I I I ± 28 fF C PROBE :140 pF RF CAVITY ROOT CURRENT RF DIPLEXER c 29 MHZ TRANSMISSION LINE CURRENT c RF CAVITY ROOT CURRENT c 80 MHZ TRANSMISSION LINE CURRENT c tx* tx* F i g . 4.29 23 MHZ REFERENCE FROM RF SOURCE MODULE AMPLITUDE DETECTOR LIMITER T 0 2 3 m z MODULATOR LIMITER T Q 8 B ^ MODULATOR LIMITER PHASE DETECTOR tx* PHASE DETECTOR tx. AMPLITUDE DETECTOR PHASE DETECTOR tx. LIMITER fC - 1 HZ _ 23 MHZ * AMPLITUDE [0 TO 10 V) 23 MHZ PHASE 1-2.5 TO 2.0 V] 69 MHZ PHASE [-2.0 TO 2.9 V) _ 89 MHZ * AMPLITUDE [0 TO 10 V] ^ 23 MHZ TUNING ERROR [-2.0 TO 2.0 V] LIMITER LIMITER PHASE DETECTOR tx. fe - 1 HZ ^. B9 MHZ TUNING ERROR [-2.0 TO 2.0 V) New R F C o n t r o l S y s t e m - R F D e t e c t i o n S y s t e m 'dee ± 2 8 fF C PROBE :140 pF ro AMPLITUDE RF DETECTOR MODULE PHASE DIGITAL VOLTMETER HP 3468B 48 v CAMAC GPIB INTERFACE RFGPIB SPARK DETECTOR RF PRESENCE DETECTOR CAMAC INTERRUPT GATE RFINPUT THRESHOLD REF -•>• TO AMPLITUDE MODULATOR ERRORVIEW 3 • EHRORVIEW 1, PHASE ERRORVIEW 2 i ERRORVIEW 3, CAMAC ADC RFADC TO PHASE MODULATOR REF COMPENSATION AMPLIFIERS F i g . 4.30 RF Detector / RF Control System Interface to compromise the required s t a b i l i t y of the measurement chain. Alternative voltage measurement schemes have been developed at other i n s t i t u t i o n s which may be of some use in RF system development and perhaps during actual RF system operation at TRIUMF should the signal from the capacitive dee voltage prove inadequate during high energy resolution modes of operation. Measurement of the accelerating voltage by i t s e f f e c t on the ion beam was discussed in section 4.4.1. A related scheme i s based on measurement of the X-ray spectrum generated by electrons liberated by ionization of residual gas in the dee gap region or emitted by a s t r a t e g i c a l l y placed filament and subsequently accelerated into the opposite dee by the accelerating f i e l d . Measurement of the end-point of the X-ray spectrum gives the maximum potential encountered by the free electrons. The scheme i s b r i e f l y discussed in section D.6. A t h i r d technique which has been described by Massey et a l [39] [41] [42] i s based on the measurement of the effect of the accelerating f i e l d on linear electrooptic materials using a laser. The scheme was o r i g i n a l l y developed for remote measurement of 60-Hz voltages, currents, and transients in a power l i n e substation environment by power u t i l i t i e s . Wyss [40] recently reviewed the use of the technique at radio and microwave frequencies for the measurement of antenna radiation patterns. The advantages (and disadvantages) of the scheme are described in the references. 240 The organization of the new RF detection system i s shown in f i g s . 4.29 and 4.30. Each detection c i r c u i t consists of four p r i n c i p a l components: a. the capacitive or inductive RF cavity probes; b. the transmission l i n e leading from the RF cavity to the RF control system, which, in the case of TRIUMF, has considerable length (over 200 f e e t ) ; c. an RF f i l t e r or diplexer to separate the fundamental and t h i r d harmonic components; d. an RF amplitude or phase detector to convert the RF signal to the DC signal that w i l l be monitored by a voltmeter (or analog to d i g i t a l converter) or applied to a summing amplifier to generate the control loop error signal, as shown in f i g . 4.30. 4.4.3 RF F i l t e r s The voltages developed across the loops and probes in the radio frequency cavity and associated transmission l i n e s are composite signals with both fundamental and t h i r d harmonic components. The signals must be f i l t e r e d and the unwanted component removed before they are passed to the appropriate RF amplitude and phase detectors. Techniques for designing and constructing RF f i l t e r s for such purposes are well known and have been described in many references including Matthaei and C r i s t a l [43], Malherbe [44], Williams [45], Christian [46], and DeMaw [47] [48], Although i t i s expected that the root and loop current signal f i l t e r s w i l l use standard low pass and high 241 pass LC f i l t e r s , the signals from the dee voltage probe w i l l be passed through a diplexer [43] so that both components of the accelerating voltage are sampled at the same voltage probe. Prototype f i l t e r s were assembled for development work but design and construction of the f i n a l versions w i l l await completion of RF amplitude and phase detector development. Ground loops and various forms of electromagnetic interference which may contaminate the dee voltage signal are an especially serious consideration at a f a c i l i t y with many high power c i r c u i t s in the v i c i n i t y of the signal paths, such as TRIUMF. E l e c t r i c a l design considerations which are useful in c o n t r o l l i n g such interference have been described by Sigg [6], Mack [49], and White [50]. 4.4.5 The RF Amplitude Detector The RF amplitude detector produces a DC voltage that i s proportional to the amplitude of the accelerating voltage. It usually consists of a precision r e c t i f i e r (or peak detector, depending on the design) followed by a low pass f i l t e r . Although a li n e a r transfer function i s c e r t a i n l y desirable, i t i s not necessary since the basic concern i s s t a b i l i t y , not absolute accuracy. The major concern in the design of the amplitude detector i s the ef f e c t of temperature fluctuations on i t s transfer function which are primarily due to the temperature dependence of the v-i c h a r a c t e r i s t i c s of semiconductor diodes [51]. A precision r e c t i f i e r can be assembled by placing the diode in the 242 feedback loop of an operational amplifier. The threshold voltage i s divided by the open loop gain of the amplifier and i s thus v i r t u a l l y eliminated [52] [53] [54]. U n t i l recently, however, th i s technique could only be used at low frequencies (less than 100 kHz) and could not be applied to RF detectors. Gummer (in the existing control system) and Durieu (in the new control system) were forced to use voltage doubling peak detectors with complex feedback c i r c u i t s . The threshold voltage was eliminated by using an op amp based feedback c i r c u i t to force nearly the same current to flow through the r e c t i f y i n g diodes and two reference diodes. The output of the voltage doubler was raised by an amount nearly equal to the voltage drop across the reference diodes. Unfortunately, although t h i s c i r c u i t (shown in f i g . 4.31) l i n e a r i z e s the detector's transfer function, i t does not provide temperature compensation. DeAgro [55] recently described an op amp based precision r e c t i f i e r which could detect signals in a linear region over a 60 dB dynamic range for roughly 11 kHz bandwidth signals of c a r r i e r frequencies ranging from 1 MHz to 4 MHz when using a simple RC succession f i l t e r . DeAgro belives that " m u c h h i g h e r c a r r i e r f r e q u e n c i e s ( u p t o 2 0 0 M H z ) c o u l d b e a c h i e v e d w h e n u s i n g s t a t e o f t h e a r t t y p e o p -a m p s . " Such op amps have been described by Evans [56]. They employ an unusual (and patented) c i r c u i t configuration to eliminate the gain bandwidth product and permit their use at RF frequencies in excess of 100 MHz. 243 Spark Detection The existing control system r e l i e s on l e v e l detectors to detect the loss of resonator voltage associated with a spark or the onset of multipactoring. A new spark detector has been designed that senses the rate of resonator voltage decrease. Under normal conditions, the rate of resonator voltage decrease or increase in response to a step function i s l imited by the energy stored in the cavity. The pole in the response i s set by the cavity's q u a l i t y factor. The discharge associated with a spark or multipactor spoils the Q of the cavity and the dee voltage w i l l decay with a much faster time constant. The prototype spark detector i s shown in f i g . 4.32. The spark i s detected by C1 in p a r a l l e l with R2 and R3. The properties of the p a r a l l e l RC c i r c u i t are described by Clement and Johnson [57], In summary, t h i s c i r c u i t presents an i n f i n i t e impedance to a voltage decaying with the same time constant. A posit i v e voltage that decays faster than the set time constant w i l l induce a negative voltage on R1. This w i l l t r i p the comparator and set the data f l i p f l o p . The f l i p f l o p can be read, set, or reset through the CAMAC dual output register and dual interrupt gate as shown in f i g . 4.10 and f i g . 4.32. In practice, the time constant of the spark detector w i l l be set about 10 to 15 percent lower than the expected cavity time constant to prevent accidental t r i g g e r i n g . A related measurement can be used to measure the 2kk q u a l i t y factor of the RF cavity while the RF system i s in pulsed mode. This value i s required in order to corr e c t l y set the resonator pole cancellation zero. The quality factor Q i s measured on the f a l l i n g edge of the pulse by observing the time in t e r v a l T required for the dee voltage to f a l l from a given voltage VT to a smaller voltage V2. If the measurement i s performed under vacuum, both voltages must be above the multipactoring threshold. V2 = V1 exp -( u> T/2Q ) (4.12) ln (V2/V1) = - (oiT/2Q) (4.13) Q = co T / [2 ln (V1/V2) ] (4.14) 4.4.5 RF Phase Detectors RF phase detectors in common use f a l l into three categories: double balanced mixers [58] [59] [61], d i g i t a l phase detectors [58] [60] [61] [62], and discriminators [58] [63]. Several phase detectors were tested as part of t h i s design study but none were evaluated in d e t a i l . In summary, double balanced mixers are perhaps the least expensive and easiest to use but often suffer from a temperature dependent voltage o f f s e t . Their output i s a function of both the amplitude and the phase of the two input signals. Although they are probably acceptable for use in the cavity tuning c o n t r o l l e r , i t i s doubtful that they would be acceptable in the t h i r d harmonic phase regulation loop. 245 D i g i t a l phase detectors based on exclusive or gates and other configurations are increasingly popular in many applications since they have a linear rather than sinusoidal transfer c h a r a c t e r i s t i c . The output amplitude of d i g i t a l phase detectors i s independent of the amplitude of the input signal although d i g i t a l phase detectors are very sensitive to the duty factor of the input signals. Unfortunately, d i g i t a l phase detectors tend to be noisier than their analog counterparts. Phase regulation in the SIN t h i r d harmonic RF system i s based on a discriminator phase detector [21] of the type described by Pedersen [63]. Prototype RF phase detectors shown in f i g . 4.33 and the RF l i m i t e r shown in f i g . 4.34 were assembled and shown to be function properly but they were not evaluated in d e t a i l . The prime function of the RF l i m i t e r ( f i g . 4.34) i s to ensure that the signal input to the d i g i t a l phase detector has a constant amplitude and a 50% duty cycle. The transfer functions of the phase detectors were measured by creating a s l i g h t difference between the frequencies of the two input signals which, by d e f i n i t i o n , corresponds to a constant rate of change in the r e l a t i v e phase between the two signals. The transfer c h a r a c t e r i s t i c s of the protototype d i g i t a l phase detectors shown in f i g . 4.35 were measured in such a manner. Although the transfer c h a r a c t e r i s t i c at 23 MHz i s quite l i n e a r , the transfer c h a r a c t e r i s t i c at 69 MHz i s not,for as yet unknown reasons. 246 IN OUT F i g . 4.31 Prototype RF Control System - RF Amplitude Detector +5V _1_ INPUT FHOM AMPLITUDE DETECTOR CI :3.3nF R2 4K7 . R3 CAMAC DUAL OUTPUT REGISTER RF0UT1 LM311 R5 •—vW-220K R4 IK R6 4K7 +5V J L R7 IK R8 4K7 Wf RESET/ON R9 2K2 VOLTAGE COMPARATOR 74LS74 TO MODE SELECTOR SPARk DETECTED CAMAC DUAL INTERRUPT GATE RFINPUT F i g . 4.32 Prototype RF Control System - Spark Detector 247 I N P U T A O U T P U T I N P U T B (a) HJS-55 MERRIMAC Rl 120 R2 120 MBD701 -•I Dl R3 150 R4 150 MBD701 - M D2 (b) F i g . 4.33 Prototype RF Control System - RF Phase Detectors (a) XOR d i g i t a l phase detector (b) discriminator-type analog phase detector IV) -p-+5V C2 O.luF R3 -vW-560 10115 10115 0. luF 10115 Rl 51 R2 470 V BB" R5 C4 ^ 470 0. luF R4 •VW-560 'BB' 10115 C5 •T O.luF G3 ^ 0.luF R6 470 R7 470 F i g . h.Jk RF Limiter based on ECL Quad Line Receiver V e r t i c a l 500 m V / d i v H o r i z o n t a l 50 u s e c / d i v V e r t i c a l 200 m V / d i v H o r i z o n t a l 100 u s e c / d i v Transfer Chara c t e r i s t i c s - Prototype D i g i t a l RF Phase Detectors (a) RF f r e q u e n c y - 2 3 . 2 3 0 7 6 MHz V ± n - 1 .5 V (peak ) (b) RF f r e q u e n c y - 69 .69030 MHz V ± n - 0 . 6 V (peak ) f , „ . - f , - 1980 Hz i n p u t 1 i n p u t 2 2 5 0 4 .5 DEVELOPMENT OF THE FINAL RF CONTROL SYSTEM DESIGN The purpose of the prototype RF control system was to demonstrate safe operation of the test f a c i l i t y RF system with a flat-topped accelerating voltage and provide a vehicle for evaluating control system design a l t e r n a t i v e s . The preliminary design objectives were met and operational experience with the f i r s t prototype suggested many improvements and enhancements to the reference design which should be incorporated into the f i n a l design. RF System Control Panel Durieu suggested that the status of the RF control system be presented on a software-driven video display terminal and thus eliminate the need for a hard-wired front panel. Such a scheme i s used in the TRIUMF central control system. Cyclotron operators gain access to various data and set points by switching between a large number of display screens. Experience with a s i m p l i f i e d version of the proposed RF console designed by D. Michelson and B. Evans and implemented using an Intel 8085-based TRIMAC suggested that such a scheme i s impractical, p a r t i c u l a r l y during RF system development. The amount of information that can be displayed on a single 80 column by 25 l i n e screen i s su r p r i s i n g l y small and the time and e f f o r t required to switch between screens while c o n t r o l l i n g a real time task is rather uncomfortable for the system operator. This problem w i l l be solved by off loading most of the display requirements to a hardware mimic panel which w i l l display 251 (under l o c a l control processor c o n t r o l ) : a. the selected operating mode, e.g. driven/self-excited, pulsed RF drive/continuous RF drive; b. the status of the RF system, e.g. dee voltage amplitude(s) and r e l a t i v e phase(s), magnitude of cavity tuning error, RF on, RF enabled, RF disabled or spark detected; and, c. c o n t r o l l e r settings, e.g. selected gain and response of compensation amplifiers, feedback control loop reference values. This w i l l simplify the design of the video display screen considerably and, most importantly, w i l l eliminate the need for the system operator to rapidly switch back and forth between display screens during execution of a given task. For similar reasons, i t has been decided to allow operators to vary set points using a rotary encoder or "soft knob" rather than through the video display terminal keyboard. A soft knob i s e s s e n t i a l l y a d i g i t a l version of a potentiometer which can be more eas i l y interfaced to a microcomputer than a potentiometer could. Hardware Issues The prototype control system was b u i l t in a half-height Eurocard chassis which proved to be rather confining. It i s suggested that the f i n a l design specify f u l l - h e i g h t Eurocards or Nuclear Instrumentation Modules (NIM's) instead. This would ease the problem of redesigning or adding more fu n c t i o n a l i t y to individual modules in the 2 5 2 future. The applications l i t e r a t u r e [28] [64] [65] [66] [67] provided much useful information regarding component selection and design of c i r c u i t layouts. The layout of the interconnections between modules in the prototype RF control system i s suggested by the block diagram in f i g . 4.36. The power supply bus and low frequency/high l e v e l signals passed between modules are carried on the signal processing crate's backplane. A l l modules interface with the l o c a l control processor through the analog c o n t r o l l e r module. The d i g i t a l input/output connectors are standard miniature D connectors. Analog signals are input/output through two pin LEMO connectors. Both sets of connectors are mounted on the rear panel of the crate. RF input/output i s performed through single pin LEMO connectors mounted on the front panel of the modules. RF signals are passed between modules through coaxial patch cables. This arrangement has proven to be extremely sati s f a c t o r y although the analog c o n t r o l l e r s and associated interfaces are somewhat cramped when mounted on just one single-height Eurocard. Single pin LEMO connectors were chosen over SMA connectors because they connect and disconnect with much greater ease but problems with poor connections have suggested that SMA connectors would have been a much better choice. The .pulse width modulator used to pulse the RF drive during system start-up should be implemented in hardware 253 C a v i t y T u n i n g E r r o r D e t e c t o r R o o t Loop L o c a l C o n t r o l P r o c e s s o r I CAMAC D i g i t a l I n t e r f a c e Bus DAC ADC P a r a l l e l I /O RF A m p l i -t u d e & P h a s e D e t e c t o r S t e p p i n g M o t o r C o n t r o l l e r &<§> Power S u p p l y Bus A m p l i -t u d e C o n t r o l -l e r P h a s e C o n t r o l -l e r RF M o d u l a -t o r S e l f - E x c i t e d I n p u t 23 MHz P h a s e R e f e r e n c e Dee V o l t a g e P r o b e RF S o u r c e 23 MHz IT Power S u p p l y + 15 VDC + 5 VDC oCRATE I /O R e a r P a n e l O L F SIGNALS oPOWER SUPPLY B a c k p l a n e ORF SIGNALS F r o n t P a n e l To 23 MHz A m p l i f i e r To 69 MHz C r a t e M a s t e r O s c i l l a t o r Fig-. 4 c 3 6 Routing of Signals Between Functional Modules -23 MHz Analog Signal Processing Crate instead of software. This would reduce the complexity of and timing constraints on the RF control system software without s i g n i f i c a n t l y increasing the complexity of the hardware. D e m o n s t r a t i o n o f t h e New R F C o n t r o l S y s t e m o n t h e C y c l o t r o n The RF systems test f a c i l i t y can only be used to demonstrate that the control system operates safely and r e l i a b l y . The true test of the control system's performance must take place on the cyclotron i t s e l f under normal operating conditions. The problem of inte r f a c i n g the new control system to the cyclotron central control system has not yet been investigated although i t seems l i k e l y that the control program w i l l i n i t i a l l y mimic the e x i s t i n g control system's hardware interface in software. In a few years, however, TRIUMF plans to upgrade the central control system. This provides an opportunity to consider the existing central control - RF control system interface, i d e n t i f y necessary or desirable improvements and implement them with minimal disruption to cyclotron operating schedules. E v a l u a t i n g a n d I m p r o v i n g t h e P e r f o r m a n c e o f t h e New R F C o n t r o l S y s t e m The problem of evaluating the performance of the RF control system i s s u r p r i s i n g l y d i f f i c u l t and l i e s at the heart of the fundamental problem l i m i t i n g the s t a b i l i t y of the accelerating voltage in the cyclotron at the present time - the lack of good measures of the quantity actually being contro l l e d . While i t i s reasonably easy to s t a b i l i z e 255 the output from a given dee voltage probe to within the desired tolerance, i t i s not as easy to s t a b i l i z e the r a d i a l position of the f i n a l turn or the energy of the proton beam that f i n a l l y emerges from the cyclotron exit horn to within the same tolerance. As noted at the beginning of thi s chapter, the performance of a control loop i s ultimately limited by the qual i t y of the measurement presented to i t . It i s important that special e f f o r t be applied to the development of better beam diagnostics in general and improved accelerating voltage measurement in p a r t i c u l a r . References: [1] E.W. Blackmore, G. Dutto, M. Zach. "TRIUMF Central Region Cyclotron Progress Report." Proc 6th Int C y c l o t r o n Conf, pp 95-101 (1972). [2] K.L. Erdman, K.H. Brackhaus, R.H.M. Gummer. "Some Aspects of the Control and S t a b i l i z a t i o n of the RF Accelerating Voltage in the TRIUMF Cyclotron." Proc 6th Int C y c l o t r o n Conf, pp 444-450 (1972). [3] R.H.M. Gummer. "Accelerating Voltage S t a b i l i z a t i o n and Control in the TRIUMF Cyclotron." IEEE Trans NS-22(3): 1257-1260 (June 1975). [4] K.H.• Brackhaus. "The Generation and Control of 1.5 Megawatts of RF Power for the TRIUMF Cyclotron." PhD Dissertation, University of B r i t i s h Columbia (1975). [5] L. Durieu, CERN/TRIUMF. Private communication (August 1984). [6] P.K. Sigg. "New RF-Control Systems for the TRIUMF Cyclotron." TRIUMF Design Note TRI-DN-83-32 (August 1983). [7] L. Durieu. "Re-examination of RF System Tolerances for Separated Turn Acceleration in TRIUMF." TRIUMF Internal Report (September 1983). 256 [8] N.M. Schmidt and R.F. Farwell. "Understanding Electronic Control of Automation Systems." Dallas, Texas: Texas Instruments, chapter 3 (1983). [9] C C . Foster. "Real Time Programming - Neglected Topics." Reading, Massachusetts: Addison-Wesley Publishing Company, pp 107-146 (1981). [10] P.H. Garrett. "Analog Systems for Microprocessors and Minicomputers." Reston,Virginia: Reston Publishing Company, Inc. pp 183-202 (1978). [11] R.J. Vader and H.W. Schreuder. "New Methods for S t a b i l i z i n g Dee Voltage and Beam R.F. Phase." IEEE Trans NS-26(2):2205-2208 (A p r i l 1979). [12] J.G. Ziegler and N.B. Nichols. "Optimum Settings for Automatic Controllers." Trans ASME 64:759-768 (November 1942) [13] S.R. Trost and C. Pomernacki. "VisiCalc for Science and Engineering." Berkeley, C a l i f o r n i a : Sybex, Inc. pp 113-115 (1983). [14] C.L. Smith, A.B. Corripio, and J. Martin, J r . "Controller Tuning from Simple Process Models." Instrumentation Technology pp 39-75 (December 1975). [15] P. von Rossen, K. Euler, and F. Hinterberger. "A New Method for High Precision Dee Voltage S t a b i l i z a t i o n . " Proc 10th Int Conf on Cyclotrons and t h e i r Appli cati ons, pp 371-372 (1984). [16] R.S. Burge, TRIUMF. Private communication (September 1985). [17] J.R. Richardson. 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Peschardt, and J. Sladden. "The LEP Radio-Frequency Low Power System." IEEE 7>a/2sNS-32(5) .-2032-2034 (October 1985). [25] D. Howard. "A V e r s a t i l e RF Controller" IEEE Trans NS-32 (5):2876-2878 (October 1985). [26] C. Bowick. "RF C i r c u i t Design." Indianapolis: Howard W. Sams & Co. Inc., pp 160-161 (1982). [27] B. Blood. "Interfacing with MECL 10,000 Integrated C i r c u i t s . " Pheonix, Arizona: Motorola Semiconductor Products, Inc. Application Note AN-720 (1974). [28] "MECL System Design Handbook." Pheonix, Arizona: Motorola Semiconductor Products, Inc. (1982). [29] V.F. Kroupa, ed. "Frequency S t a b i l i t y : Fundamentals and Measurement." New York: IEEE Press (Reprint Volume 1983). [30] R.V. Garver. "Microwave Diode Control Devices." Dedham, Massachusetts: Artech House, Inc. pp 215-287 (1976). [31] "IF Baseband Signal Processing Components." Merrimac Industries, Inc. Catalog M-80-3 p 54-55 (1980). [32] "TRW RF Semiconductors." Lawndale, C a l i f o r n i a : TRW Electronic Components, Inc. (1979 Catalog). [33] L. Durieu, CERN/TRIUMF. Private communication (1984). [34] M.K. Craddock and J.R. Richardson. "Magnetic F i e l d Tolerances for a Six-Sector 500 MeV H- Cyclotron." TRIUMF Report TRI-67-2, p 13 (December 1968). [35] K.L. Erdman. Discussion following: K.L.Erdman, K.H. Brackhaus, and R.H.M. Gummer. "Some Aspects of the Control and S t a b i l i z a t i o n of the RF Accelerating Voltage in the TRIUMF Cyclotron." Proc 6th Int C y c l o t r o n Conf, p 450 (1972). [36] D.A. Dohan, TRIUMF. Private communication (1985). [37] R. Hohbach. "Results of Recent RF Work (Summer 1983) and Proposed Future Action." TRIUMF Design Note TRI-DN-83-37 (September 1983). 258 [38] "Kapton Polyimide Film - Type H: Summary of Properties." B u l l e t i n H-1D, E.I. DuPont de Nemours & Co, Inc. Industrial Films D i v i s i o n . [39] G.A. Massey, San Diego State University. Private communication, 16 March 1984. [40] J.C. Wyss. "Photonic Sensors for Electromagnetic F i e l d Measurements." IEEE Ant Propagat Soc Newsletter 26(5):5-9 (October 1984). [41] G.A. Massey, D.C. Erickson, and R.A. Kadlec. "Electromagnetic F i e l d Components: Their Measurement Using Linear Electro-Optic and Magneto-optic E f f e c t s . " Appli ed Opt i cs 14:2712-2719 (November 1975). [42] G.A. Massey, J.C. Johnson, and D.C. Erickson. "Laser Sensing of E l e c t r i c and Magnetic F i e l d s for Power Transmission Applications." Proc SPIE 88:91-96 (1976). [43] G.L. Matthaei and E.G. C r i s t a l . "Theory and Design of Diplexers and Multiplexers." in Advances in Microwaves, Vol 2, L. Young, ed. New York: Academic Press (1967). [44] J.A.G. Malherbe. "Microwave Transmission Line F i l t e r s . " Dedham, Massachusetts: Artech House, Inc., pp 232-262 (1979). [45] A.B. Williams. "Electronic F i l t e r Design Handbook." New York: McGraw-Hill Book Co. (1981). [46] E. Ch r i s t i a n . " L C - F i l t e r s : Design, Testing, and Manufacturing." New York: John Wiley & Sons (1983). [47] M.F. DeMaw. "P r a c t i c a l RF Communications Data for Engineers and Technicians." Indianapolis: Howard W. Sams (1978). [48] M.F. DeMaw. "Ferromagnetic-Core Design and Application Handbook." Englewood C l i f f s : Prentice-Hall, Inc. (1981). [49] R.A. Mack. " E l e c t r i c a l Interference, Grounding, and Shielding." Cambridge Electron Accelerator, Harvard University Report CEAL-TM-170 (30 March 1967). [50] J . White. "Wiring of Data Systems for Minumum Noise." IEEE Trans RFI-5-.77-82 (March 1963). [51] J . Millman. "Microelectronics - D i g i t a l and Analog C i r c u i t s and Systems." New York: McGraw-Hill Book Co. pp 26-56 (1979). 259 R.C. Dobkin. "Precision AC/DC Converters." National Semiconductor Linear Brief 8 (August 1969). R.C. Dobkin. "True rms Detector." National Semiconductor Linear Brief 25 (June 1973). W. Jung. "Op Amp Cook Book." Indianapolis: Howard W. Sams, Inc. (1977) W.C. DeAgro. "Wide Dynamic Range Linear Detection." RF Design 8(9):29-30,32 (September 1985). S. Evans. "A New Approach to Op Amp Design." RF Design 8(9):56-57,59 (September 1985). P.R. Clement and W.C. Johnson. " E l e c t r i c a l Engineering Science." New York: McGraw-Hill Book Co. Inc. pp 254-256 (1960). W.H. Hayward. "Introduction to Radio Frequency Design." Englewood C l i f f s : Prentice-Hall, Inc. pp 318-323 (1982). "29 Most Frequently Asked Questions About Phase Detectors." Brooklyn, New York: M i n i - C i r c u i t s (1980). H.M. B e r l i n . "Design of Phase-Locked Loop C i r c u i t s , with Experiments." Indianapolis: Howard W. Sams & Co. Inc., pp 24-51 (1978). J. Briaud. "RF-Phase Detector Modules." Proc 10th Int Conf on Cyclotrons and t h e i r Applications, pp 364-366 (1984). CR. Jackson. "Selecting the Right F i l t e r s for D i g i t a l Phase Detectors." in C i r c u i t s and Software for E l e c t r o n i c s Engineers, H. Bierman, ed. New York: McGraw-Hill Book Co. pp 98-99 (1983). B.O. Pedersen. "Phase-Sensitive Detection with Multiple Frequencies." IRE Trans I n s t r , pp 349-355 (December 1960). "Analog C i r c u i t Design Seminar." Analog Devices (A p r i l 1982). "Data Conversion Seminar." Analog Devices ( A p r i l 1982). "Applications Manual for Operational Amplifiers - A Library of P r a c t i c a l Feedback C i r c u i t s . " Dedham, Massachusetts: Philbrick/Nexus Research (1968). [67] D.E. Pippenger and E.J. Tobaben. "Linear and Interface C i r c u i t s Applications - Volume 1." Dallas, Texas: Texas Instruments, Inc. pp 2-1 to 3-24 (1985). 260 CHAPTER 5 SUMMARY AND CONCLUSIONS 5.1 SUMMARY This thesis has described certain aspects of the t h i r d harmonic RF flat-topping project at TRIUMF. The following major points were discussed: Chapter One - advances in cyclotron design that have been applied to the TRIUMF cyclotron and the advantages of "flat-topping" or otherwise modifying i t s accelerating voltage Chapter Two - how a "flat-topped" accelerating voltage waveform can compensate for certain second order e f f e c t s including energy dispersion and longitudinal space charge forces within the beam packet and deviations from isochronism in the cyclotron's phase history and thus make i t easier to accelerate the ion beam in separated turns at the extraction radius and the required tolerances on the amplitude and phase s t a b i l i t y of the accelerating voltage Chapter Three - the problems, including c o n f l i c t with operating schedules and residual r a d i o a c t i v i t y in the cyclotron vacuum tank, that are associated with developing new RF system components using the cyclotron i t s e l f and the need to use replicas of the RF system, such as the 1 : 1 0 scale model 261 and the RF systems test f a c i l i t y , for development work - c r i t e r i a for designing cavity tuning mechanisms and the size, location, and mechanical design of loops for coupling RF power to the fundamental and t h i r d harmonic modes in the test f a c i l i t y cavity and the cyclotron - description and experimental evaluation of a prototype cavity tuning mechanism based on d e f l e c t i o n of the dee l i n e r (ground panel) - a procedure for test f a c i l i t y RF system start-up and cavity tuning under operating conditions similar to those in the cyclotron, based on knowledge of the e f f e c t s of multipactoring, constraints on the operation of RF power amplifiers, and experience with the e x i s t i n g cyclotron RF system; - demonstration of a flat-topped accelerating voltage in the TRIUMF RF systems test f a c i l i t y cavity at operational dee voltage levels (100 kV - fundamental and 11 kV - t h i r d harmonic) Chapter Four - the need to replace the existing RF control system and the conceptual design of a new control system - a description of the four modes of RF system operation and the new RF system control program - the design and testing of a prototype version of the new cyclotron RF control system and aspects of the design that should be investigated or corrected before a second prototype i s constructed 262 - problems associated with the measurement of the accelerating voltage and the current state of development of the RF detector section of the new RF control system Supporting material i s contained in the four appendices. 5.2 STATUS OF THE THIRD HARMONIC RF FLAT-TOPPING PROJECT PRIOR TO THE COMMENCEMENT OF THIS DESIGN STUDY The RF flat-topping project at TRIUMF has evolved in three d i s t i n c t phases: Phase One (circa 1969): A proposal to f l a t - t o p the accelerating voltage in the TRIUMF cyclotron based on exciting both the fundamental and t h i r d harmonic accelerating modes in the same RF cavity was presented. Phase Two (circa 1972): Prochazka presented a design study of the possible tuning mechanisms and cavity coupling and matching c i r c u i t s that would be required to implement such a scheme. Phase Three (circa 1983): The concept of t h i r d harmonic flat-topping was resurrected, for reasons of both need (plans to extract H- from the cyclotron) and opportunity (the RF Resonator Replacement Program). By 1983, when the current design and development e f f o r t began, two important milestones had already been accomplished: a. After over forty years, a t h i r d harmonic RF system had f i n a l l y (1981) been successfully i n s t a l l e d in a cyclotron (at the Swiss In s t i t u t e for Nuclear Research in Zurich) and i t was confirmed that a 263 flat-topped accelerating voltage could sub s t a n t i a l l y improve a cyclotron's operating c h a r a c t e r i s t i c s and the properties of i t s ion beam; and, b. Construction and testing of the f i r s t major component of TRIUMF's t h i r d harmonic RF system, a 100 kilowatt 69 MHz RF power amp l i f i e r , had been completed. In 1983, the task of solving the three remaining problems began: a. The design of improvements to the mechanical structure of the TRIUMF cyclotron RF cavity, including redesign of the cantilevered hot arms, central region, and flux guides that were planned under the terms of the resonator replacement program and solutions to the problems with coupling between the fundamental (23 MHz) mode in the cyclotron RF cavity and p a r a s i t i c modes in the beam gap ("RF Leakage"); b. The design of modifications to the TRIUMF cyclotron RF cavity that are required for t h i r d harmonic RF system operation, including new fundamental and t h i r d harmonic tuning mechanisms and the t h i r d harmonic coupling loop and matching section; and, c. The improvements to the existing fundamental RF control system that would be required to achieve 26k maximum benefit from the t h i r d harmonic RF system and the design of the new t h i r d harmonic RF control system based on the fundamental system; This thesis project was p r i n c i p a l l y concerned with the las t two issues. 5.3 RESULTS OF THIS STUDY 5.3.1 Demonstration of a Flat-topped Accelerating Voltage in the RF Systems Test F a c i l i t y The major objective of th i s work, demonstration of a flat-topped accelerating voltage in the TRIUMF RF systems test f a c i l i t y , was accomplished under Dr. T.A. Enegren's dire c t i o n and with the assistance of the RF systems group. These tests confirmed that the fundamental and t h i r d harmonic modes could be excited simultaneously and r e l i a b l y in an evacuated RF cavity that i s mechanically similar to the cyclotron RF cavity. Methods and procedures for tuning the RF cavity, coupling power into the RF cavity, and dealing with sparking and multipactoring, were demonstrated. The dee l i n e r d e f l e c t i o n tuning scheme provided s u f f i c i e n t tuning range to tune the cavity to the desired fundamental and t h i r d harmonic frequencies under normal operating conditions. Multipactoring, or multiple impact secondary electron emission, can be a stubborn obstacle to the successful operation of accelerator RF c a v i t i e s . In contrast to 265 breakdown phenomena, which set an upper l i m i t to the accelerating voltage that can be r e l i a b l y maintained in an accelerator RF cavity, multipactoring sets a lower l i m i t to the accelerating voltage that can be r e l i a b l y maintained in an accelerator RF ca v i t y . Although multipactor discharges in the test f a c i l i t y cavity were e a s i l y avoided at the fundamental frequency by pulsing the RF drive above the multipactoring threshold voltage - approximately 27 k i l o v o l t s during pulsed operation - quickly enough, they were impossible to avoid when the th i r d harmonic mode was driven alone. Fortunately, however, i t was possible to suppress t h i r d harmonic multipactoring by f i r s t exciting the fundamental mode in the RF cavity. This e f f e c t i v e l y sets the t h i r d harmonic multipactor threshold voltage to zero. I f , as expected, t h i s result i s shown to apply to the cyclotron RF cavity, the turn-on procedure for th i r d harmonic mode w i l l be far simpler than for the fundamental mode since pulsing the RF drive above a threshold w i l l not be required. 5.3.2 The New RF Control System The second major objective of this work, design and development of a prototype version of the new RF control system, was accomplished under Dr. L. Durieu's d i r e c t i o n with technical assistance from technicians in the Technology and Administration D i v i s i o n , p a r t i c u l a r l y Mr. P. Bennett. 266 The prototype control system performed well during operational tests conducted on the RF systems test f a c i l i t y and, i f the second prototype incorporates the minor improvements suggested by these tests (see section 4.3), i t should e a s i l y meet the design objectives that have been set for i t . 5 . 4 F U R T H E R WORK Much work remains to be done before TRIUMF w i l l be able to demonstrate a flat-topped accelerating voltage in the 520 MeV cyclotron. 5 . 4 . 1 P a r a s i t i c M o d e s i n t h e C y c l o t r o n B e a m G a p The most c r i t i c a l issue i s the problem with p a r a s i t i c modes in the beam gap which 1:10 scale model studies suggest are even more troublesome at t h i r d harmonic than they are at the fundamental frequency. Resolving t h i s problem has occupied the bulk of the attention of the Cyclotron Development and RF Systems groups at TRIUMF during the past two years. 5 . 4 . 2 M e c h a n i c a l D e s i g n o f M o d i f i c a t i o n s t o t h e C y c l o t r o n R F C a v i t y Once the modifications to the RF cavity that are required for t h i r d harmonic operation, including the modifications to the flux guides and central region, and the new cavity tuning and coupling mechanisms, have been l a i d out by accelerator RF s p e c i a l i s t s , the p r i n c i p a l constraints and d i f f i c u l t i e s associated with implementing these modifications a r i s e from: 267 a. the large amounts of RF power involved (1.2 megawatts at 23 MHz, up to 100 kilowatts at 69 MHz) and the resulting problems with heat removal; b. mechanical vibrations that are excited by the flow of cooling water in the dees and other structures; c. the r e s t r i c t i o n s placed on the mechanical design of RF system components by the radiation environment, the need to choose materials that keep contamination of the vacuum to an absolute minimum, and the very small volume within the cyclotron vacuum tank that i s allocated to the RF cavity. The mechanical engineering problems associated with the redesign of the RF cavity are not t r i v i a l and must be solved by experienced mechanical engineers working with RF s p e c i a l i s t s . 5.4.3 A Second Prototype RF Control System A second prototype version of the new RF control system w i l l be designed and constructed. The minor d e f i c i e n c i e s that have been encountered during tests of the f i r s t prototype w i l l be corrected. The second prototype w i l l be used to evaluate the control system design in the cyclotron environment aft e r extensive testing at the RF systems test f a c i l i t y . Before a second prototype i s constructed, however, a dedicated l o c a l control processor must be selected to 268 replace the Cyclotron Development VAX-11/730 and a prototype version of the RF control system software must be written to replace the r e l a t i v e l y crude test programs currently implemented. U n t i l the second prototype has been b u i l t and debugged using the RF systems test f a c i l i t y , i t w i l l not be possible to test the effectiveness of Durieu's compensation scheme for regulating the amplitude and phase of the accelerating voltage. In the meantime, however, i t would be useful to investigate methods for tuning the analog c o n t r o l l e r for optimum performance once i t has been i n s t a l l e d , as discussed in sect ion 4.1. 5.4.4 RF Detection Ultimately, the performance of the new radio frequency control system w i l l be limited by the accuracy of the measurement of the accelerating voltage amplitude and phase that i s presented to the analog c o n t r o l l e r . Measurement of the amplitude and phase of the accelerating voltage to the precision required for acceleration in separated turns (+ 80 ppm - 23 MHz amplitude, + 660 PP m ~ 69 MHz amplitude, + 0.12 degrees - 69 MHz phase) w i l l be d i f f i c u l t because of problems associated with mechanical v i b r a t i o n of the cantilevered hot arms and variations in the temperature of various components of the RF detection sub-system. The development of the RF detection sub-system i s not yet complete. Although working prototypes of the detectors 269 have been b u i l t and shown to function properly, many aspects of detector performance, including their precision and temperature s t a b i l i t y , have not yet been determined with accuracy. Suggestions for the next stage in the development of the new RF detectors were discussed in section 4.4. 5.5 CONCLUSIONS Successful demonstration of a flat-topped accelerating voltage in the test f a c i l i t y cavity suggests that i f the problems with p a r a s i t i c modes in the beam gap can be solved, then i t w i l l be possible to f l a t - t o p the accelerating voltage in the TRIUMF cyclotron. If the problem with the accelerating voltage amplitude and phase measurement can be resolved, i t should be possible to achieve acceleration in separated turns near the extraction radius. Development of a procedure for tuning the accelerating voltage control loops, similar to the Ziegler-Nichols technique but applicable to type 2 (double integral) c o n t r o l l e r s , should be investigated. References: [1] T. Enegren, L. Durieu, G. Dutto, D. Michelson, R. P o i r i e r , and R.E. Worsham. "Recent Prototype Studies and Measurements Toward Third Harmonic Flat-topping at TRIUMF." 1984 European Conference on Progress in Cyclotrons: Aachen, West Germany. (November 1984) [2] T. Enegren, L. Durieu, D. Michelson, and R.E. Worsham. "Development of Flat-topped RF Voltage for TRIUMF." IEEE Trans NS-32(5):2936-2938 (October 1985). 270 APPENDIX A THE TRIUMF MESON FACILITY - 1985 Sponsored by: University of Alberta University of B r i t i s h Columbia University of V i c t o r i a Simon Fraser University Address: Telephone: Telex: Director: TRIUMF i s Canada's national f a c i l i t y for intermediate energy nuclear physics. The concept of a cyclotron based meson factory originated with J.R. Richardson of UCLA in the early 1950's. A University of C a l i f o r n i a (UCLA) proposal [ l ] to build a pion f a c i l i t y based on an H-cyclotron was defeated in the early 1960's by a r i v a l proposal from Yale University to build the 800 MeV / 1 mA proton l i n e a r accelerator at Los Alamos. In 1966, a scaled down version of the UCLA proposal - a meson workshop c a l l e d TRIUMF [2] - was submitted to the Canadian Atomic Energy Control Board by four u n i v e r s i t i e s in western Canada. TRIUMF was funded for design and development in A p r i l 1968 by the Atomic Energy Control Board. Construction of the 520 MeV H- cyclotron began in July 1970. The f i r s t f u l l energy proton beam was extracted on 15 December 1974. This was 271 quickly followed by the f i r s t simultaneous extraction of two proton beams with d i f f e r e n t energies on 20 February 1975 [3]. F u l l intensity (100-microampere) proton beams were f i r s t extracted on a routine basis in 1978. The f i r s t f u l l year of the TRIUMF science program was 1982. A beam production summary i s presented in f i g . A.I. Much of the TRIUMF cyclotron's v e r s a t i l i t y , e s p e c i a l l y compared to her s i s t e r meson fac t o r i e s at Los Alamos and Zurich, comes from using the cyclotron to accelerate negative hydrogen ions rather than protons. Beam extraction by electronic stripping (conversion of H- to H+ by passing the beam through a p y r o l i t i c graphite stripping f o i l ) i s nearly 100 percent e f f i c i e n t and can be used to extract protons from a wide range of orbit r a d i i , hence beam energies. This is in marked contrast to the si t u a t i o n encountered when extraction of a proton beam i s i n i t i a t e d by e l e c t r o s t a t i c d e f l e c t i o n : a. extraction i s very i n e f f i c i e n t unless turns are well separated at the extraction radius; and, b. extraction can only be accomplished at a single radius. At TRIUMF, the energy of the proton beams delivered to the two major beamlines (BL 1 and BL 4) i s continuously variable between 180 and 520 MeV. This p a r t i c u l a r feature presents some unique opportunities for studies of nuclear 'structure at intermediate energies because a major change in beam energy can be accomplished in about two hours, a great 272 60 -o r 2 0 8 /xA mAh -300 • DATA AT SEPT. 20 / 1984 1975' 1977 h/yr h-6000 -4000 -2000 F i g . A.l TRIUMF - Summary of Beam Production (1975 - 1984) reduction over the time required with a t r a d i t i o n a l variable energy machine [4]. A minor beamline (BL 2C) i s used to transport low energy protons (less than 100 MeV) to isotope production targets. Several beams with very d i f f e r e n t energies and currents can be extracted simultaneously. Beam current s p l i t r a t i o s of greater than 100 000 to 1 can be r e l i a b l y supported during routine beam delivery. The average strength of the main magnetic f i e l d must increase r a d i a l l y to maintain isochronism because the ef f e c t i v e mass of the protons increase with energy. Normally, however, a r a d i a l l y increasing magnetic f i e l d v e r t i c a l l y defocuses the beam, allowing i t to st r i k e the vacuum tank and other structures. The azimuthal variations and the s p i r a l shape of the TRIUMF main magnet provide v e r t i c a l focusing to compensate for t h i s e f f e c t . In TRIUMF, at high energies, the s p i r a l angle focusing term is up to f i f t e e n times larger than the term contributed by the azimuthal or "Thomas" va r i a t i o n s . When the H- ion beam reaches r e l a t i v i s t i c v e l o c i t i e s , the loosely bound second electron can be removed from the H-ion by the e l e c t r i c f i e l d r e s u l t i n g from the Lorentz transformation of the main magnetic f i e l d 'into the rest frame of the ion. The neutral hydrogen atoms that result from electromagnetic stripping are not guided by the cyclotron magnetic f i e l d and are l o s t . At higher energies, p a r t i c u l a r l y between 450 and 500 MeV, t h i s mechanism ) 274 accounts for a large f r a c t i o n of the ion beam lost during the acceleration process. The neutral hydrogen atoms st r i k e the wall of the vacuum tank at high v e l o c i t y , making i t radioactive. Residual r a d i o a c t i v i t y in the cyclotron vacuum tank (5 - 10 rads/hour) complicates the problems of modification and maintenance. To l i m i t electromagnetic stripping and consequent a c t i v a t i o n of the cyclotron vacuum tank, the peak magnetic f i e l d strength i s made quite low. This, in turn, implies that the f i n a l orbit radius, and therefore the cyclotron RF cavity, w i l l be large. This has conferred upon TRIUMF the d i s t i n c t i o n of being the world's largest cyclotron but unfortunately i t s size has made i t s mechanical design, p a r t i c u l a r l y that of the RF cavity, extremely d i f f i c u l t . The experimental f a c i l i t i e s and beamlines are shown in f i g . A.2. The cyclotron currently feeds two major beamlines: a. Beamline 1 - Meson H a l l ; b. Beamline 4 - Proton H a l l ; and one minor beamline: c. Beamline 2C - Isotope Production; and a host of secondary beamlines fed by various production targets. Beamline 2A, which has not yet been constructed, w i l l be used to guide 450 MeV negative hydrogen ions from the cyclotron to the KAON Factory accumulator r i n g . The cyclotron i s the heart of the f a c i l i t y . An 275 -xl P R O T O N HALL E X T E N S I O N 4 2 M e V ISOTOPE P R O D U C T I O N C Y C L O T R O N SERVICE N A N N E X E X T E N S I O N I O N S O U R C E 3 f P O L A R I Z E D I O N SOURCE M E S O N H A L L SERVICE A N N E X F i g . A.2 TRIUMF - Beamlines and Experimental F a c i l i t i e s a r t i s t ' s conception of the cyclotron was shown in f i g . 1.3. The cyclotron i s shown in plan view in f i g . A.3, and in cross-section in f i g . A.4. A cross section of the cyclotron vault and service annex i s shown in f i g . A.5. Of p a r t i c u l a r interest i s the distance signals from beam diagnostic or RF probes must travel before reaching the primary service l e v e l ( l e v e l 264), the RF room, or the central control room. HISTORY AND STATUS Begin Design: July 1966 Begin Model Tests: December 1966 End Design: October 1968 Begin Construction: January 1970 F i r s t Beam: December 1974 Accelerator Cost: $12,000,000 (1974) F a c i l i t y Cost: $50,000,000 (1984) Funded by: Atomic Energy Control Board National Research Council of Canada TRIUMF U n i v e r s i t i e s ACCELERATOR STAFF, OPERATION AND DEVELOPMENT 15 S c i e n t i s t s 19 Engineers 55 Technicians 22 Craftsmen 2 Graduate Students (MASc) 17 Cyclotron Operators The cyclotron operates 24 hours per day, seven days per week during scheduled beam production. One day per week i s set aside for cyclotron and beam development or planned maintenance. Six days per week are devoted to beam production for experimental users. 277 F i g . A.3 TRIUMF 520 MeV Cyclotron - Plan View Key to Figurei 1 Beam Exit Port -to Beam line k and Proton Hall 2 Cryopanel 3 Magnet Sector Number 1 k Cyclotron Central Region - Centre Post - Spiral Inflector 5 RADIO FREQUENCY CAVITY Resonator Segment lj-1,6 6 Cyclotron Vacuum Tank and Access Ports 7 Stripping Fo i l and Probe The main magnet has six pairs of pole faces called sectors. They are numbered from 1 to 6 in clock-wise direction. The RF cavity is divided into 80 segments and 8 flux guides. They are identified by quadrant (num-bered from 1 to U in clock-wise direction) ( upper or lower, and by sequence (numoerea irom 1 to 10 outwards from the central re-gion) eg. kL6, item 5 above OPERATING MODE: CYCLOTRON CLOSED -MAINTENANCE MODE: CYCLOTRON OPEN-PRIMARY SUPPORT STRUCTURE SECONDARY SUPPORT BEAMS I I I I I meter ELEVATING JACKS I foot F i g . A ,k T R I U M F 520 M e V C y c l o t r o n - C r o s s - s e c t i o n a l V i e w IV) CO o ION S O U R C E C E N T R A L CONTROL ROOM R F S Y S T E M S S E R V I C E A N N E X B A S E M E N T . C Y C L O T R O N / V A U L T C Y C L O T R O N '15m F i g . A.5 Cyclotron Vault and Service Annex - Cross-sectional View Annual Budget for Operation and Development of Cyclotron and Experimental F a c i l i t i e s : $24,000,000 (1984) Funded by National Research Council of Canada RESEARCH STAFF Users: 90 in house 168 outside Graduate Students: 35 Annual Budget for Research A c t i v i t i e s : $3,351,875 (1984) Funded by Natural Sciences and Engineering Research Council CYCLOTRON MAIN MAGNET Diameter of Pole Face: 17.17 metres Weight Iron: 4000 tons C o i l s : 170 tons Cooling System: Closed loop active water - AlACW (Aluminum Active Cooling Water) Extraction Radius: 7.80 metres Number of Sectors: 6 Maximum S p i r a l Angle: 72° Trimming C o i l s : 55 c i r c u l a r - used to compensate for deviations from isochronism 13 harmonic - used to help suppress betatron o s c i l l a t i o n s Magnet C o i l Conductor: Aluminum 281 Power Requirements Main C o i l s : 1,270 kilowatts (maximum) Current S t a b i l i t y : 0.7 x 10 6 Trim C o i l s : 68 kilowatts (maximum) Current S t a b i l i t y : 0.1 % f u l l scale Magnet Parameters at 0.72 x 10^ Ampere-turns H i l l s : F i e l d Strength = 5.71 kilogauss (maximum) Magnet Gap = 0.53 metres Valleys: F i e l d Strength = 1.25 kilogauss (minimum) Magnet Gap = i n f i n i t e Average F i e l d at Extraction Radius: 4.6 kilogauss B max <B> 1 .26 The highest possible ion energy, 520 MeV, i s set by a bending l i m i t rather than a focusing l i m i t . ACCELERATION SYSTEM 2 dee system - 180 degrees apart Beam Aperture: 8 centimetres DC Bias: 0 k i l o v o l t s The RF system operates on the f i f t h harmonic of the ion rotation frequency: Range: 23.055 to 23.070 MHz _ g Frequency S t a b i l i t y : + 1 x 10 282 Maximum Dee Voltage: 85 k i l o v o l t s (fundamental) 0 (t h i r d harmonic) Minimum Gap: 2.5 centimetres (in central region) peak to peak noise Dee Voltage S t a b i l i t y : 4 x 10~ 4 = peak RF voltage Maximum Energy Gain per Turn: 340 keV Beam Phase S t a b i l i t y : + 2 degrees RF Power Requirement: 800 kilowatts (fundamental) RF Cavity Type: Room Temperature, Doubly Reentrant, Highly Flattened, Quarter-Wave Coaxial Cavity RF Cavity Quality Factor: 5800 (unloaded) at 23 MHz RF Cavity Dimensions (including Beam Gap): 16.5 metres x 6.5 metres x 0.3 metres A major upgrade to the cyclotron RF system i s currently being planned. The mechanical c h a r a c t e r i s t i c s of the dees w i l l be improved and provision for t h i r d harmonic f l a t -topping of the accelerating voltage w i l l be b u i l t into the RF cavity and the RF control system. VACUUM SYSTEM — 8 Operating Pressure: 5 x 1 0 Torr Pumps: 2 - Helium-cooled cryopanels (T = 20 K) Area: 1.2 square metres 2 8 3 4 -1 1 10 inch d i f f u s i o n pump 8 inch d i f f u s i o n pump 30 000 litres/second turbo pump ION SOURCES Ehlers PIG H-Lamb Shi f t polarized H-others under development INJECTION SYSTEM 40 metre long 300 keV beam l i n e with e l e c t r o s t a t i c bends and dipoles EXTRACTION SYSTEM Protons —> Electron stripping of H- in 25 um p y r o l i t i c graphite f o i l Negative Hydrogen Ions (proposed) —> E l e c t r o s t a t i c d e f l e c t i o n following excitation of coherent r a d i a l o s c i l l a t i o n to increase separation between f i n a l turns FACILITIES FOR RESEARCH Shielded Area, fixed: 2350 square metres 17 targets in 12 experimental stations 10 stations may be served at the same time Computing F a c i l i t i e s - not including cyclotron control systems: 1 - 48 megabyte Amdahl 5840 with double accelerator (UBC Computing Centre) 1 - VAX 8600 (clustered) 1 - VAX-11/780 1 - VAX-11/730 284 several VAX-11/750's and PDP-11/xx for data a c q u i s i t i o n and experiment control several assorted PC's, LSI-11/23, TRIMAC for single users Other Experimental F a c i l i t i e s : Polarized Fast Neutron Beam Thermal Neutron Source Biomedical Pion Ir r a d i a t i o n F a c i l i t y Isotope and Spallation F a c i l i t i e s PARTICLE Primary Beam H- -> p Polarized H--> Polarized p CHARACTERISTIC BEAMS ENERGY RANGE (MeV) MAXIMUM CURRENT (uA) 70 - 90 180 - 520 180 - 520 10 208 1 78% p o l a r i z a t i o n Secondary Beams Pions 15 - 300 10 - 10 BEAM PROPERTIES Pulse Width: 25 RF degrees @ 150 uA of 500 MeV H- ions Phase Excursion (maximum): 20 RF degrees @ 100 uA of 500 MeV H- ions Extraction E f f i c i e n c y : 99.95% @ 100 uA of 500 MeV H- ions Energy Resolution AE/E: 0.3% @ 100 uA of 500 MeV H- ions 285 0. 1: @ 8 uA of 500 MeV H- ions Emi ttance (7T mm mrad): 3.0 a x i a l 3.0 r a d i a l }@ 100 uA of 500 MeV H- ions OPERATING PROGRAMS Basic Nuclear Physics Biomedical Applications S o l i d State Physics Isotope Production High Intensity Unpolarized Beam - 70 % of production time Low Intensity Polarized Beam - 30 % of production time See also: Fig 1.3 520 MeV Cyclotron - A r t i s t ' s Conception Table II Internal Beam Char a c t e r i s t i c s and Cyclotron Design Parameters REFERENCES [1] [2] [3] [4] [5] "Pion F a c i l i t y - A High Energy Cyclotron for Negative Ions." UCLA Report (1964). E.W. Vogt and J . J . Burgerjon, ed. "TRIUMF Proposal and Cost Estimate." University of B r i t i s h Columbia (1966). J.R. Richardson. "The Status of TRIUMF." Proc 7th Int Conf on Cyclotrons and t h e i r Applications, pp 41-48 (1975). J.R. Richardson, E.W. Blackmore, G. Dutto, C.J. Kost, G.H. MacKenzie, and M.K. Craddock. "Production of Simultaneous Variable Energy Beams from the TRIUMF Cyclotron." IEEE Trans NS-22(3):1402-1407 (June 1975). J.M. Cameron, P. Kitching, and D.A. Hutcheon, ed. "Prospects for High Resolution Studies with A Proton Beam Between Ep = 200-500 MeV." TRIUMF Report TRI-80-2, pp 1-2 (October 1980). 286 D. A. Dohan, G.H. MacKenzie, G. Dutto "Cyclotron Data Sheets - Entry No. 8: TRIUMF." Proc 1 0 t h Int Conf on Cyclotrons and their Applications, p 634 (1984). E. W. Vogt. "Report to the NRC Committee on TRIUMF." (Ap r i l 1983). G.H. Mackenzie. "Studies for a TRIUMF Kaon Factory." INS KIKUCHI Winter School on Accelerators for Nuclear Physics, 29 Jan - 2 Feb 1984. R. Baartman, E.W. Blackmore, J. Carey, D. Dohan, G. Dutto, D. Gurd, R.E. Laxdal, G.H. Mackenzie, D. Pearce, R. P o i r i e r , and P.W. Schmor. "Status Report on the TRIUMF Cyclotron." Proc 1 0 t h Int Conf on Cyclotrons and their Appli cat i ons, pp 203-206 (1984). 287 APPENDIX B THE TRIUMF CYCLOTRON'S RADIO FREQUENCY SYSTEM B.l INTRODUCTION The TRIUMF cyclotron's radio frequency system was described in f i v e conference publications and two graduate theses between 1972 and 1975: Erdman et a l [1], Erdman et a l [2], Prochazka (PhD) [3], Brackhaus (PhD) [4], P o i r i e r and Zach [5], Gummer [6], and Gummer, P o i r i e r , and Zach [7]. The RF system consists of three basic units, as shown in f i g . B.1: a. the radio frequency cavity - the structure within which the fundamental and t h i r d harmonic accelerating voltages are developed; b. the two radio frequency power amplifiers - the fundamental power source, capable of de l i v e r i n g 1.8 megawatts at 23 MHz and the t h i r d harmonic power source, capable of d e l i v e r i n g 100 kilowatts at 69 MHz; and, c. the radio frequency control system - p r i n c i p a l l y a feedback regulation system (supervised by a f i n i t e state machine) which controls system start-up, cavity tuning mechanisms, and the r e l a t i v e phase and amplitude of the accelerating voltages. The exi s t i n g RF system can support only a fundamental ( i . e . sinusoidal) accelerating voltage. The entire RF 2 8 8 23.1 MHZ 69.3 MHZ 2 mW • RF MODULATORS AMPLITUDE G PHASE 2 mW CENTRAL CONTROL SYSTEM - interface to cyclotron operators - error logging RADIO FREQUENCY CONTROL SYSTEM - RF system start-up - regulation of accelerating voltage amplitude and phase by control of RF drive - control of RF cavity tuning 23 MHZ (B X 4CX250. OOOA) TUNING CONTROL 1.8 MW (MAXIMUM) RF POWER AMPLIFIERS 69 MHZ (1 X 4CX100. 000E) 2 mW 100 kW (MAXIMUM) PT DETECTION AMPLITUDE G PHASE RF" DETECTION CAVITY TUNING ERROR PHASE BETWEEN TRANSMISSION LINE AND RF CAVITY CURRENTS CYCLOTRON RADIO FREQUENCY CAVITYI 340 keV energy gain/turn \ DEE VOLTAGE PROBE F i g . B.l Components of TRIUMF's Radio Frequency System cavity and the RF control system must be substantially modified or replaced before a flat-topped accelerating voltage can be excited in the cyclotron in both a safe and r e l i a b l e manner (the f i r s t objective) and with s u f f i c i e n t s t a b i l i t y to permit separated turn acceleration and single turn extraction (the ultimate objective). The two RF power amplifiers are not expected to require substantial modification. Some minor changes might be needed, however, given that the 23 MHz transmitter, shown in f i g . B.2, w i l l be required to generate almost 40 per cent more power (1.2 Megawatts from 900 kilowatts) than i t does now and the 69 MHz transmitter, which consists of a 1.6 kilowatt common cathode driver followed by a 100 kilowatt 4CW100,000E common gr i d power amplifier, has not yet been used to drive a radio frequency cavity - i t has only been successfully tested using a water-cooled matched load. B.2 THE CYCLOTRON RADIO FREQUENCY CAVITY B.2.1 General Description The radio frequency cavity of a c l a s s i c a l cyclotron i s a foreshortened quarter-wave coaxial structure. A hollow, semi-circular accelerating electrode, c a l l e d the dee, forms the t i p of the inner conductor of the capacitively-loaded quarter-wave stub. The remainder of the inner conductor of the coaxial structure, c a l l e d the dee stem, extends beyond the magnet gap to the shorting plane and provides mechanical support for the dee. The s h i e l d of the coaxial structure i s referred to as the dee l i n e r . It is usually attached 290 F R E Q U E N C Y S Y N T H E S I Z E R I N P U T S E L E C T D R I V E R S T A G E S 2mW 20W 1 B 0 0 W 1 1 V D E E 2 8 f F 1 4 0 p F 1 2 0 k V i R F M O D U - —^  L A T O R 5 C X 1 5 0 0 A I P A 4 C W 1 0 0 . O O O E I N P U T P U L S E A R C S E N S I N G M A N U A L O N / O F F R E M O T E O N / O F F (dee voltage level sense) F E E D B A C K C O N T R O L A M P L I F I E R S R E F D A C 30kW E»— 3 0 k W P O W E R D I V I D E R A N D P H A S E R 30kW 3 0 k W A M P L I T U D E D E T E C T O R 4 C L A S S C G R O U N D E D G R I D P O W E R A M P L I F I E R S P A # 4 (2) 4 C W 2 5 0 . O O O A 9 0 0 k W P A # 3 (2) 4 C W 2 5 0 , O O O A PA #1 J2)_ _4CW25J)._000A | 2 0 k V 2 . 6 M W P O W E R S U P P L Y C R O W B A R 1.8 MW TO R F CAVITY S A F E T Y C U T O F F F i g . B.2 TRIUMF RF System - 23 MHz Power Source d i r e c t l y to the walls of the vacuum tank. Like the dee stem, the dee l i n e r i s almost always water cooled. The accelerating voltage i s developed between two dee structures which are excited in a push-pull mode. The cyclotron i l l u s t r a t e d in Rossi's patent ( f i g . 1.1) i s a t y p i c a l example of such a structure. Examples of various dee structures used in more modern cyclotrons, but based on similar p r i n c i p l e s , were presented by Riedel [8]. The f i r s t cyclotrons to employ ph y s i c a l l y large RF systems were the synchrocyclotrons b u i l t in the late 1940's. Although the c l a s s i c a l RF system described above was very successful in small cyclotrons, i t had some serious shortcomings when scaled upward in siz e . When the dees are large, i t i s d i f f i c u l t to guarantee the absence of harmful p a r a s i t i c modes in the cavity or severe non-uniformities in the dee voltage p r o f i l e . Such structures are very d i f f i c u l t to model numerically and one must resort to the use of expensive scale models that do not always f a i t h f u l l y mimic the behaviour of the real ca v i t y . The large dee structure may also be d i f f i c u l t to support mechanically. Problems with mechanical vibrations, p a r t i c u l a r l y those excited by the flow of cooling water in the dee, may be d i f f i c u l t to suppress. Erdman [9] reported that some cyclotron f a c i l i t i e s have begun to use insulators to provide some mechanical support for certain structures but, in general, insulators have always been considered to be far too f r a g i l e and unreliable for general use. Additional problems with 292 sparking, multipactoring phenomena (see also Appendix D), f a i l u r e of e l e c t r i c a l contacts that are required to support large RF currents, and d i f f i c u l t i e s in achieving the desired accelerating voltage amplitude and phase s t a b i l i t y frustrated early RF system designers. These problems continue to frustrate many contemporary designers: Now, as then, the only solutions for the majority of RF problems l i e in better mechanical engineering of the system, problems which conventionally-trained e l e c t r i c a l engineers are not usually well prepared to address at the l e v e l of expertise required. Problems associated with the design of RF systems for large cyclotrons were reviewed by Bieth [10] at a recent cyclotron conference. Conventional dee structures have only moderately high qu a l i t y factors. E x c i t a t i o n of a given accelerating voltage requires that a r e l a t i v e l y large amount of RF power be supplied to the ca v i t y . Conventional cyclotron dees have been replaced with high Q resonant c a v i t i e s at SIN [11] [12]. V i r t u a l l y a l l of the problems l i s t e d above were eliminated, giving SIN a very high energy gain per turn of 2000 keV with RF power requirements of about 1 megawatt di s t r i b u t e d among four fundamental RF c a v i t i e s . This compares quite favourably with TRIUMF's single RF cavity which requires almost 1 megawatt of power to support an energy gain per turn of only 340 keV. 293 Structures in which the accelerating electrodes are located in the magnet v a l l e y s , (e.g. SIN's RF c a v i t i e s ) rather than between the magnet poles (e.g. TRIUMF's RF c a v i t y ) , offer another advantage: they reduce the size of the magnet gap and hence the amount of power that must be supplied to the magnet. In the TRIUMF Proposal and Cost Estimate [13], i t was reported that two such structures were considered for TRIUMF but "the proposed magnet system does not lend i t s e l f to either, due to the narrow valleys and large s p i r a l angles ..." The basic design of TRIUMF's radio frequency cavity was suggested by K.R. MacKenzie of UCLA. It i s a doubly reentrant structure based on two quarter-wave stubs separated by a short gap. As such, i t resembles a conventional cyclotron dee except that the dee stem i s perpendicular to, rather than p a r a l l e l to, the dee gap. The constant dee to l i n e r spacing from the accelerating gap to the root distinguishes i t from a conventional dee, however. The cavity's i n t e r i o r i s l i n e d with copper. It has an i n t e r i o r surface area of 430 square metres which encloses a volume of 21.5 cubic metres. At 23 MHz, the cavity has an unloaded qu a l i t y factor of about 5800; t h i s r i s e s to approximately 10,000 at 69 MHz. The cyclotron's large size permits the RF cavity to be completely accommodated within the 16.5-metre diameter cyclotron vacuum tank. Unfortunately, the sheer size of the cavity and i t s placement between opposing magnet pole pieces have immensely 29k complicated the mechanical design of the cavity r e s u l t i n g in some persistent operational d i f f i c u l t i e s which must be corrected (see section B.2.2). The evolution cf TRIUMF's radio frequency cavity i s shown in f i g . B.3: The radio frequency cavity resembles two c a p a c i t i v e l y -loaded quarter-wave stubs mounted t i p to t i p . Such a cavity (a) i s described as being doubly reentrant. The accelerating gap i s the short gap region between the two inner conductors. The inner conductors are hollow to permit ions to pass through the accelerating f i e l d s . Such a cavity can be excited in either a push-push or push-pull mode. Only the push-pull mode i s useful for p a r t i c l e acceleration; The structure (b) i s then highly flattened so that i t w i l l f i t between the pole pieces of the cyclotron main magnet. The small volume of the cavity compared to i t s surface area gives the structure only a moderate quality factor; The cavity (c) is broken into 80 segments to permit easy i n s t a l l a t i o n and removal for repair or replacement. Forty segments are attached to the l i d of the vacuum tank and the remaining forty are mounted on -the floor of the vacuum tank. The ground panels (dee l i n e r ) , which form the outer conductor of the coaxial structure, are r i g i d l y attached to the wall of the vacuum tank. The hot arms (dee/dee stem), which form the inner conductor, are cantilevered panels supported by a structure referred to as 295 GUIDE F i g . B.3 E v o l u t i o n of the TRIUMF RF C a v i t y from a Quarter-Wave Stub a strongback. The t i p s of the hot arms are shaped to support the correct f i e l d shape in the short gap for v e r t i c a l focusing of the beam by the accelerating f i e l d , as depicted in f i g . 2.1; and, The r e s u l t i n g resonant cavity (d) i s mounted in the cyclotron vacuum tank. The volume occupied by the RF cavity structure, including the beam gap between the resonator hot 3 arms i s 0.3 m X 16.5 m X 6.5 m or 32.2 m , including the beam gap between the hot arms. The beam aperture i s approximately 0.1 m X 16.3 m. B.2.2 The RF Resonator Replacement Program Soon after the cyclotron was commissioned, i t became apparent that among the many minor problems [7] with the RF system that were quickly dealt with there were two serious problems that could not be as e a s i l y solved: The pneumatically actuated tuning f o i l s located in the root of each resonator segment began to f a i l and had to be removed from service. Although i t was suspected that the f a i l u r e was due to a f a u l t in the production of the material from which the tuning f o i l s were manufactured and not due to a basic flaw in the actual design, the defective f o i l s were not replaced. The RF cavity i s now tuned by varying the temperature and pressure of the resonator cooling water to allow small (less than 4 kHz) changes in the cavity's resonant frequency. This "temporary" solution has performed well, on the whole for over a decade but from time to time t h i s tuning mechanism has exhibited e r r a t i c behaviour 297 that has resulted in cyclotron down time; and, The alignment of the cantilevered panels that form the inner conductor of the RF cavity's coaxial structure (the "hot arms") i s more c r i t i c a l than was suspected during their design and development. When the panels are not well aligned, an RF voltage develops between upper and lower t i p s . This couples the push-pull accelerating mode in the RF cavity to p a r a s i t i c modes in the supposedly f i e l d - f r e e region (the "beam cavity") between and behind the cantilevered panels. The main str u c t u r a l supports for the "hot arms", referred to as strongbacks, are not water-cooled and have a tendency to droop when heated by RF power dissipated by these p a r a s i t i c modes. This, in turn, tends to worsen the misalignment. The problem has largely been controlled by very careful alignment of the hot arms but adjustment i s only possible from within the vacuum tank with the l i d of the vacuum tank raised. Through t r i a l and error, the hot arms have been s u f f i c i e n t l y well aligned to permit r e l i a b l e cyclotron operation but p a r a s i t i c modes in the beam cavity s t i l l i n terfere with the operation of beam diagnostic equipment such as the non-intercepting beam phase probes and the two low energy (LE) probes. To l i m i t damage to structures in the beam gap/cavity caused by power dissipated by the p a r a s i t i c modes, the RF system i s run with a dee voltage of just 85 k i l o v o l t s (fundamental only) rather than the 100 k i l o v o l t s o r i g i n a l l y planned. 298 Serious concern for the st r u c t u r a l inadequacies of the the cantilevered panels and their mechanical supports led to the decision to replace them by late 1987. The RF Resonator Replacement Program [14] i s concerned with design of new support structures for the panels, suppression of p a r a s i t i c modes in the beam gap, and the replacement of the existing tuning mechanism with a new one that has greater r e l i a b i l i t y and a greater tuning range than the ex i s t i n g mechanism. Such mechanical upgrade programs are not uncommon at accelerator f a c i l i t i e s . Sigg [15] described similar improvements to the SIN injector cyclotron RF system, including design and i n s t a l l a t i o n of new tuning mechanisms and dee voltage pickups that resulted in a major increase in both performance and r e l i a b i l i t y . Although the program at TRIUMF emphasizes the improvement of cyclotron r e l i a b i l i t y , i t has been recognized that attention paid to selected mechanical factors can s i g n i f i c a n t l y improve beam qu a l i t y and improve cyclotron performance. Spe c i f i c matters that have recently received increased attention include: a. lowering the amplitude of panel vibrations excited by the flow of cooling water; b. improving the dee voltage p r o f i l e by redesigning the geometry of the central region (number one) resonator segments and the flux guides - the structures joining the upper and lower number ten resonators to complete the coaxial structure of 299 the radio frequency cavity; and, c. incorporating features necessary to support t h i r d harmonic flat-topped operation of the radio frequency system. B.3 PROPOSED RF STRUCTURES FOR FLAT-TOPPING AT TRIUMF The replacement of a l l eighty resonator hot arms in 1986/87 w i l l be an expensive undertaking of great magnitude. It provides an ideal opportunity to make the changes to the radio frequency cavity that are necessary to permit t h i r d harmonic flat-topped operation which, as related in the f i r s t chapter, has become increasingly desirable at TRIUMF. Three methods for supporting a t h i r d harmonic accelerating mode in TRIUMF have been suggested: a. mounting a separate cavity in the vacuum tank behind the main radio frequency cavity, as described by Laxdal [16]; b. mounting a second dee within the exi s t i n g dee, as proposed for the Indiana University Cyclotron F a c i l i t y (IUCF) [17] and the Ganil heavy ion f a c i l i t y near Caen [18]; and, c. making the main RF cavity harmonically resonant so that i t w i l l support both the fundamental and t h i r d harmonic modes, as proposed by Erdman et a l [19], B.3.1 Separate Ca v i t i e s for Fundamental and Third Harmonic Modes The use of a p h y s i c a l l y separate cavity to support the t h i r d harmonic mode i s the scheme most often c i t e d in 300 proposals to f l a t - t o p cyclotrons and i s the only proven scheme to date. Its physical i s o l a t i o n from the fundamental radio frequency cavity gives i t some advantages over the other two schemes: a. The tuning mechanisms for fundamental and t h i r d harmonic are completely independent of each other; and, b. The fundamental and t h i r d harmonic power amplifiers are e l e c t r i c a l l y isolated from each other and special transmission l i n e f i l t e r s to iso l a t e them are therefore not required. Excitation of a separate flat-topping cavity in vacuum could present some problems, however: Because i t does not accelerate (or, perhaps more properly, decelerate) ions along the same accelerating gap as the fundamental does, Blosser [20] noted that "a phase dependent disturbance of the r a d i a l betatron o s c i l l a t i o n can be excited," as was encountered during a Michigan State University design study of a similar system. This p o s s i b i l i t y would have to be checked with o r b i t simulation studies before a decision to proceed with t h i s option was taken; The accelerating voltage supported by a t h i r d harmonic cavity may not be very much higher than the multipactoring threshold, as i s the case with the SIN flat-topping cavity. This could make i t very d i f f i c u l t to maintain an appropriate dee voltage, as was i n i t i a l l y the case at SIN. 301 The layout of the 590 MeV ring cyclotron at SIN i s shown in f i g . B.4. The sector magnets, fundamental RF c a v i t i e s , and the t h i r d harmonic flat-topping cavity are i d e n t i f i e d . A flat-topping cavity that was proposed for TRIUMF i s shown in f i g . B.5. TRIUMF i s not seriously considering the use of a separate cavity for flat-topping at the present time because the accelerating voltage would not be flat-topped in the central region and inner r a d i i of the cyclotron so only some of the advantages of flat-topping would be achieved. Also, space in the region behind the radio frequency cavity i s at a premium given the alte r n a t i v e extraction task force's plans to i n s t a l l magnetic channels, RF boosters, and RF deflectors in the same area [21]. B.3.2 Dee Within A Dee The dee within a dee, as depicted in f i g . B.6, was proposed by Rickey et a l [17] for second • harmonic f l a t -topping of the Indiana University 200 MeV cyclotron. It has most of the advantages of a separate cavity but few of the disadvantages, p a r t i c u l a r l y in the TRIUMF case, but i t introduces new problems of i t s own. It was b r i e f l y considered at TRIUMF but, as with IUCF, severe mechanical and e l e c t r i c a l problems were anticipated and the concept was abandoned. In p a r t i c u l a r , support and alignment of the inner dee i s not a t r i v i a l problem because of the mechanical complexity of such a large doubly cantilevered structure: a. a mechanism for adjusting the v e r t i c a l alignment of the cantilevered panel forming the inner dee 302 72 MeV LAYOUT OF THE 590 MeV RING CYCLOTRON AT SIN H g = gap T E | Q | RF CAVITY Key to Figure i 1 Fundamental RF Cavity (1 of 4) 2 Third Harmonic RF Cavity 3 Sector Magnet (1 of 8) Net Energy Gain per Turni 2 MeV F i g . B.4 The Flat-topping System of the 590 MeV Ring Cyclotron at SIN Cavity Dimensions I n s t a l l a t i o n i n Cyclotron (next to proposed f i f t h harmonic booster cavity) F i g . B.5 A Third Harmonic Flat-topping Cavity f o r the TRIUMF Cyclotron Pig. B.6 A Third Harmonic Dee within the Fundamental Dee would be very complex; b. the inner dee could not be e a s i l y observed from the cyclotron periscope during o p t i c a l surveys; c. insulators, which could be i n s t a l l e d to make the structure r i g i d and robust, are too f r a g i l e and unreliable to be used as str u c t u r a l components; The geometric complexity of the structure makes i t d i f f i c u l t to predict undesirable interaction between a mode in a primary cavity and p a r a s i t i c modes in the other two c a v i t i e s ; and, Multipactoring could s t i l l cause some d i f f i c u l t y at t h i r d harmonic, as i s the case for a phy s i c a l l y separate cavity . B.3.3 Harmonically Resonant Radio Frequency Cavity Excitation of the t h i r d harmonic accelerating mode in the same cavity as the fundamental was proposed for TRIUMF by Erdman et a l [19] in 1969 and has recently been investigated for use in other p a r t i c l e accelerators by Schriber [22] of Los Alamos National Laboratory and Hess, Schettman, and Smith [23] of Stanford University. The concept as realized in TRIUMF's RF cavity i s shown schematically in f i g . B.7. Note that the two modes are driven out of phase with respect to each other. This concept i s preferred by TRIUMF because: a. i t supports a flat-topped accelerating voltage along the entire accelerating gap; 306 F i g . B.7 The Fundamental and Third Harmonic Accelerating Modes i n TRIUMF's Radio Frequency Cavity b. i t makes e f f i c i e n t use of valuable space in the cyclotron; and, c. i t avoids the mechanical complexity of a supplementary dee within the primary dee. Harmonically resonant c a v i t i e s have several disadvantages, however: a. Tuning mechanisms for fundamental and t h i r d harmonic often cannot be made t o t a l l y independent; b. The fundamental and t h i r d harmonic power amplifiers may not be e l e c t r i c a l l y isolated from each other; and, in the case of the TRIUMF cyclotron, c. The t h i r d harmonic mode's voltage p r o f i l e along the accelerating gap in the main RF cavity i s very non-uniform; d. The t h i r d harmonic mode in the main RF cavity i s clo s e l y coupled to p a r a s i t i c modes in the beam cavity; and, e. Exciting the t h i r d harmonic mode in a 3X/4 cavity w i l l require three times as much power as exciting an equivalent X/4 cavity. Tests described in greater d e t a i l in chapter three suggest that the presence of the fundamental mode with s u f f i c i e n t amplitude w i l l e f f e c t i v e l y bias the cavity against multipactor discharges caused by the t h i r d harmonic mode. If thi s result i s also found to apply to the TRIUMF cavity, the t h i r d harmonic start-up procedure w i l l be 308 considerably simpler than the start-up procedure for the fundamental mode or for the t h i r d harmonic mode in a separate cavit y . B.4 RE -DES IGN OF THE RADIO FREQUENCY CAV ITY A 1:10 scale model of the cyclotron vacuum tank and radio frequency cavity was constructed in late 1982 for studies of the excitation (and suppression) of p a r a s i t i c modes in the beam cavity. I n i t i a l r esults of the investigation were reported by Susini et a l [24], P o i r i e r et a l [25], and Fong et a l [26]. Recent work using the model has shown that the geometry of the central region and flux guides play a major role in determining both the voltage p r o f i l e along the accelerating gap ( f i g . B.8) and the coupling of energy into p a r a s i t i c modes supported by the beam gap/cavity ( f i g . B.9). For the voltage p r o f i l e along the accelerating gap to be uniform, the energy in the accelerating modes must propagate only in the di r e c t i o n perpendicular to the dee gap. This implies that a l l longitudinal sections of RF cavity, as i l l u s t r a t e d in f i g . B.7, must be one-dimensionally or lo n g i t u d i n a l l y resonant at the same frequency. In the exis t i n g cyclotron RF cavity, the central region and flux guides are not lo n g i t u d i n a l l y resonant at the same frequency as the rest of the RF cavity. As a re s u l t , energy in the cavity propagates p a r a l l e l to, as well as perpendicular to, the dee gap. The voltage p r o f i l e shown 309 in f i g . B.8 shows the ef f e c t of the higher gap capacitance in the central region and lower gap capacitance in the flux guides: the dee voltage drops off towards the outer r a d i i . The voltage drop off at t h i r d harmonic i s e s p e c i a l l y pronounced and must be corrected. As discussed e a r l i e r , minimizing the coupling of energy from the fundamental mode in the RF cavity to p a r a s i t i c modes in the beam gap cavity has been one of the most d i f f i c u l t problems associated with RF system operation at TRIUMF. The basic geometry of the sit u a t i o n i s shown in f i g . B.9. Although misalignment of the hot arms has been found to be a major factor in determining the amount of energy coupled into the p a r a s i t i c modes, i t i s also been found that coupling can be s i g n i f i c a n t l y reduced by covering the s l o t s between resonator segments and the upper and lower flux guides. Typical p a r a s i t i c mode f i e l d d i s t r i b u t i o n s in one quadrant of the beam gap cavity and a voltage p r o f i l e of the mode, as measured at the point d i r e c t l y behind the root of the RF cavity, are shown in f i g . B.9. The power coupled into the p a r a s i t i c modes is not large - only a few thousand watts at most - but i t causes serious problems in a region where i t was not anticipated that any RF f i e l d s would e x i s t . At best, the p a r a s i t i c modes interfere greatly with beam diagnostic probes; at worst, they can cause severe s t r u c t u r a l damage to equipment in the beam gap and the resonator segments themselves. 310 Centre Post Accelerating Gap Flux Guide OkV Root F i g . B.8 Voltage P r o f i l e along the Cyclotron Accelerating Gap The LANL program SUPERFISH was used to generate t h i s p l o t of l i n e s of constant dee voltage i n one octant of the TRIUMF radio frequency cavity. The centre post, accelerating gap, and fl u x guides are c l e a r l y v i s i b l e . The SUPERFISH model, although crude , shows the drop i n dee voltage towards the extraction r a d i i . RF t CAVITY > ^ G R O U N D P A N E L ] t I I 1 I • ! • T 1 i t iii * © ! " 1' I 1 Y Y * t r . p t t t • ROOT j | |T BEAM GAP/CAVITY D E E G A P P a r a s i t i c Voltage Measurement Point P a r a s i t i c Mode Voltage with reBpect to maximum dee voltage dB TYPICAL PARASITIC MODE FIELD DISTRIBUTION One Quadrant TYPICAL PARASITIC MODE VOLTAGE PROFILE One Quadrant F i g . B . 9 Measurements of the F i e l d D i s t r i b u t i o n i n P a r a s i t i c Modes Excited i n the Cyclotron Beam Gap Although i t has been demonstrated that proper alignment of the cantilevered panels greatly l i m i t s the amount of power that i s coupled into the p a r a s i t i c modes in the beam cavity, i t would be desirable to have some sort of supplementary mechanism to help suppress the p a r a s i t i c modes. Various schemes have been investigated during the past three years. Attempts to s h i f t the frequency of the p a r a s i t i c modes have been largely unsuccessful at reducing the coupling, as have attempts to dampen the modes with lossy RF surfaces. Although i t has been shown that covering a l l the s l o t s between cantilevered panel segments and the upper and lower fluxguides w i l l s ubstantially reduce the coupling, the cyclotron i t s e l f has not yet been modified because studies have not yet been completed. An active suppression scheme resembling neutralization that was proposed by Worsham [27] is also currently being investigated. Unfortunately, results obtained from model studies have not always been found to hold when applied to the cyclotron, p a r t i c u l a r l y with respect to p a r a s i t i c mode studies at the fundamental frequency. Studies of the t h i r d harmonic mode and i t s p a r a s i t i c s have also proven to be more challenging than expected. B.5 SUMMARY O F T H E T E C H N I C A L O B J E C T I V E S O F T H E R A D I O FREQUENCY S Y S T E M U P G R A D E B.5 . 1 R a d i o F r e q u e n c y C a v i t y The redesign of the radio frequency cavity has, thus far, been p r i n c i p a l l y concerned with improvement of the mechanical s t a b i l i t y and alignment of the cantilevered 313 panels [28] to reduce cantilevered panel vibrati o n and to reduce the power coupled to p a r a s i t i c modes in the beam cav i t y . The central region, flux guides, and cavity tuning mechanisms w i l l be redesigned to permit automatic retuning of the RF cavity to the fundamental and t h i r d harmonic drive frequencies simultaneously over a sp e c i f i e d range and to keep variations and asymmetries in the accelerating voltage p r o f i l e below s p e c i f i e d l i m i t s . F i n a l l y , cavity coupling loops, transmission l i n e f i l t e r s , and matching sections for coupling fundamental and t h i r d harmonic radio frequency power into the cavity must be designed, constructed, and i n s t a l l e d . Design of the new RF cavity components w i l l r e f l e c t the special problems associated with i n s t a l l a t i o n and alignment of hardware in the cyclotron vacuum tank, p r i n c i p a l l y the need to r e s t r i c t the exposure of technicians to the highly radioactive cyclotron i n t e r i o r . It i s planned to remotely handle and i n s t a l l hardware wherever possible. B.5.2 Radio Frequency C o n t r o l System It has been decided to replace the exis t i n g RF control system. The design of the new control system centers around four p r i n c i p a l issues: The new RF control system w i l l have provision for c o n t r o l l i n g the t h i r d harmonic RF system was well as the fundamental RF system; 314 The new RF control system w i l l have a more open architecture and be more a c c e s s i b i l e for experimentation, development, and expansion than the exi s t i n g system. The control program that manages the system start-up procedure w i l l be implemented in software rather than hardware to be more f l e x i b l e ; The major improvement in accelerating voltage s t a b i l i t y that i s necessary for single turn extraction requires very precise measurement of the amplitude and phase of the dee voltage. Measurement of the fundamental mode amplitude to within 80 parts per m i l l i o n and the r e l a t i v e phase between the fundamental and t h i r d harmonic modes to within 5 picoseconds (0.12 degrees at 69 MHz) w i l l be d i f f i c u l t due to problems with temperature variations in components of the detector system; and, Two major disturbances, hot arm (dee) vibration and pulsing of the beam at 1 kHz, l i m i t the s t a b i l i t y of the accelerating voltage in the radio frequency system's present configuration. Ideally, these disturbances w i l l be greatly reduced at their sources as part of the radio frequency system upgrade, but i t w i l l probably be necessary to suppress their e f f e c t s on the accelerating voltage by increasing the gain and bandwidth of the feedback compensation amplifiers. The exi s t i n g compensation network does not adequately suppress these disturbances and must be replaced. 3 1 5 R e f e r e n c e s : [I] K.L. Erdman, R. P o i r i e r , O.K. Fredriksson, J.F. Weldon, W.A. Grundman. "TRIUMF Amplifier and Resonator System." Proc 6th Int Cyclotron Conf, pp 451-458 (1972). [2] K.L. Erdman, K.H. Brackhaus, R.H.M. Gummer. "Some Aspects of the Control and S t a b i l i z a t i o n of the RF Accelerating Voltage in the TRIUMF Cyclotron". Proc 6th Int Cyclotron Conf, pp 444-450 (1972). [3] A. Prochazka. "The Design of the RF System for the TRIUMF Cyclotron." PhD Dissertation, University of B r i t i s h Columbia (1972). [4] K.H. Brackhaus. "The Generation and Control of 1.5 Megawatts of RF Power for the TRIUMF Cyclotron." PhD Dissertation, University of B r i t i s h Columbia (1975). [5] R. P o i r i e r and M. Zach. "The TRIUMF RF System." IEEE Trans NS-22(3):1253-1256 (1975). [6] R.H.M. Gummer. "Accelerating Voltage Control and S t a b i l i z a t i o n in the TRIUMF Cyclotron". IEEE Trans NS-22 ( 3 ): 1 257- 1 260 (1975). [7] R.H.M. Gummer, R.L. P o i r i e r , and M. Zach. "TRIUMF RF System - I n i t i a l Operating Problems and Their Solutions." Proc 7th Int Conf on Cyclotrons and their Applications, pp 167-170 (1975). [8] J . Riedel. "R.F. Systems." IEEE Trans NS-26(2):2133-2136 (April 1979). [9] K.L. Erdman. "Special Aspects of Cyclotron RF Systems." Proc 7th Int Conf on Cyclotrons and their Applications, pp 141-145 (1975). [10] C. Bieth. " C r i t i c a l Features of RF Systems for Large Cyclotrons." Proc 10th Int Conf on Cyclotrons and their Appli cat i ons, pp 294-298 (1984). [II] B. Bischof. "The RF-System of the Flattop-Acceleration Structure in the SIN 590-MeV-Ring-Cyclotron." IEEE Trans NS-26(2):2186-2189 ( A p r i l 1979). [12] S. Adam, W. Joho, P. Lanz, H. Leber, N. Schmid, U. Schryber. " F i r s t Operation of a Flat-top Accelerating System in an Isochronous Cyclotron." IEEE Trans NS-28(3) :272l-2723 (June 1981). [13] E.W. Vogt and J . J . Burgerjon, ed. "TRIUMF Proposal and Cost Estimate." University of B r i t i s h Columbia (1966). 316 [14] TRIUMF F a c i l i t y Development Plan. "RF Resonator Replacement Program." 12 July 1984. [15] P.K. Sigg. "Improvements in the SIN-Injector RF System." Nucl Inst Met h 155:1-10 (1978). [16] R.E. Laxdal. "Separate 3rd Harmonic Flattopping Cavity." TRIUMF Memorandum dated 15 August 1984. [17] M.E. Rickey, M.B. Sampson, and B.M. Bardin. "General Design Features of the Indiana University 200 MeV Cyclotron." IEEE Trans NS-16(3):397-404 (December 1968). [18] C. Bieth, A. Joubert, G. Rastoix, and J. Riedel. "The Ganil Accelerating System." Proc 7th Int Conf on Cyclotrons and their Applications, pp 163-166 (1975). [19] K.L. Erdman, A. Prochazka, O.K. Fredriksson, R. Thomas, and W.A. Grundman. "A Square-Wave RF System Design for TRIUMF." Proc 5th Int Cyclotron Conf, pp 105-110 (1969). [20] H.G. Blosser. "Future Cyclotrons." Proc 6th Int Cyclotron Conf, pp 16-32 (1972). [21] M. Zach, G. Dutto, R.E. Laxdal, G.H. Mackenzie, J.R. Richardson, R. T r e l l e , R.E. Worsham. "The H- High Intensity Beam Extraction System for TRIUMF." IEEE Trans NS-32(5):3042-3044 (October 1985). [22] S.O. Schriber. "Room Temperature Cavities for High-Beta Accelerating Structures." Proc Conf on the Future Possibilities for Electron Accel er at ors, C h a r l o t t e s v i l l e , VA (1979). [23] C.E. Hess, H.A. Schwettman, and T.I. Smith. "Harmonically Resonant Cavities for High Brightness Beams." IEEE Trans NS-32(5):2924-2926 (October 1985). [24] A. Susini, D. Dohan, T. Enegren, K. Fong, V. Pacak, R. P o i r i e r . "An Investigation of RF Problems at TRIUMF." TRIUMF Design Note TRI-DN-82-27 (December 1982). [25] R. P o i r i e r , D. Dohan, G. Dutto, T.A. Enegren, K. Fong, and V. Pacak. "Determination of the RF Leakage F i e l d in the Vacuum Tank of the TRIUMF Cyclotron." IEEE Trans NS-30U):351 4-351 6 (August 1983). [26] K. Fong, D.A. Dohan, V. Pacak, R. Hutcheon. "Model Study on the Reduction of RF Leakage in the TRIUMF Cyclotron." IEEE Trans NS-32(5):2939-2942 (October 1985). 3 1 7 [27] R.E. Worsham, TRIUMF. Private comunication (1985). [28] G. Stanford, R. Worsham, K. Fong, and S. Hutton. "A New and Improved RF Resonator Segment for the TRIUMF Cyclotron." IEEE Trans NS-32(5):2942-2944 (October 1985). 318 APPENDIX C CALCULATION AND MEASUREMENT OF THE PROPERTIES OF ACCELERATOR RF CAVITIES C l INTRODUCTION This appendix serves as a brief survey of the tools used by R F engineers to calculate and measure the properties of accelerator R F c a v i t i e s . Problems associated with the mechanical construction of c a v i t i e s and d r i v i n g them with high power while under vacuum are discussed in Appendix D. C.2 DESIGN OF THE RF CAVITY A n a l y t i c a l Methods The properties of the normal modes in electromagnetic resonant c a v i t i e s have been studied extensively since the Second World War. A n a l y t i c a l methods in R F cavity design are well known and have been described by Slater [1], Harrington [2], Johnson [3], and many others. Most accelerator R F c a v i t i e s are derived from the c l a s s i c a l "short-gap" coaxial cavity as described by Mavrogenes and Gallagher [4]. There are obvious exceptions, however, such as the loaded c i r c u l a r waveguide used in the Stanford Linear Accelerator and the T E 1 Q 1 R F c a v i t i e s used in the 5 9 0 MeV ring cyclotron at SIN. Computer Aided Design The l a s t twenty years have seen rapid development of 319 computer aided design tools to a s s i s t in the design of accelerator RF c a v i t i e s . MESSYMESH (see ref 1 in [13]), announced in 1961, and LALA [5], announced in 1966, were among the f i r s t of the so-called "Helmholtz solvers" used to calculate the resonant frequency and f i e l d d i s t r i b u t i o n of axisymmetric RF c a v i t i e s with s i m p l i f i e d geometries. SUPERFISH [6], announced in 1976, i s perhaps the best known and the most popular Helmholtz solver used in accelerator RF engineering. SUPERFISH uses f i n i t e element methods to analyze axisymmetric RF c a v i t i e s or guided wave structures with constant cross-section. It calculates the frequencies of the dominant and some higher order modes, RF e l e c t r i c and magnetic f i e l d d i s t r i b u t i o n , and the qu a l i t y factor and shunt impedance of the cavity. Other programs of a similar nature have been described in the l i t e r a t u r e but are not as widely used. A number of enhancements and post-processors have been developed for SUPERFISH during the past ten years [7] -[11] including ULTRAFISH [8], a generalization of SUPERFISH that solves for resonant modes with azimuthal va r i a t i o n s , and a post processor for SUPERFISH c a l l e d MARC [10] that calculates the temperature d i s t r i b u t i o n and dimensional changes in linac RF structures. Both steady state and transient behaviour can be examined with the computer model. Despite their wide acceptance, SUPERFISH and similar programs sometimes discourage users because they f a i l to converge or to give correct answers and because the 320 documentation that accompanies them i s often incomplete. Nevertheless, in the hands of experienced users they can y i e l d useful results and reduce the need for the construction of r e l a t i v e l y expensive prototypes and scale models. Weiland [13] recently reviewed the current state of computer modelling of two- and three-dimensional c a v i t i e s . He noted that although the modelling of two-dimensional and c y l i n d r i c a l l y symmetric structures has achieved a certain maturity in recent years, three dimensional codes are s t i l l in t heir infancy and are not yet r e l i a b l e enough for routine use by non-experts. Great progress i s being made; Weilland expects that "provided that computer size and speed continues to increase, these 3D-codes w i l l also be a common tool soon." Experimental Methods It i s often necessary to experimentally determine the f i e l d d i s t r i b u t i o n in an RF. cavity, e s p e c i a l l y i f the cavity has an unusual or complex geometry. A loop or probe can be used to sample the f i e l d near the walls of the cavity. If the f i e l d does not vary in the di r e c t i o n perpendicular to the cavity wall, as in the case of both the accelerating and the p a r a s i t i c modes in TRIUMF (see Appendix B), then such loops and probes can give an accurate picture of the f i e l d d i s t r i b u t i o n in the cavity. In c a v i t i e s with complex geometries, the f i e l d d i s t r i b u t i o n can be mapped by measuring the s h i f t in the cavity's resonant frequency as a 3 2 1 d i e l e c t r i c bead i s pulled through i t , as described by Maier and Slater [14], The a v a i l a b i l i t y of inexpensive microcomputers has made i t possible to automate the technique at reasonable cost. Automated bead p u l l e r s have recently been described by Hepburn and Michel [15] and Bernier, Sphicopoulos, and Gardiol [16]. C.3 CALCULATION AND MEASUREMENT OF THE PROPERTIES OF THE CAVITY COUPLING NETWORK When designing the RF coupling network, the r e l a t i v e l y complex f i e l d theory description of the cavity can be replaced by an equivalent c i r c u i t model, as described by Beringer [17]. The theory of cavity coupling mechanisms as presented by Harrington [2] and Johnson [3] i s well known; more p r a c t i c a l considerations s p e c i f i c to accelerator RF c a v i t i e s were recently presented by Botha and Van der Merwe [18]. Experimental techniques for measuring the parameters of the equivalent c i r c u i t of the cavity and i t s coupling mechanisms were described by Ginzton [19] in the late 1950's. In recent years the RF network analyzer [20] [21] has replaced the slo t t e d l i n e used by Ginzton and has made the measurement of microwave networks much faster and more accurate than ever before. The measurement techniques employed when using network analyzers to characterize general networks are well known [22], Kajfez and Hwan [23] recently described an updated version of Ginzton's method for measuring the q u a l i t y and coupling factors of coupled 322 c a v i t i e s using a network analyzer. There has been much interest in developing error models and c a l i b r a t i o n procedures to make network analyzer measurements as accurate as possible. Representative work has been described by F i t z p a t r i c k [24] [25]. References: [ 1 ] J.C. Slater. "Microwave E l e c t r o n i c s . " Princeton, NJ: D. Van Nostrand Co. Inc. (1950). [2] R.F. Harrington. "Time-Harmonic Electromagnetic F i e l d s . " New York: McGraw-Hill Book Co., pp 381-446 (1961). [3] C C . Johnson. " F i e l d and Wave Electrodynamics." New York: McGraw-Hill Book Co., pp 213-243 (1965). [4] G. Mavrogenes and W.J. Gallagher. "Coaxial Cavities with Beam Interaction." IEEE Trans NS-32(5):2778-2780 (October 1985). [5] H.C. Hoyt, D.D. Simmonds, and W.F. Rich. "Computer Designed 805 MHz Proton Linac C a v i t i e s . " Rev Sci Instr 37(6):755-762 (June 1966). [6] K. Halbach and R.F. Holsinger. "Superfish - A Computer Program for Evaluation of RF Cavities with C y l i n d r i c a l Symmetry." P a r t i c l e A c c e l e r a t o r s 7:213-222 (July 1976). [7] D.W. Reid, A. Harvey, G.W. Rodenz, and R.F. Holsinger. "Superfish." Proc 1981 L i n a c Conf, pp 99-102 (1981). [8] R.L. Gluckstern, R.F. Holsinger, K. Halbach, G.N. Minerbo. " U l t r a f i s h - Generalization of Superfish to m > 1." Proc 1981 Linac Conf, pp 102-107 (1981). [9] S.O. Schriber and R.F. Holsinger. "Additions and Improvements to the RF Cavity Code Superfish." Proc 1983 Linac Conf (1983). [10] J . McKeown and J.-P. Labrie. "Heat Transfer, Thermal Stress Analysis, and the Dynamic Behaviour of High Power RF Structures." Proc 1983 L i n a c Conf (1983). 323 [11] "Poisson Group Programs User's Guide." Los Alamos National Laboratory (14 Feb 1981). [12] J.L. Warren, G.P. Boicourt, M.T. Menzel, G.W. Rodenz, and M.C. Vasquez. "Revision of and Documentation for the Standard Version of the Poisson Group Codes." IEEE Trans NS-32(5):2870-2872 (October 1985). [13] T. Weiland, "Computer Modelling of Two- and Three-Dimensional C a v i t i e s . " IEEE Trans NS-32(5):2738-2742 (October 1985). [14] L.C. Maier and J.C. Slater. "Determination of F i e l d Strength in a Linear Accelerator Cavity." J Appl Phys 23:78-83 (January 1952). [15] J.D. Hepburn and W.L. Michel. "A Microprocessor-Controlled Bead Puller For RF Cavity Measurements." Proc 1981 Linac Conf, pp 56-58 (1981). [16] L.G. Bernier, T. Sphicopoulos, and F.E. Gardiol. "An Automatic System for the Measurement of the F i e l d D i s t r i b u t i o n in Resonant C a v i t i e s . " IEEE Trans IM-32:462-466 (1983). [17] R. Beringer. "Resonant Cavities as Microwave C i r c u i t Elements," in P r i n c i p l e s of Microwave C i r c u i t s , C.G. Montgomery, R.H. Dicke, and E.M. P u r c e l l , ed. New York: McGraw-Hill Book Co. pp 207-239 (1948). (October 1985). [18] A.H. Botha and F.S. Van der Merwe. "Design of C i r c u i t s for Coupling Power Amplifiers to Resonators." IEEE Trans NS-26(3):3962-3964 (1979). [19] E.L. Ginzton. "Microwave Measurements." New York: McGraw-Hill Book Co. Inc., pp 391-461 (1957). [20] M.A. Maury, J r . "Automated Network Analyzer Microwave Measurements Past, Present & Future." Microwave J 25(4):l8-28 (April 1982). [21] J . F i t z p a t r i c k . "A History of Automatic Microwave Network Analyzers." Microwave J 25(4):43-56 ( A p r i l 1982). [22] "HP 8505A RF Network Analyzer Basic Measurements." Hewlett Packard Application Note 219. Palo Alto: Hewlett-Packard Co. (1978). [23] D. Kajfez and E.J. Hwan. "Q-Factor Measurement with Network Analyzer." IEEE Trans MTT-32(7):666-670 (July 1984). 324 [24] J. F i t z p a t r i c k . "Error Models for Systems Measurement." Microwave J 21(5):63"66 (May 1978). [25] J. F i t z p a t r i c k . "Automatic Network Analyzer Accuracy: How to Get I t , Lose I t , Then Regain I t . " Microwave Systems News (5):77-93 (May 1980). \ 325 APPENDIX D OBSTACLES TO SUCCESSFUL EXCITATION OF ACCELERATOR RF CAVITIES "The 37 inch cyclotron which ran just before the 2nd World War was the f i r s t machine with a f a i r l y large r . f . structure. I n i t i a l l y the engineering was done by people with expertise in high power broadcast transmitters, employing the standard methods used to feed antennas. However, i t soon became apparent that a high energy storage, high Q dee which frequently sparked did not look l i k e an antenna load, and d i f f e r e n t methods had to be found to drive i t . So, very quickly a nuclear p h y s i c i s t (K. Mackenzie) had to learn how to be a cyclotron r . f . engineer." [1] D.l INTRODUCTION It has been recognized for some time that the unusual load c h a r a c t e r i s t i c s of an evacuated resonant cavity complicate the design and operation of accelerator radio frequency systems. Several phenomena that a l t e r the q u a l i t y factor and resonant frequency of an accelerator cavity during RF system operation are described in t h i s appendix. Other problems that a f f e c t RF system performance, such as mechanical vibration of the accelerating electrodes [2] [3], are not discussed here although they are ultimately very important considerations, e s p e c i a l l y in the case of TRIUMF. Problems Associated With Source/Load Mismatch Normally, every e f f o r t i s made to ensure that the input impedance of the accelerating cavity i s matched to output impedance of the transmitter and transmission l i n e 326 that drive i t . In practice, however, an accelerating cavity often presents a highly variable load to the transmitter. Before discussing the causes of such var i a t i o n s , the problems that are encountered when a high power transmitter i s driven into a poorly matched load w i l l be described. They include: a. the transmitter may not be able to supply enough power to maintain the desired accelerating voltage i f the cavity's resonant frequency d r i f t s too far from the driv i n g frequency; b. the f i n a l tetrode or triode could be damaged by dis s i p a t i o n of excessive power r e f l e c t e d back from the load [ 4 ] ; c. breakdown and sparking could occur: i . at the peaks of the voltage standing wave in the transmission l i n e ; or, i i . in one of the several capacitors in the anode c i r c u i t ; d. the phase and amplitude response of the transmitter may be severely affected by the change in the load [5]. Provided s u f f i c i e n t fault protection i s b u i l t into the transmitter, the limited output c a p a b i l i t y of the transmitter i s of far greater concern than the p o s s i b i l i t y of damage to the tube when attempting to drive an accelerating cavity that hasn't been tuned to the driving frequency. At best, the system w i l l be unable to excite the 327 desired accelerating p o t e n t i a l ; at worst, the transmitter w i l l be overdriven and w i l l be shut down by the appropriate safety interlock or c i r c u i t breaker. Although power g r i d tubes are designed to stand considerable abuse, excess anode d i s s i p a t i o n w i l l damage the tube i f i t occurs long enough to overheat the envelope and seal structure. The control and screen grids in power grid tubes are less rugged than the anode, however, and must be protected by appropriate hardware interl o c k s . The maximum power that each gri d can dissipate i s indicated on the tube's data sheet and should not be exceeded for time inter v a l s greater than one second [4], Sparking, the other problem p a r t i c u l a r l y associated with high lev e l s of ref l e c t e d power, can generally be avoided by selection of appropriately rated components and transmission l i n e s . The accelerating cavity presents a highly variable load to a d r i v i n g amplifier because the cavity i s a resonant structure with a f a i r l y narrow bandwidth. Its input impedance, Z.n, i s given by: R o : (D . D CO coft ' + i n , <«„ " «?> using the standard Foster model of a loop coupled cavity as described in chapter three. The input impedance of the cavity i s a function of: 328 a. the r a t i o of the cavity's resonant frequency to the frequency of the signal d r i v i n g i t ; and, b. the cavity loading, which determines the shunt resistance and qual i t y factor of the cavity. Many factors can cause changes in the accelerating cavity's geometry and loading and thereby change i t s resonant frequency, i t s qu a l i t y factor, and i t s input impedance, including: a. thermal expansion and contraction of the cavity structure, which w i l l greatly a f f e c t the cavity's resonant frequency; b. e l e c t r i c a l discharge phenomena such as sparking which w i l l lower the cavity's q u a l i t y factor and which may also s l i g h t l y detune the cavity; and, c. external loading of the cavity, which in the case of an accelerating cavity i s mostly due to the energy passed to, or taken from, the ion beam being accelerated. These problems occur, to varying degrees, in a l l large and mechanically complex c a v i t i e s that are excited with high power for use in s c i e n t i f i c or i n d u s t r i a l applications. There i s , however, one p a r t i c u l a r l y d i f f i c u l t problem that i s peculiar to radio frequency c a v i t i e s operating in vacuum: d. multipactoring, referred to as "ion-lock" in some early reports, a mechanism by which gases at very low pressure break down when subjected to high 329 frequency e l e c t r i c f i e l d s of much smaller magnitude than are normally associated with sparking and more conventional discharge phenomena. In a radio frequency cavity which i s under vacuum, multipactoring sets a lower l i m i t to: a. the accelerating voltage that can be r e l i a b l y maintained; and, b. the rate at which the accelerating voltage must increase while i t i s passing through the multipactoring region i f no special multipactoring suppression mechanisms are b u i l t into the cavity . Once the c h a r a c t e r i s t i c s of multipactoring in a p a r t i c u l a r cavity are understood, i t can usually be: a. avoided with special operating procedures, e.g. pulsing the accelerating voltage through the multipactoring region to a sustainable l e v e l ; or, i f necessary, b. suppressed at certain power l e v e l s by modifying either the cavity's geometry or i t s surfaces. In severe cases, however, such as when multipactoring problems were encountered during the development of the SIN ring cyclotron's t h i r d harmonic flat-topping cavity [6], multipactoring can be a stubborn obstacle to successful operation. 330 D.2 EXCITATION OF AN ACCELERATOR RF CAVITY IN AIR Excit i n g a t y p i c a l accelerator RF cavity with low power le v e l s in a i r demands application of only the most elementary concepts of cavity tuning, coupling and load matching. Driving the accelerating potential to near the maximum value allowed by the breakdown strength of a i r (approximately 30 k i l o v o l t s per centimetre) i s e a s i l y accomplished provided that consideration i s given to: a. the change in the resonant frequency of the cavity as RF surface currents heat the cavity and cause i t to expand; and, b. preventing damage to the RF cavity and the RF power amplifier in the event a spark develops in the RF cavity or in the transmission l i n e feeding i t . Temperature Effects The most important consideration in the mechanical design of an accelerating cavity i s heat evacuation. In the absence of a cooling system, an RF cavity under vacuum can only lose heat through radiation because: a. convection cooling cannot occur; and, b. in a vacuum, there i s v i r t u a l l y no conduction of heat between two surfaces in contact i f they are of ordinary roughness. 331 Because heat evacuation by radiation i s very i n e f f i c i e n t , a cooling system must be incorporated into the resonator design. Water i s normally used as the working f l u i d . To evacuate W watts of RF power, F litres/second of water flow i s required, given a desired temperature d i f f e r e n t i a l between the output and input of A T Celcius: For example, to maintain a fiv e degree difference between the RF systems test f a c i l i t y cavity cooling water i n l e t and return while the cavity i s being excited at f u l l power (40 kilowatts), almost 120 l i t r e s of cooling water must pass through the cavity every minute. In the cyclotron, the cooling water requirement jumps to almost 4600 litres/minute due to i t s greater size and greater power d i s s i p a t i o n . As the RF drive power increases, the temperature of the cavity w i l l increase. This w i l l generally lower the cavity's resonant frequency. A homogeneous cavity made of one kind of metal w i l l have a thermal tuning c o e f f i c i e n t proportional to the linear c o e f f i c i e n t of expansion of the metal because the resonant frequency f of the cavity i s approximately inversely proportional to the linear dimensions of the cavity [7], F = W AT/ 4186 (D.2) or, A T = W F/ 4186 (D.3) X 0 oc 1 (D.4) dX 0/dT = dl/dT = a (D.5) df/f - a f dT (D.6) 332 In a re-entrant cavity, the change in gap capacitance that accompanies the thermal expansion of the central conductor dominates the thermal tuning c o e f f i c i e n t i f the outer conductor i s r i g i d l y attached to an external structure such as the cyclotron vacuum tank. A C g a p / C g a p ~ A l / d ( D ' 7 ) where: C g a p * s t* i e capacitance of the short gap 1 i s the length of the inner conductor d i s the length of the gap The resonant frequency of the cavity i s an eigenvalue of the transcendental equation: Z^  tan (col/c) = 1/coC- ^ (D.8) where: co i s the resonant frequency ZQ i s the c h a r a c t e r i s t i c impedance of the dee stem After c a l c u l a t i n g the t o t a l d i f f e r e n t i a l and making some substitutions, i t has been shown [8] that as a f i r s t order approximation, A f / f - A l/1 (D.9) A f / f - a AT (D. 10) Many accelerator RF c a v i t i e s , including the NSLS accelerating c a v i t i e s at Brookhaven [9] and the Tevatron accelerating c a v i t i e s at Fermilab [10], are automatically tuned (at least in part) by varying the temperature of the cavity cooling water. 333 TABLE VIII Linear C o e f f i c i e n t of Expansion of Various Metals Metal Linear C o e f f i c i e n t of Expansion,a (per degree Celcius at 20 C) Copper 16.7 x 10 6 Mild Steel 10.5 - 11.6 x 10 6 I nvar 0.9 x 10 6 Aluminum 25.5 x 10 6 Sparking K i l p a t r i c k [11] has described a spark as: "an occurrence in time at which there is a spontaneous, abrupt, and complete (to f i r s t order) d i s s i p a t i o n of e l e c t r i c a l l y stored energy for a given voltage across a gap between metal electrodes." Although a spark might quench i t s e l f almost immediately, i t can develop extremely rapidly into an arc, fed by copious emission of electrons from the cathode metal. If the current flow in the arc i s s u f f i c i e n t l y high, l o c a l heating of the electrodes in the v i c i n i t y of the arc can result in vaporization of metal from the electrodes and damage to the structure. A spark can s l i g h t l y detune the cavity but i t s major eff e c t i s to s p o i l the cavity's q u a l i t y factor. Thus, when 33k a spark occurs, a power amplifier that was c r i t i c a l l y coupled to the RF cavity during normal operation i s suddenly tremendously under coupled to the cavity, i . e . the real part of the input impedance of the cavity i s far lower than the c h a r a c t e r i s t i c impedance of the transmission l i n e feeding i t . As a re s u l t , large amounts of power are r e f l e c t e d back to the power amplifier rather than supporting the mode, an obviously undesirable s i t u a t i o n . If a spark occurs when the accelerator cavity i s excited in a i r , i t i s most e a s i l y detected by the sudden jump in power being re f l e c t e d back to the transmitter. The usual procedure i s to monitor the ref l e c t e d power and simply shut down the RF drive momentarily i f sparking becomes excessive or the re f l e c t e d power exceeds a s p e c i f i e d l i m i t . In general, however, few sparks or d i f f i c u l t i e s are encountered when an RF accelerating cavity i s driven with moderates amount of power in a i r . D.3 EXCITATION OF AN ACCELERATOR RF CAVITY UNDER VACUUM Vacuum Breakdown Although the exact mechanism of breakdown in vacuum i s not well understood, several phenomena which can lead to breakdown, including cathode f i e l d emission and steady cathode emission currents, have been i d e n t i f i e d [12]. Operational experience has shown that when the dee voltage i s kept above a minimum l e v e l , sparking occurs very infrequently in the TRIUMF RF cavity as predicted by 335 application of K i l p a t r i c k ' s c r i t e r i o n [1.1] to the TRIUMF geometry. Peter et a l [13] recently showed that K i l p a t r i c k ' s c r i t e r i o n i s usually too conservative so the safety margin i s far greater than was o r i g i n a l l y assumed. Experience has also shown that system start-up under vacuum, i . e . r a i s i n g the dee voltage from zero to above the previously mentioned "minimum l e v e l " , i s much more d i f f i c u l t than system start-up in a i r . Following exposure to a i r , the surfaces of the RF cavity are heavily contaminated by water adsorbed from the a i r and o i l from various sources. Although much of the contamination i s spontaneously released when the cavity i s pumped down to vacuum, some contaminants remain. When RF power i s i n i t i a l l y applied to the cavity, the surface currents heat the cavity, releasing some contaminants. They lower the breakdown threshold of the gap, which results in a spark. As the accelerating potential i s raised, and the cavity heats up, more contaminants are released and more sparking occurs u n t i l the bulk of the contaminants have been released and normal operation can begin. This process i s referred to either as cavity surface conditioning or cavity surface processing. A spark i s usually extremely faint unless i t i s causing damage to the cavity, i . e . i t has become a destructive arc. Although sparks can often be seen by a c a r e f u l observer, sudden upward jumps in the vacuum pressure or the r e f l e c t e d 336 power, or sudden downward jumps in the accelerating voltage are more r e l i a b l e indications that contaminants are being released and discharges are occurring. On detection of a spark, the RF control system removes the RF drive signal from the power amplifier. After a short i n t e r v a l , i . e . long enough to pump the offending contaminants from the cavity, the RF drive i s reapplied. If the i n t e r v a l i s far shorter than the RF cavity's thermal time constant, then f u l l power can be reapplied immediately without concern for thermal d r i f t of the cavity's resonant frequency. Sparking may reoccur immediately after power i s reapplied to the cavity. Depending on the nature and size of the gas release, i t may take several such cycles of excitation and pumping to restore normal operation. Sparking w i l l occur more frequently during system start-up in a cavity under vacuum than i t does in a i r but t h i s does not s i g n i f i c a n t l y complicate the procedure for dr i v i n g a cavity under vacuum. Spark detection and recovery are usually automated to simplify system operation, as described in section 4.2 of chapter four. Multipactoring, on the other hand, does s i g n i f i c a n t l y complicate the procedure for e x c i t i n g an accelerator cavity under vacuum. In contrast to breakdown phenomena, which set an upper l i m i t to the accelerating voltage that can be r e l i a b l y maintained in the cavity, multipactoring sets a lower l i m i t to the accelerating voltage that can be r e l i a b l y maintained in the c a v i t y . 337 D.4 MULTIPACTORING Multipactoring, or multiple impact secondary electron emission, i s a discharge phenomenon peculiar to radio frequency systems operating in a vacuum. Unlike sparking, which i s the result of an exponential m u l t i p l i c a t i o n of free electrons induced by the f i e l d which i s subsequently enhanced by thermal emission from cathode surfaces, multipactoring i s a resonant phenomenon in which free electrons are generated by a variety of sources - perhaps ionization of residual gas or f i e l d emission - and accelerated toward a surface where they cause emission of secondary electrons on impact. Multipactoring can occur i f : a. the mean free path of electrons i s longer than the gap between the two surfaces, i . e . the vacuum i s s u f f i c i e n t l y good; and, b. the dimensions of the gap and the voltage and frequency of the applied f i e l d are such that an electron emitted by one surface w i l l : i . accelerate towards the opposite surface, traversing the gap in an odd multiple of one-half an RF period; i i . s t r i k e the opposite surface with s u f f i c i e n t energy to eject secondary electrons which w i l l , in turn, accelerate back towards the f i r s t surface, repeating the cycle; This cycle is described as c l a s s i c a l or "two-point" multipactoring. In some radio frequency c a v i t i e s , an anomolous form of multipactoring [29] referred to as "one-3 3 8 point" or "r e f l e x " multipactoring can occur. In t h i s case, an electron traces a complex and non-conservative path through the cavity in such a way that i t returns to i t s point of o r i g i n with s u f f i c i e n t v e l o c i t y to cause secondary emission and satisy the condition for p e r i o d i c i t y . A multipactor discharge consists of a thin electron cloud which i s driven back and forth across the gap in response to the applied RF voltage. As in the case of sparking, multipactoring s p o i l s the quality factor of the cavity. This decouples the cavity from the power amplifier and transmission l i n e , r e s u l t i n g in excessive reflected power. Once multipactoring becomes established, an increase in the RF power applied to the cavity w i l l only result in more re f l e c t e d power. The accelerating voltage w i l l increase only marginally. Multipactoring in radio frequency c a v i t i e s frequently exhibits unexpected and unpredictable behaviour. In some cases, multipactoring appears to "burn i t s e l f out" after several hours of constant RF drive, a phenomenon which i s often accompanied by an iridescent colouring of the cavity walls. At TRIUMF, however, multipactoring i s avoided by pulsing on the RF power fast enough to drive the accelerating voltage through the multipactoring range before a multipactor discharge loads down the resonant mode. In general, i t becomes easier to pulse through multipactoring as the q u a l i t y of the vacuum improves. 339 H i s t o r i c a l Summary Multipactoring was f i r s t recognized by Philo T. Farnsworth [14] in 1929. This was followed by investigations of multipactoring between the plates of a condensor (capacitor) by G i l l and von Engel [15], Lax et a l [16], and Hatch and Williams [17]. By the late 1950's, multipactoring was reasonably well understood although much of the fine d e t a i l remained (and remains) to be worked out. Harmon [18] presented a brief review of the multipactoring mechanism, as developed by several authors, in a single framework. In the microwave f i e l d , a number of devices based on the multipactor e f f e c t were proposed, including a multipactor TR box described by Forrer and Milazzo [19], a high l e v e l microwave r e c t i f i e r proposed by Keenan [20], an L-band electron gun proposed by Gallagher [21], and a multipactoring electron gun for high duty lin a c s described by Liska [22 ]. In both the accelerator and the high power microwave tube f i e l d , however, most e f f o r t s were directed toward the development of means for the prevention and suppression of multipactoring, e s p e c i a l l y in accelerator c a v i t i e s . Hatch [23] presented a comprehensive review (1962) of both the theory of multipactoring and means for suppression. Although multipactoring has always been troublesome for designers and operators of accelerator radio frequency 340 systems, t r i a l and error (and some clever insight) provided means for dealing with the problem in the room temperature RF systems developed in the 1940's, 50's and 60's. In the 1970's, problems with multipactoring i n : a. superconducting RF c a v i t i e s for linear accelerators [24]; and, b. t h i r d harmonic RF c a v i t i e s for cyclotrons [6]; reminded accelerator designers that multipactoring i s s t i l l a fundamental problem. In a report issued by Oak Ridge National Laboratory in December 1977 [25], i t was remarked that: "The many ramifications of multipactoring are only now being discovered and explored." Unfortunately, multipactoring in accelerator radio frequency c a v i t i e s has proven extremely d i f f i c u l t to characterize. Smith [26], Bischof [27] and Gallagher [28] have applied s t r i c t l y a n a l y t i c a l descriptions to the problem but t h e i r models have some shortcomings. In p a r t i c u l a r , as Smythe noted, in reference to multipactoring in the Berkeley 88-inch cyclotron [26], in any quarter-wave accelerating cavity with a constant dee-to-liner spacing, there i s always a space that s a t i s f i e s the multipactoring condition because the voltages in the cavity extend from maximum down to zero. It has been suggested that RF magnetic f i e l d s provide a sweeper action that ultimately quenches two point multipactoring as the multipactoring range rushes away from the RF e l e c t r i c f i e l d maximum and towards the RF magnetic f i e l d maximum. Unfortunately, the complexity of the algebra 341 and the integrals associated with the solution of the kinematic equations make i t d i f f i c u l t to set up a closed form sol u t i o n . Experimental work, as reviewed by Gallagher [28] and F a r r e l l and Gallagher [29], has been of much greater p r a c t i c a l value. Computer programs have been developed by Ben-Zvi, Crawford and Turneaure [30] and Boni et a l [31], among others, which trace the path of a multipactoring electron in a cavity based on knowledge of the f i e l d d i s t r i b u t i o n within the c a v i t y . Although some results have been encouraging, users of such programs note that in practice, such simulations have met with mixed success in predicting the multipactoring thresholds in real c a v i t i e s [32], Suppression of Multipactoring Multipactoring can be made less troublesome in a number of ways: The accelerating voltage can be made to r i s e quickly enough that electron loading cannot build up fast enough to quench i t . Smith [26] suggested that i f the accelerating voltage increased at a minimum rate of 1 kilovolt/microsecond through the multipactoring range, the multipactoring range could e a s i l y be passed through. It has also been suggested that the accelerating voltage should not remain in the multipactoring region for longer than f i v e RF periods but th i s figure i s probably highly dependent on the q u a l i t y of the vacuum. Experience has shown that i t i s far easier to break through multipactoring in a hard vacuum than 3k2 i t i s to break through multipactoring in a soft vacuum; A DC bias of a few hundred vo l t s placed across the multipactoring surfaces [23] w i l l suppress multipactoring but i s awkward to implement; The geometry of the cavity or structures within the cavity such as the location of coupling loops can be altered [31], a successful strategy in stubborn cases; or The surface of the cavity can be coated with a material having a low secondary emission c o e f f i c i e n t , such as aquadag, as was done at SIN [6]. The rate at which the accelerating voltage can be made to r i s e during a pulse-on is constrained by the energy storage capacity of the RF cavity which i s usually described in terms of i t s quality factor: V (t) = V (1 - e " W t / 2 Q ) (D.14) acc o where Q i s the loaded q u a l i t y factor of the system. In very high Q systems, such as SIN, i t i s not possible to pulse on the accelerating voltage fast enough to overcome multipactoring due to thi s fundamental l i m i t a t i o n . Other methods, such as simply allowing the multipactor discharge to burn i t s e l f out, must be used to raise the accelerating voltage above the multipactoring threshold. In systems with moderate Q's, such as TRIUMF, one can pulse the accelerating voltage to a f i n a l value that i s higher than the multipactoring threshold so the accelerating voltage w i l l r i s e at an acceptable rate through the multipactoring range. 343 The upper l i m i t to the multipactoring range i s proportional to the square of the RF frequency and the square of the distance between the surfaces. Thus as the RF frequency i s increased, as for example i n the t h i r d o r f i f t h harmonic booster c a v i t i e s (with gaps of similar dimension to the fundamental cavity) planned f o r the c y c l o t r o n , the multipactoring problem worsens greatly. A Simplified Model of the Multipactor E f f e c t A s i m p l i f i e d model of multipactoring i s presented in f i g . D.1 and the supplement that accompanies i t . In such models, i t i s generally assumed that two electrodes ( p a r a l l e l metal plates) are separated by a vacuum. An alternating potential i s applied between plates. The model is designed to answer the question, "Under what conditions (voltage, frequency, s t a r t i n g phase, gap spacing) can a multipactor discharge be sustained?" In general, one requires a source of residual ions and a mean free path longer than gap length, d. The surfaces must have secondary emission c o e f f i c i e n t s greater than 1. The electron clouds must traverse the gap in an exact odd multiple of half cycles. Starting phase with respect to the peak of the RF waveform i s a free parameter, however. The model has serious l i m i t a t i o n s , however. It takes no account of either a s t a t i c external magnetic f i e l d (this could be accounted for by increasing the e f f e c t i v e gap length, however) or an RF magnetic f i e l d . It assumes that the v e l o c i t y of secondary emission i s zero and takes no 3kk 1 m i Li Li L («>1) - e E -© - e - © E . F i g . D.l Evolution of a Multipactor Discharge A S i m p l i f i e d Model of the Multipactor E f f e c t A multipactor discharge can be sustained between the plates of a p a r a l l e l plate capacitor or s i m i l a r structure provided: - the mean f r e e path i s greater than the width of the gap - the secondary emission c o e f f i c i e n t of the metal surfaces <5 i s greater than one - the r e l a t i o n s h i p between the magnitude of the applied RF p o t e n t i a l V , i t s frequency to, and the gap width d must be such than an electron can, depending on i t s s t a r t i n g phase with respect to f i e l d r e v e r s a l <j>, traverse the gap i n an odd multiple of half c y c l e s . - there i s a source of r e s i d u a l ions to begin the cascade process Summary of the Kinematics of Multipactoring F = m d 2x = eE sin(ut + 4>) d t 7 d 2x dt 2" eV sin(o)t + <|>) md dx = (cos 4> - cos(cot + <(>)); dx = 0 @ t = 0 dt u>md dt 345 Supp lemen t t o F i g . D . l A S i m p l i f i e d M o d e l o f t h e M u l t i p a c t o r E f f e c t ( c o n t i n u e d ) eV x = o (cot c o s <|> - s i n (a ) t + <(>) + s i n <)>); x = 0 @ t = 0 u/md ait - ( 2 n - l ) i r ; t h e c o n d i t i o n must be c y c l i e eV d = o ( 2 n - l ) i T c o s <j> - s i n ( ( 2 n - l ) i T + <)>)+ s i n d> co^md V * w 2 m d 2 V y _ o ; t h e a p p l i e d v o l t a g e a t w h i c h a m -2 s i n <j> + i T ( 2 n - l ) c o s (j) m u l t i p a c t o r d i s c h a r g e c a n be s u s t a i n e d g i v e n ID, d , n , <f> V / V i s p l o t t e d a s a f u n c t i o n o f <f> f o r v a r i o u s v a l u e s o f n m o i n f i g . D . 2 . N o t e : - i f <j> < 0 ° t h e n t h e E f i e l d i s i n i t i a l l y r e t a r d i n g - i f <|> > 90° t h e n t h e e l e c t r o n w i l l n o t be a b l e t o make i t a c r o s s t h e gap t o t h e o p p o s i t e e l e c t r o d e - i f 4 i s s u c h t h a t dV /dd) = 0 , t h e n V (<))) i s t h e minimum V m * m m - V m a y i s t h e maximum v o l t a g e a t w h i c h a m u l t i p a c t o r d i s c h a r g e c a n be s u s t a i n e d V = V 12 ; 4 = 9 0 ° max o V m i n = V ( 2 S i n * + ^ ^ n - 1 ) c o s ; 4> - t a n _ 1 ( 2 / ( 2 n - l ) T r ) V and V a r e p l o t t e d as a f u n c t i o n o f d/\ ( f o r v a r i o u s v a l u e s o f n) max m i n i n f i g . D . 3 . 346 F i g . D.2 Multipactoring Threshold Voltage as a Function of S t a r t i n g Phase R e f : F i g . D . l and s u p p l e m e n t oo o > o 0 -r 0.00 F i g . D.3 0.005 0.010 0.015 0.030 0.035 0.040 i r 0.020 0.025 d/x Multipactoring Threshold Voltage as a Function of Gap Length and RF Wavelength Ref: F i g . D.l and supplement account of the secondary emission c h a r a c t e r i s t i c s of the metal surfaces. It does not predict the f i n a l multipactor saturation current. Its most serious shortcoming i s that i t does not resolve Smythe's paradox, i . e . in a rectangular or coaxial cavity, there i s always a space that s a t i s f i e s the multipactoring condition becuase the voltage goes from maximum down to zero in a continuous function. However, an a n a l y t i c a l extension of the model to resolve the paradox would be very unwieldy. The multipactoring voltage as a function of star t i n g phase angle i s presented in f i g . D.2. A graph for estimating the multipactoring voltage thresholds over a t y p i c a l range of gap dimensions and RF frequencies i s presented in f i g . D . 3 . D.5 OBSERVATIONS OF MULTIPACTORING IN THE TRIUMF RF SYSTEMS TEST FACILITY Observations of' multipactoring in the TRIUMF RF systems test f a c i l i t y are presented in f i g . D.4. The leading and f a l l i n g edges of a 1 millisecond pulsed RF drive signal were observed at the output of the cavity's dee voltage probe as the amplitude of the pulse was varied above and below the multipactoring theshold. Once above the multipactoring threshold, the RF drive was switched to continuous output. Multipactoring Threshold - Continuous RF Drive The RF drive l e v e l was slowly lowered u n t i l a multipactor discharge began. This occurred when the dee 349 V e r t i c a l 25 kV/div Horizontal 200 usec/div (a) Dee Voltage i n RF Systems Test F a c i l i t y Cavity During Pulsed Mode Operation F a l l of Dee Voltage Following Removal of RF Drive I n i t i a l Rise of Dee Voltage Through Multipactoring Region dV/dt > 375 V/usec 0 • r m V m m • Note onset of a multipactor discharge (rapid decrease i n decay time constant) at j u s t under 3 - 4 k i l o v o l t s . mmm • ^ • • • • • • 1 V e r t i c a l 2.5 kV/div Horizontal 20 usec/div (h) Expanded View of Rising and F a l l i n g Edges of RF Pulse i n (a). F i g . D.4 Observations of Multipactoring i n the TRIUMF RF Systems Test F a c i l i t y Cavity 350 voltage suddenly dropped from approximately 5 k i l o v o l t s to several tens of v o l t s . This threshold i s far higher than the threshold predicted by the s i m p l i f i e d model of multipactoring. Assuming that the model i s b a s i c a l l y correct, and considering Smythe's observation, the r e s u l t s suggest that the multipactor discharge zone moves back along the transmission l i n e towards the root u n t i l i t reaches a point approximately 50 centimetres from the root. At t h i s point the trajectory of the multipactoring electrons is probably so disturbed by the influence of the RF magnetic f i e l d that the resonant condition required to sustain multipactoring i s no longer s a t i s f i e d . Inspection of f i g . 2.1, the cross-section of the accelerating gap, shows that many of the e l e c t r i c f i e l d l i n e s in the accelerating f i e l d follow paths that are longer than the s p e c i f i e d dee gap. The s i m p l i f i e d model c l e a r l y shows that at a given frequency, longer gaps have a higher multipactor threshold voltage. This might explain why the multipactor threshold voltage i s higher than i n i t i a l l y expected, but i t does not resolve Smythe's paradox. These results suggest that the s i m p l i f i e d model that i s commonly used [26]-[28] to describe multipactoring in quarter-wave accelerating c a v i t i e s : a./ does not completely describe the true physical s i t u a t i o n ; and, 3 5 1 b. in the case of the TRIUMF RF systems test f a c i l i t y , gives predicted thresholds that are too low by a factor of f i v e . A search of the multipactor l i t e r a t u r e did not f i n d any quantitative solution or description of multipactoring in a quarter-wave stub although i t has been mentioned in passing by two people: Bischof [27] and Smythe responding to Smith [26]. This suggests that an investigation of t h i s rather fundamental problem i s long overdue. Multipactoring Threshold - Pulsed RF Drive The multipactoring threshold during operation with the RF drive pulsed on for 1 millisecond every 25 milliseconds was also measured. The threshold, approximately 27 k i l o v o l t s , was c. factor of f i v e greater than the continuous drive case. The result i s e a s i l y explained. The rate of voltage r i s e , as described'by equation (D.14), decreases as the voltage approaches i t s f i n a l value. As noted e a r l i e r , the dee voltage must pass through the multipactor zone before s u f f i c i e n t electron loading to s p o i l the N cavity Q builds up. The results suggest that given an unloaded qual i t y factor of about 5800 at 23 MHz, d v a e e / d t i s not high enough to overcome multipactor loading i f the multipactor zone has not been passed through by the time - 1/5 V f i n a l * 352 Anomalous RF R e c t i f i c a t i o n in Connection with Multipactoring During the RF flat-topping tests described in chapter three, apparent r e c t i f i c a t i o n of the RF during cavity surface conditioning was often observed on an oscilloscope monitoring the output of the dee voltage probes. This was a very unexpected observation. A search of the l i t e r a t u r e showed that F.G. Tinta [33] observed a similar e f f e c t in the Nevis syncrocyclotron. Conditions for RF r e c t i f i c a t i o n can appear when an asymmetry i s created in the available current c a r r i e r s around electrodes. In his case, the multipacting gap was biased. Tinta suggested that, " i n biased multipacting gaps, the energy of electrons h i t t i n g opposite electrode surfaces i s not symmetrically d i s t r i b u t e d and t h i s may result in asymmetric secondary electron y i e l d . " D.6 X-RAY EMISSION BY ACCELERATOR RF CAVITIES The hard vacuum and high voltages present in accelerator RF c a v i t i e s combine to make them copious producers of X-rays. During RF system development work, these X-rays can be a serious safety hazard and special precautions must be taken. The X-rays can be put to good use in a scheme used to c a l i b r a t e the dee voltage probes in both the MSU [34] and IUCF [35] cyclotrons. In t h i s technique, the peak dee voltage i s inferred from a measurement of the end-point of the bremsstrahlung continuum made using a S i ( L i ) X-ray detector. M i l l e r and Kashy [34] suggest that the accuracy of the X-ray c a l i b r a t i o n technique i s probably around 1 to 2 3 5 3 percent for voltages above 30 k i l o v o l t s . Such a c a l i b r a t i o n technique would be very useful at TRIUMF where the absolute c a l i b r a t i o n of the capacitive dee voltage probes has recently been questioned. References: [1] J. Riedel. "R.F. Systems." IEEE Trans NS-26(2):2133 (April 1979). [2] R.A. Tennant. "Accelerator Vibration Issues." IEEE Trans NS-32(5):2868-2869 (October 1985). [3] D. Dohan, G. Dutto, K. Fong, R. Laxdal, V. Pacak, R. P o i r i e r , R. Worsham, and M. Zach. "Requirements for a New Resonator Structure at TRIUMF." Proc 10th Int Conf on Cyclotrons and t h e i r Applications, pp 357-359 (1984). [4] "Care and Feeding of Power Grid Tubes." San Carlos, C a l i f o r n i a : Varian, Eimac D i v i s i o n , pp 42-45 (1967). [5] D.R. Vaughan, G.E. Mols, D.W. Reid, and J.M. Potter. "A High-Power, Solid-State RF Source for Accelerator C a v i t i e s . " IEEE Trans NS-32(5):2857-2859 (October 1985). [6] S. Adam, W. Joho, P. Lanz, H. Leber, N. Schmid, U. Schryber. " F i r s t Operation of a Flattop Accelerating System in an Isochronous Cyclotron." IEEE Trans NS-28(3):2721-2723 (June 1981). [7] International Telephone and Telegraph Co. "Reference Data for Radio Frequency Engineers." Indianapolis: Howard W. Sams & Co, Inc. pp 23-20 - 23-21 (1968). [8] A. Prochazka. "Detuning of the Resonator." TRIUMF Design Note TRI-DN-71-34 (September 1971). [9] R.B. McKenzie-Wilson. "Tuning System for Capacitively Loaded X/4 Accelerating Cavity." IEEE Trans NS-32 (5):2786-2787 (October 1985). [10] Q. Kerns, C. Kerns, H. M i l l e r , S. Tawzer, J . Reid, R. Webber, and D. Wildman. "Fermilab Tevatron High Level RF Accelerating Systems." IEEE Trans NS-32(5):2809-281 1 (October 1985). 3 5 ^ [11] W.D. K i l p a t r i c k . "A C r i t e r i o n for Vacuum Sparking Designed to Include Both RF and DC." University of C a l i f o r n i a Radiation Laboratory - UCRL Report 2321 (4 September 1953). [12] T.W. Dakin. "Insulating Gases" in Standard Handbook for E l e c t r i c a l Engineers, Donald G. Fink, ed. New York: McGraw-Hill, (1977) pp 4-131 - 4-138. [13] W. Peter, R.J. Faehl, A. Kadish, and L.E. Thode. " C r i t e r i a for Vacuum Breakdown in RF C a v i t i e s . " IEEE T r a n s NS-30(4):3454-3456 (August 1983). [14] P.T. Farnsworth. "Television by Electron Image Scanning." J. F r a n k l i n I n s t 218:411-444 (1934). [15] E.W.B. G i l l and A von Engel. "Starting Potentials of High-Frequency Gas Discharges at Low Pressure." P r o c Roy Soc A192:446~463 (1948). [16] B. Lax, W.P. A l l i s , and S.C. Brown. "The Effect of Magnetic F i e l d on the Breakdown of Gases at Microwave Frequency." J Appl Phys 21:1297-1304 (1950). [17] A.J. Hatch and H.B. Williams. "The Secondary Electron Resonance Mechanism of Low-Pressure High-Frequency Gas Breakdown." J Appl Phys 25:417-423 (1954). [18] G.S. Harmon. "Multipacting Mechanism as The Orgin of Breakdown in High-Frequency E l e c t r i c a l Discharges in Gases." Am J Phys 47(8):722-726 (August 1979). [19] M.P. Forrer and C Milazzo. "Duplexing and Switching with Multipactor Discharges" Proc IRE 50:442-45? (1962) . [20] P. Keenan. "A High Level Microwave R e c t i f i e r " Report of S c i e n t i f i c Research Laboratory, Lockheed-California Co. (1964). [21] W.J. Gallagher. "The Multipactor Electron Gun." P r o c IEEE 57(1):94-95 (1969). [22] D.J. Liska. "Multipactoring-Electron Gun for High Duty Linacs." Proc IEEE 59(8):1253-1254 (1971). [23] A.J. Hatch. "Suppression of Multipacting in P a r t i c l e Accelerators." Nucl Inst Methods 41:261-271 (1966). [24] J . H a l b r i t t e r . "Electron Loading of Superconducting RF C a v i t i e s . " P a r t i c l e Accel er at or s 3: 163-174 ( 1972). 355 [25] "The Role of Electron Accelerators in U.S. Medium Energy Nuclear Science." Oak Ridge National Laboratory Report ORNL/PPA-77/4, p23 (December 1977). [26] B.H. Smith. "The Radio Frequency System of the Berkeley 88-inch Cyclotron." Nucl Inst Methods 18,19:184-193 (1962). [27] B. Bischof. "Rise Time of the Multipactoring Process in Cavities of P a r t i c l e Accelerators." Nucl Inst Methods 97:81-86 (1971). [28] W.J. Gallagher. "The Multipactor E f f e c t . " IEEE Trans NS-26(3):4280-4282 (June 1979). [29] S.R. F a r r e l l and W.J. Gallagher. "Further Notes on the Multipactor E f f e c t . " IEEE Trans NS-32(5):2900-2902 (October 1985). [30] I. Ben-zvi, J.F. Crawford, and J.P. Turneaure. "Electron M u l t i p l i c a t i o n in C a v i t i e s . " IEEE Trans NS-20:54-58 (1973). [31] R. Boni, V. Chimenti, P. Fernandes, R. Parodi, B. Spataro, and F. T a z z i o l i . "Reduction of Multipacting in an Accelerator Cavity." IEEE Trans NS-32(5):2815-2817 (October 1985). [32] J.M. Brennan, State University of New York - Stoney Brook. Private Communication, (1985). [33] F.G. Tinta. "Multipacting Related Processes and RF Breakdown Protection by using Bias Pulsing at the Nevis Synchrocyclotron." Proc 7th Int Conf on Cyclotrons and their Appli cat i ons, pp 151-155 (1975). [34] P. M i l l e r and E. Kashy. "Absolute Calibration of the Dee Voltage by X-Ray Endpoint." Proc 7th Int Conf on Cyclotrons and their Applications, pp 171-174 (1975). [35] R.E. Pollock. "Status Report on the Indiana University Cyclotron F a c i l i t y . " Proc 7th Int Conf on Cyclotrons and their Applications, pp 27-32 (1975). 356 

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