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The measurement of radial recovery in a hydrogen spark discharge. Butter, Donald Alexander McNaughton 1963

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THE MEASUREMENT OF RADIAL RECOVERY IN A HYDROGEN SPARK DISCHARGE by DONALD ALEXANDER McNAUGHTON BUTTER B.Sc. University of Glasgow 1961 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the department of PHYSICS We accept this thesis as conforming to the required standard. THE UNIVERSITY OF BRITISH COLUMBIA October 1963 In presenting 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 m e n t of the requirements for an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I f u r t h e r agree that per-mission for extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s representativeso I t i s understood that copying, or p u b l i -c a t i o n of t h i s t h e s i s for f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of The U n i v e r s i t y of B r i t i s h Columbia,. Vancouver. 8, Canada. ABSTRACT A m e t h o d i s d e s c r i b e d w h e r e b y t h e r e c o v e r y o f p l a s m a s f o r m e d b y t h e p a s s a g e o f a h i g h c u r r e n t d i s c h a r g e a c r o s s a s p a r k gap h a s b e e n m e a s u r e d as a f u n c t i o n o f b o t h t h e t i m e a f t e r t h e p a s s a g e o f a c u r r e n t p u l s e and t h e r a d i a l d i s t a n c e f r o m t h e c e n t r e o f t h e s p a r k g a p . j The e f f e c t s o f p r e s s u r e , c u r r e n t m a g n i t u d e and c u r r e n t w a v e f o r m on t h e r e c o v e r y t i m e o f t h e t e s t gap w e r e i n v e s t i g a t e d and i t was f o u n d t h a t a l l o f t h e m e x e r t e d p o w e r f u l b u t e x p l i c a b l e i n f l u e n c e s on t h e r e c o v e r y . Gas t e m p e r a t u r e s w e r e d e r i v e d u s i n g t h e a s s u m p t i o n t h a t t h e P a s c h e n Law h o l d s t r u e d u r i n g t h e f i n a l r e c o v e r y p e r i o d . i i ACKNOWLEDGEMENTS I w i s h t o t h a n k D r . R . J . C h u r c h i l l f o r h a v i n g s u g g e s t e d t h e p r o b l e m a n d f o r h i s c o n t i n u e d i n t e r e s t i n i t s d e v e l o p m e n t . I am g r a t e f u l f o r t h e t e c h n i c a l a s s i s t a n c e and a d v i c e o f M e s s r s . A . F r a s e r and P . Haas o f t h e P h y s i c s W o r k s h o p a n d f o r t h e . v a l u a b l e h e l p g i v e n b y M r . J . T u r n e r i n e l e c t r o n i c s . I w i s h t o o , t o t h a n k my w i f e , J e a n , f o r h e r h e l p i n t h e d r a w i n g o f t h e c i r c u i t s and t h e i l l u s t r a t i o n s h e r e i n ; M r . P . H o l b o r n e o f t h e U n i v e r s i t y E x t e n s i o n D e p a r t m e n t f o r h i s h e l p w i t h t h e P h o t o g r a p h y ; a n d M i s s H . F e l d e r o f t h e P h y s i c s O f f i c e o f t h e U n i v e r s i t y o f W e s t e r n O n t a r i o f o r h e r t y p i n g b f t h e w o r k . The w o r k was p a r t i a l l y s u p p o r t e d b y a r e s e a r c h g r a n t f r o m t h e . A t o m i c E n e r g y C o n t r o l B o a r d o f C a n a d a . i i i TABLE OF CONTENTS •ABSfFt&C.T i i ACKNOWLEDGEMENTS i i i L I S T OF ILLUSTRATIONS v i L I S T OF T A B L E S : . . ' v i i i CHAPTER 1 INTRODUCTION 1 CHAPTER 2 PRELIMINARY DISCUSSION AND D E F I N I T I O N S . 4 2 . 1 The r e s t r i k i n g v o l t a g e wave f o r m . 4 2 . 2 S p a r k b r e a k d o w n , s t a t i s t i c a l l a g , a n d i n i t i a l p o i n t s i n t h e r e c o v e r y c u r v e s 4 2 . 3 T e m p e r a t u r e d e r i v a t i o n 7 CHAPTER .3 EXPERIMENTAL METHOD 9 3 . 1 Summary o f t e c h n i q u e 9 3 . 2 O v e r a l l o p e r a t i o n o f r e c o v e r y m e a s u r i n g s y s t e m . . 9 3 . 3 E x p e r i m e n t a l p r e c a u t i o n s 11 CHAPTER 4 APPARATUS 16 4 . 1 The s p a r k chamber 16 4 . 2 M e a s u r e m e n t o f c u r r e n t 18 4 . 3 R e s t r i k i n g v o l t a g e g e n e r a t o r a n d t r i g g e r u n i t . . . 20 4 . 4 The p o t e n t i a l d i v i d e r f o r m e a s u r i n g r e s t r i k i n g v o l t a g e 20 i v V 4.5 The probe for measuring radial dependence of recovery 23 4.6 Spark chamber and test gap 23 4.7 The damping resistor 23 CHAPTER 5 PRELIMINARY EXPERIMENTS 27 5.1 Dependence of reignition voltages of the test gap on certain parameters 27 CHAPTER 6 MEASUREMENT OF RADIAL RECOVERY 42 6.1 Recovery of the test gap 42 CHAPTER 7 CONCLUSIONS 53 7.1 Conclusions 53 BIBLIOGRAPHY 54 L I S T OF ILLUSTRATIONS F i g u r e 1 I d e a l i z e d o s c i l l o s c o p e t r a c e 5 2 O b s e r v e d t r a n s i e n t s 6 3 . B l o c k d i a g r a m o f a p p a r a t u s 10 4 S c h e m a t i c c i r c u i t f o r m a i n bank • 12 5 S p a r k chamber 17 6 E q u i v a l e n t c i r c u i t o f R o g o w s k i G o i l and i n t e g r a t o r 19 7 T , I wave f o r m s . 21 8 The p o t e n t i a l d i v i d e r 22 9 C a l i b r a t i o n o f p o t e n t i a l d i v i d e r 22 10 The p r o b e 24 11 The s p a r k chamber and e l e c t r o d e s y s t e m 25 12 B r e a k d o w n v o l t a g e s v s . p r e s s u r e 28 13 T e s t gap r e c o v e r y i n h y d r o g e n a s a f u n c t i o n o f p r e s s u r e (4 kA s u r g e c u r r e n t ) 30 14 N o r m a l i z e d r e i g n i t i o n c h a r a c t e r i s t i c s f o r h y d r o g e n (4 k A s u r g e c u r r e n t ) 31 15 D e r i v e d gas t e m p e r a t u r e s f o r t e s t gap (4 kA s u r g e c u r r e n t ) . 32 16 T e s t gap r e c o v e r y i n h y d r o g e n a s a f u n c t i o n o f p r e s s u r e (40 k A s u r g e c u r r e n t ) 34 v i v i i F i g u r e 17 N o r m a l i z e d r e i g n i t i o n c h a r a c t e r i s t i c s f o r h y d r o g e n (40 k A s u r g e c u r r e n t ) 35 18 D e r i v e d gas t e m p e r a t u r e s f o r t e s t gap (40 kA s u r g e c u r r e n t ) . 36 19 T e s t gap r e c o v e r y i n h y d r o g e n as a f u n c t i o n o f p r e s s u r e (88 kA s u r g e c u r r e n t ) , 38 20 N o r m a l i z e d r e i g n i t i o n c h a r a c t e r i s t i c s f o r h y d r o g e n (88 kA s u r g e c u r r e n t ) ; * ; 39 21 D e r i v e d g a s t e m p e r a t u r e s f o r t e s t gap (88 kA s u r g e - c u r r e n t ) . 40 22 T e s t gap r e c o v e r y i n h y d r o g e n . . . ; 44 23 R e i g n i t i o n c h a r a c t e r i s t i c f o r p r o b e i n h y d r o g e n ( p r o b e d i s t a n c e f r o m t e s t gap ( r = 2 c m . ) . 46 24 R e i g n i t i o n c h a r a c t e r i s t i c f o r p r o b e i n h y d r o g e n ( r = 3 c m . ) . 47 25 R e i g n i t i o n c h a r a c t e r i s t i c f o r p r o b e i n h y d r o g e n ( r = 4 c m . ) . 48 26 R e i g n i t i o n c h a r a c t e r i s t i c f o r p r o b e i n h y d r o g e n ( r = .5 c m . ) . 49 27 R e i g n i t i o n c h a r a c t e r i s t i c f o r p r o b e . i n . h y d r o g e n ( r = 6 c m . ) . 50 28 R e i g n i t i o n c h a r a c t e r i s t i c s f o r p r o b e i n h y d r o g e n 51 29 D e r i v e d g a s t e m p e r a t u r e s 52 L I S T OF TABLES T a b l e I R e i g n i t i o n v o l t a g e s , p e r c e n t a g e b r e a k d o w n , .and d e r i v e d t e m p e r a t u r e s a t d i f f e r e n t p r e s s u r e s i n h y d r o g e n f o r a 4 kA u n i d i r e c t i o n a l s u r g e c u r r e n t 29 I I R e i g n i t i o n v o l t a g e s , p e r c e n t a g e b r e a k d o w n , a n d d e r i v e d . t e m p e r a t u r e s a t d i f f e r e n t p r e s s u r e s i n h y d r o g e n f o r a 40 kA u n i d i r e c t i o n a l s u r g e c u r r e n t 33 I I I R e i g n i t i o n v o l t a g e s , p e r c e n t a g e b r e a k d o w n , a n d d e r i v e d t e m p e r a t u r e s a t d i f f e r e n t p r e s s u r e s i n h y d r o g e n f o r a n o s c i l l a t o r y s u r g e c u r r e n t w i t h a p e r i o d o f 13 ^ . s e c . . . . . 37 XV M i n i m u m r e i g n i t i o n v o l t a g e s o f t h e t e s t gap 43 V M i n i m u m r e i g n i t i o n v o l t a g e s o f p r o b e a t d i f f e r e n t r a d i a l d i s t a n c e s f r o m t e s t gap .• 45 v i i i CHAPTER 1 INTRODUCTION The most widely studied parameter of.the spark discharge i s the sparking potential. It has been investigated as a function of different parameters by a host of researches^. The f i r s t quanti-tative treatment of the electric spark was accomplished by J. S. Townsend in 1903, who regarded the current i n a gap below break-down as being caused by a continual ionization by c o l l i s i o n process, and from his analysis was able to predict that subject to certain conditions, a self sustained discharge maintained by positive ion bombardment of the cathode would result. Although later researchers have shown that the Townsend treatment i s over simplified, i t non the less i s a useful guide i n research into the properties of spark gaps. In the early days of research into the behaviour of spark gaps agreement of experimental results obtained by differnt researches was often poor, due to failure to provide adequate triggering electrons i n a reasonable time, absence of humidity control, im-purities i n the gas, and disagreement on a criterion for a spark 2 b r e a k d o w n . F u r t h e r m o r e a n y s p a r k d i s c h a r g e may w e l l l e a v e p a r t o f e i t h e r e l e c t r o d e j a g g e d , t h u s l o w e r i n g t h e v a l u e o f t h e p o t e n t i a l n e c e s s a r y t o c a u s e t h e n e x t b r e a k d o w n . I t i s h a r d l y s u r p r i s i n g , t h e r e f o r e t h a t g r a v e d i s c r e p a n c i e s h a v e a r i s e n a n d w i l l c o n t i n u e t o a r i s e i n t h i s s u b j e c t u n l e s s c a r e i s t a k e n t o e l i m i n a t e o r a t l e a s t t o m i n i m i z e t h e s e e f f e c t s . I t s h o u l d be. b o r n e i n m i n d t o o t h a t c o n -s i s t e n c y a n d r e p r o d u c i b i l i t y o f e x p e r i m e n t a l r e s u l t s o f a s i n g l e e x p e r i m e n t e r i s i n no way a m e a s u r e o f t h e a c c u r a c y o f t h e r e s u l t s . The d i f f i c u l t i e s i n h e r e n t i n t h i s t y p e o f w o r k , however; do n o t d e t r a c t f r o m t h e v a l u a b l e u s e s t o w h i c h s p a r k g a p s h a v e b e e n p u t when made t o o p e r a t e a s s w i t c h e s . Used as s w i t c h e s , s p a r k g a p s h a v e become i n c r e a s i n g l y p r o m i n e n t i n s e v e r a l f i e l d s o f p h y s i c s and e l e c t r i c a l e n g i n e e r i n g . I n . t h e t e s t i n g o f t h e e f f e c t s o f s u r g e 4 c u r r e n t s on c e r t a i n h i g h v o l t a g e e q u i p m e n t , B e l l a s c h i , u s e d s p a r k g a p s t o c o n n e c t l u m p e d c a p a c i t o r b a n k s t o t h e t e s t s p e c i m e n s . McCann a n d C l a r k " ' , and l a t e r McCann e t a l . ^ , i n v e s t i g a t e d d i e l e c t r i c r e c o v e r y o f g a p s p f l e n g t h v a r y i n g f r o m 3 " t o 1 1 " by means o f two s y n c h r o n i z e d i m p u l s e g e n e r a t o r s . They f o u n d t h a t t h e d i e l e c t r i c r e c o v e r e d " i n a somewhat e x p o n e n t i a l m a n n e r " and t h a t t h e s m a l l e r gaps r e c o v e r e d more r a p i d l y t h a n t h e l a r g e r o n e s , t h i s phenomenon b e i n g a t t r i b u t e d 7 8 t o t h e c o o l i n g e f f e c t o f t h e e l e c t r o d e s . L a t e r , E d e l s a n d C r a w f o r d > d e f i n e d c o n d i t i o n s o f f r e e r e c o v e r y o f a gap a f t e r t h e p a s s a g e o f l o w c u r r e n t a r c s u n d e r v a r i o u s . e x p e r i m e n t a l c o n d i t i o n s . W i t h t h e a d v e n t o f t h e r m o n u c l e a r r e s e a r c h , t h e s p a r k gap h a s g a i n e d p r o m i n e n c e i n many s w i t c h i n g a p p l i c a t i o n s , and t h e p o s s i b i l i t y o f i t s u s e i n r e c u r r e n t s w i t c h o p e r a t i o n s , s t i m u l a t e d i n t e r e s t i n t h e r e c o v e r y 3 properties of spark gaps after the passage of high current discharges. This problem was treated by Churchill^, using a technique similar to 8 Q that defined by Edels and Crawford ' , who measured the reignition voltage as a function of time after the passage of currents varying from 100 k.a. to 235 k.a. with different gap lengths and gaseous dielectrics. It was noted that the mass of the electrodes had a powerful, effect "on the mechanisms of dielectric recovery and that the "best reproducibility" of results was found for hydrogen with an average scattering of about 3%. Basically, this thesis i s a continuation of Churchill's work. It describes the recovery characteristics of the fixed test gap for "different pressures, peak currents, and current waveforms. It goes further and investigates the behaviour of the recovering dielectric at different radial distances from the test gap by means of an auxilliary moveable spark gap. From the data of the reignition voltages thus obtained, the temperature profiles were deduced. CHAPTER 2 PRELIMINARY DISCUSSION AND D E F I N I T I O N S 2.1 The r e s t r i k i n g v o l t a g e w a v e f o r m T h e : ! v o l t a g e a p p l i e d a c r o s s t h e s p a r k gap t o t e s t t h e r e c o v e r y -a f t e r s p a r k b r e a k d o w n i s o f u n i t f u n c t i o n f o r m . T h a t i s t o s a y t h e a p p l i e d v o l t a g e r i s e s . f r o m z e r o t o some m a g n i t u d e i n n e g l i g i b l e t i m e and i s t h e n m a i n t a i n e d a t t h i s m a g n i t u d e . The a d v a n t a g e s o f u s i n g a u n i t f u n c t i o n v o l t a g e t o i n v e s t i g a t e t h e r e c o v e r y p r o p e r t i e s o f a d i e l e c t r i c a r e d i s c u s s e d b y E d e l s a n d C r a w f o r d . S p a r k b r e a k d o w n w i l l be o b s e r v e d on t h e f l a t p o r t i o n o f t h e u n i t f u n c t i o n v o l t a g e so t h a t a c h e c k may be k e p t on t h e t i m e l a g t o b r e a k d o w n a s m e a s u r e d b y t h e l e n g t h o f t h e f l a t p o r t i o n . 2.2 S p a r k b r e a k d o w n , s t a t i s t i c a l l a g , and t h e . i n i t i a l p o i n t s i n t h e r e c o v e r y c u r v e s . A s p a r k b r e a k d o w n w i l l be s a i d t o h a v e o c c u r r e d when t h e i m p e d a n c e o f f e r e d b y t h e d i e l e c t r i c b e t w e e n t h e e l e c t r o d e s o f t h e s p a r k gap a p p e a r s t o v a n i s h i n s t a n t a n e o u s l y u p o n a p p l i c a t i o n o f a s u f f i c i e n t l y h i g h v o l t a g e . When a s p a r k b r e a k d o w n o c c u r s , a t r a n s i e n t 5 w i l l be o b s e r v e d w h i c h w i l l f o l l o w c l o s e l y t h e s k e t c h i n F i g . 1 b e l o w V 0 L J T A G E TIME F i g . 1 . I d e a l i z e d o s c i l l o s c o p e t r a c e H e r e , AB i s t h e i n h e r e n t d e l a y o f t h e c i r c u i t b e t w e e n t h e t r i g g e r i n g o f t h e o s c i l l o s c o p e and t h e a p p l i c a t i o n o f t h e r e s t r i k i n g v o l t a g e , w h i l e CD g i v e s a m e a s u r e o f t h e t i m e f o r t h e t e s t gap t o b r e a k down when a v o l t a g e BC i s i m p r e s s e d a c r o s s i t . O b v i o u s l y CD c a n v a r y i n l e n g t h , and t h e q u e s t i o n a r i s e s a s t o w h a t c r i t e r i o n s h o u l d be c h o s e n f o r t h i s l e n g t h . A minimum b r e a k d o w n c r i t e r i o n was a d o p t e d so t h a t a l a g o f 50 u. s e c . o r m o r e , o c c u r r i n g n o t l e s s t h a n e i g h t a n d n o t more t h a n s i x t e e n t i m e s o u t o f t w e n t y - f o u r s h o t s was t a k e n as a s p a r k b r e a k d o w n . I t r e m a i n s now o n l y t o m e n t i o n t h e i n i t i a l p o i n t s on a n y r e c o v e r y c u r v e a t t h e v e r y s h o r t d e l a y t i m e s . A t t h e s e t i m e s , t h e g a s , b e i n g h e a v i l y i o n i z e d , d o e s n o t b e h a v e i n t h e s i m p l e way i t d o e s a t l o n g d e l a y t i m e s . T h i s i s d e m o n s t r a t e d b y t h e a p p e a r a n c e o f d i f f e r e n t t y p e s o f t r a n s i e n t s on t h e C . R . O . I n F i g . 2 a r e a s e r i e s o f p h o t o g r a p h s o f e v e r y t y p e o f t r a n s i e n t t h a t was o b s e r v e d t o g e t h e r w i t h i n t e r p r e t a t i o n s o f t h e m . A l l t h e p h o t o g r a p h s were t a k e n o n a 50 | i s e c . t i m e b a s e . A 6 Fig.- 2. Observed transients 7 a . U n i t f u n c t i o n p r o b i n g v o l t a g e . b . S p a r k b r e a k d o w n . c . F a s t c o n t i n u o u s b r e a k d o w n . H e r e t h e d i e l e c t r i c p r e s e n t s s u d d e n l y a s m a l l f i n i t e i m p e d a n c e t o t h e p a s s a g e o f c u r r e n t a c r o s s t h e g a p . The i m p e d a n c e d r o p s e v e n t u a l l y t o z e r o . d . S l o w c o n t i n u o u s b r e a k d o w n . H e r e t h e d i e l e c t r i c p r e s e n t s a l a r g e r i m p e d a n c e t o t h e p a s s a g e o f c u r r e n t . T h i s i m p e d a n c e d r o p s e v e n t u a l l y t o z e r o . e . S l o w c o n t i n u o u s b r e a k d o w n f o l l o w e d b y f a s t c o n t i n u o u s b r e a k d o w n . The i m p e d a n c e o f t h e d i e l e c t r i c c h a n g e s . f . F a s t c o n t i n u o u s b r e a k d o w n f o l l o w e d b y r e c o v e r y . The d i e l e c t r i c r e c o v e r s a f t e r h a v i n g p a s s e d c u r r e n t . g . Same a s i n f . h . S p u r i o u s b r e a k d o w n i n a p p a r a t u s . i . Same a s i n h . The i n i t i a l p o i n t i n a n y r e c o v e r y c u r v e i s t h a t p o i n t a t s h o r t d e l a y t i m e s a t w h i c h a n y o f t h e above t r a n s i e n t s a p a r t f r o m a . a n d ,b. f i r s t a p p e a r . 2 . 3 T e m p e r a t u r e D e r i v a t i o n . The t e m p e r a t u r e o f t h e g a s e o u s d i e l e c t r i c may be d e d u c e d f r o m i t s r e c o v e r y c h a r a c t e r i s t i c s b y t h e a s s u m p t i o n o f P a s c h e n ' s L a w . D u r i n g t h e f i n a l r e c o v e r y p e r i o d t h e r e i g n i t i o n c h a r a c t e r i s t i c f o r a p a r t i c u l a r gap d e p i c t s , t h e manner i n w h i c h t h e gas d e n s i t y v a r i e s w i t h t i m e . The gas d e n s i t y c o r r e s p o n d i n g t o e a c h r e i g n i t i o n v o l t a g e ^ m a y be determined by measuring the impulse breakdown voltage as a function of pressure for the test gas at ambient temperature. For a given reignition voltage V r the corresponding gas density pr i s equal to the test gas density which gives a breakdown voltage V r at ambient temperature. By setting the densities i n terms of gas pressure and temperature i t i s possible to derive the recovered gas temperature T r as T r .= TPr/P where P and T refer to the test gas conditions and P r i s the equivalent gas pressure of the recovery tests. PV = nRT = SL RT = mkT. P = ™ kT = p kT N 0 V CHAPTER ,3 EXPERIMENTAL METHOD 3 . 1 Summary o f t e c h n i q u e The t e c h n i q u e e m p l o y e d h e r e f o r t h e r e c o v e r y t i m e p r o f i l e s 9 i s s i m i l a r t o t h a t u s e d b y C h u r c h i l l , i n h i s w o r k on c u r r e n t p u l s e s . A f t e r p a s s i n g t h e s u r g e c u r r e n t t h r o u g h t h e , t e s t g a p , a p e r i o d o f t i m e i s a l l o w e d d u r i n g w h i c h t h e d i e l e c t r i c f r e e l y r e c o v e r s . Then a t a p r e d e t e r m i n e d t i m e , a f t e r d i s c h a r g e , t h e u n i t f u n c t i o n v o l t a g e i s a p p l i e d a c r o s s t h e t e s t gap a n d t h e r e i g n i t i o n v o l t a g e i s o b s e r v e d . The r e i g n i t i o n v o l t a g e was d e t e r m i n e d f o r e a c h p o i n t a c c o r d i n g t o t h e c r i t e r i a a l r e a d y d i s c u s s e d . T h i s p r o c e d u r e was r e p e a t e d a t v a r i o u s d e l a y t i m e s . 3 . 2 O v e r a l l o p e r a t i o n o f r e c o v e r y m e a s u r i n g s y s t e m The o v e r a l l o p e r a t i o n o f t h e r e c o v e r y m e a s u r i n g s y s t e m i s d e s c r i b e d w i t h r e f e r e n c e t o t h e b l o c k d i a g r a m i n F i g . 3 . The c a p a c i t o r bank and the . r e s t r i k i n g v o l t a g e g e n e r a t o r a r e c h a r g e d t o a n y d e s i r e d v o l t a g e up t o 20 K . V . b y t h e v a r i a c c o n t r o l l e d E . H . T . s e t . The h i g h c u r r e n t d i s c h a r g e i s i n i t i a t e d b y a p u l s e f r o m t h e m a i n t r i g g e r u n i t and t h e e v e n t s t a k e p l a c e i n t h e f o l l o w i n g s e q u e n c e : a v o l t a g e i n d u c e d i n a p i c k u p c o i l p l a c e d a r o u n d t h e e a r t h e l e c t r o d e 9 E.H.T. CHARGING SET s CAPACITOR BANK CURRENT PICK UP COIL AND C.R.O. / / -MANUAL CONTROLS s ELECTRONIC DELAY UNIT / SPARK CHAMBER AND ELECTRODE SYSTEM s \ > > f E.H.T. CHARGING SET \ RESTRIKING VOLTAGE GENERATOR POTENTIAL DIVIDER AND C.R.O. ? F i g . 3. Block diagram of apparatus t r i g g e r e d t h e d e l a y u n i t , w h i c h , a f t e r t h e s e t d e l a y t i m e , f i r e d t h e r e s t r i k i n g v o l t a g e g e n e r a t o r w h i c h a p p l i e d t h e u n i t f u n c t i o n v o l t a g e a c r o s s t h e t e s t g a p . F i g . 4 shows a s c h e m a t i c r e p r e s e n t a t i o n o f t h e g e n e r a t o r c i r c u i t w h i c h t o a c l o s e a p p r o x i m a t i o n i s a s e r i e s RLC c i r c u i t . C I R C U I T PARAMETERS PARAMETER VALUE Number o f c a p a c i t o r s 2 C a p a c i t a n c e o f e a c h c a p a c i t o r 5 u . . F . W o r k i n g v o l t a g e 20 K . V . Maximum e n e r g y 2 K . J . T o t a l c i r c u i t i n d u c t a n c e 0 . 4 2 u - . H . P e a k c u r r e n t 40 K . A . 3 . 3 E x p e r i m e n t a l : p r e c a u t i o n s I n w o r k i n g w i t h s p a r k g a p s , t h e f o l l o w i n g p r e c a u t i o n s m u s t be o b s e r v e d t o e n s u r e t h a t t h e w o r k i s a t a l l t i m e s b e i n g done u n d e r c a r e f u l l y c o n t r o l l e d c o n d i t i o n s . 1 . The s p a r k chamber must be v a c u u m - t i g h t . F o r , i f a i r w e r e t o l e a k i n d u r i n g a r u n , t h e n t h e w a t e r v a p o u r and d u s t t h a t a r e a s s o c i a t e d w i t h i t c o u l d a l t e r t h e r e s u l t s . F o r e x a m p l e , d u s t p a r t i c l e s on t h e e l e c t r o d e s w o u l d r e d u c e d r a s t i c a l l y t h e s p a r k i n g p o t e n t i a l b y c h a n g i n g t h e f i e l d u n i f o r m i t y w h i l e t h e p r e s e n c e o f w a t e r v a p o u r w o u l d s p o i l : t h e p u r i t y o f t h e g a s s a m p l e u s e d and t h e p a s s i n g o f h e a v y s u r g e c u r r e n t s t h r o u g h t h e h y d r o g e n - o x y g e n m i x t u r e , F i g . 4 . S c h e m a t i c c i r c u i t f o r m a i n b a n k 13 could lead to rather dangerous consequences. Accordingly, a great deal of e f f o r t was expended i n applying standard vacuum techniques ^ with the end re s u l t of a ne g l i g i b l e leak rate of 43 microns/hour. 2. The purity of the gas sample must be insured. Specially p u r i f i e d research hydrogen of 99.99% purity was used. 3. The presence of residual water vapour must be guarded against. The vacuum pump was always l e f t on overnight and the following day, before the f i r s t run, the chamber was flushed out three times with the dry research hydrogen. 4. Dust on the electrodes must be eliminated because t h i s could produce i r r e g u l a r i t i e s on the electrode surfaces so charging the uniformity of the f i e l d and lowering the sparking potential. To guard' against t h i s , a fter the flushing process 3., the capacitor bank was f i r e d s i x times without damping r e s i s t o r s . The r e s u l t i n g 88 K.A. peak current was enough to burn these i r r e g u l a r i t i e s away. Afterwards the chamber was flushed three times more. In the case of the probe, i t was allowed to spark f r e e l y 100 times and the same flushing process was then repeated. 5. Care must be taken to ensure the consistent accuracy of the r e i g n i t i o n voltage VR (Fig. 4). This was measured by a d.c. measurement of the voltage to which the r e s t r i k i n g voltage capacitor was charged by means of an avometer i n series with a 5.3 M.J"X . d.c. m u l t i p l i e r . The avometer was i n i t i a l l y calibrated to within 5% and then the amplitude of the transient wave form appearing across the 14 p o t e n t i a l d i v i d e r was m e a s u r e d f o r a v o m e t e r r e a d i n g s o f 2 . 5 K . V . , 5 K . V . , 7 . 5 K . T . , a n d 10 K . V . and t h e r e s u l t s n o t e d . A t t h e end o f e v e r y r u n t h e r e a f t e r , t h e a v o m e t e r r e a d i n g was c h e c k e d f o r t h e above f o u r v a l u e s a g a i n s t t h e t r a n s i e n t wave f o r m s . They a g r e e d t o w i t h i n 5%. T h i s m e t h o d e n s u r e s a c o n t i n u a l c r o s s c h e c k on t h e p o t e n t i a l d i v i d e r , a v o m e t e r , a n d power s u p p l y r e a d i n g s i m u l t a n e o u s l y . 6 . -A f u r t h e r i m p o r t a n t p a r a m e t e r was t h e v o l t a g e t o w h i c h t h e c a p a c i t o r bank was c h a r g e d . I t was f o u n d t h a t a f t e r t h e r u n a t . 4 K i A . t h e v o l t a g e a s r e a d on t h e power s u p p l y was d e p e n d a b l e o n l y t o a b o u t 10% o w i n g t o t h e a g e i n g o f t h e s h u n t r e s i s t o r . T h i s was overcome, b y b u i l d i n g a n o t h e r a v o m e t e r i n t o t h e e q u i p m e n t w h i c h w o u l d g i v e a d . c . m e a s u r e o f t h e v o l t a g e t o w h i c h t h e b a n k was c h a r g e d . . The two a v o m e t e r s were c h e c k e d a g a i n s t e a c h o t h e r t w i c e p e r d a y . 7 . G r e a t c a r e was t a k e n t o e n s u r e t h a t t h e s e p a r a t i o n o f t h e e l e c t r o d e s i n t h e probe, r e m a i n e d c o n s t a n t . I f i t c h a n g e d i t f o l l o w s t h a t t h e f i e l d c o n f i g u r a t i o n w o u l d change a n d h e n c e so w o u l d t h e s p a r k i n g p o t e n t i a l . A f t e r t h e gap s e p a r a t i o n h a d b e e n c h e c k e d b y means o f s l i p d i s c s whose s i z e s h a d b e e n m e a s u r e d by a m i c r o m e t e r , and a l l t h e s t a n d a r d p r e c a u t i o n s a l r e a d y m e n t i o n e d h a d b e e n t a k e n , t h e s p a r k i n g p o t e n t i a l V g was m e a s u r e d , a n d h e n c e f o r t h c h e c k e d a f t e r e v e r y t e n s h o t s , t h e c r i t e r i o n b e i n g t h a t i n f i v e c o n s e c u t i v e a t t e m p t s , a t l e a s t one w o u l d r e s u l t i n a s p a r k b r e a k d o w n . T h i s ,. c r i t e r i o n was a d o p t e d b e c a u s e i t was f o u n d t h a t a f t e r a l l t h e . above m e n t i o n e d p r e c a u t i o n s h a d b e e n t a k e n , f i v e a t t e m p t s w e r e n e e d e d t o e n s u r e a t l e a s t one s p a r k b r e a k d o w n a t a n y p r e s s u r e . 15 8 . The l a s t important parameter was the distance of the test gap from'the probing gap. This was checked immediately before and af t e r each point on the recovery curves. 9. The shape, and magnitude of the current wave form was checked after every f i f t y shots by examining the dl/dt wave form. CHAPTER 4 APPARATUS 4.1 The spark chamber The main requirements f o r a spark chamber are f a c i l i t i e s f o r v i s u a l observation, sealed surroundings f o r the test gap, and a means by which the capacitor bank could be switched on to the test gap. A sketch of the chamber i s given i n F i g . 5 and i t i s a modified version of a spark chamber used by Churchill"^;.. The walls of the chamber are ^ " thick brass. The high yoltage lead from the capacitor bank could be attached to AA where AA and DD were brass plates to which ^" diameter tungsten slugs had been soft soldered, forming the electrodes of the trigger gap GI, and they were supported by threaded lucite,supports so that with the aid of adjusting nuts, Gl and G2 could be varied. The brass plates EE and J J , had long s o l i d brass cylinders tapped at the ends, into which the threaded brass holders of the \ diameter tungsten slugs forming the anode and cathode of the test gap could be screwed, and held f i r m l y at a given separation by means of f i x i n g nuts. EE had on the side opposite to that withothe brass cylinder another tungsten slug of 5" diameter which formed the cathode of the 16 Fig. 5. Spark Chamber 18 i s o l a t i n g g a p . F F was a 1" t h i c k i n s u l a t i n g p l a t e made o f l u c i t e g r o o v e d on t h e s i d e f a c i n g i n t h e chamber t o i n c r e a s e t h e v o l t a g e t r a c k i n g p a t h and t h u s r e d u c e t h e p o s s i b i l i t y o f s p a r k i n g b e t w e e n t h e anode and t h e s p a r k c h a m b e r , w h i c h was e a r t h e d b y means o f t h r e e t h i c k c o p p e r w i r e s w h i c h w e r e c o n n e c t e d t o a l o n g b r a s s r o d w h i c h was sunk f i f t e e n f e e t i n t o t h e g r o u n d . T h i s r o d was t h e e q u i p m e n t ' s common e a r t h . 4 .2 M e a s u r e m e n t o f c u r r e n t C u r r e n t t r a c e s w e r e o b t a i n e d b y p l a c i n g a p i c k u p c o i l w i t h a b u i l t i n a n a l o g u e i n t e g r a t o r a r o u n d t h e e a r t h e l e c t r o d e o f t h e s p a r k c h a m b e r , a n d f e e d i n g t h e i n d u c e d v o l t a g e i n t o a C . R . O . The v a l u e o f t h e RC c o m b i n a t i o n was c h o s e n so a s t o be v e r y much g r e a t e r t h a n t h e n a t u r a l t i m e c o n s t a n t o f t h e c o i l . The c o i l ' s e q u i v a l e n t c i r c u i t shown i n F i g . 6 i s a n a l y z e d b e l o w . L e t V and v be t h e p o t e n t i a l s i m p r e s s e d a c r o s s t h e c o i l a n d t h e c a p a c i t o r r e s p e c t i v e l y w h e r e v = V a n d V = V ^ n . Then f o r l a r g e v a l u e s o f orCR v / V = ( 1 / j w C ) / ( R + 1/j cu- C ) = 1/j or C T h e n , ' i f T i s t h e p e r i o d o f t i m e d u r i n g w h i c h t h e s u r g e c u r r e n t p u l s e f l o w s t h r o u g h t h e s p a r k c h a m b e r ' s e a r t h e l e c t r o d e S u p p o s e now t h a t t h e c u r r e n t f l o w i n g t h r o u g h t h i s e l e c t r o d e i s I and t h a t t h e c o e f f i c i e n t o f m u t u a l i n d u c t a n c e i s M . Then Fig. 6. Equivalent circuit of Rogowski Coil and integrator 20 f T A s s u m i n g now t h a t yQ V d t i s f i n i t e t h e n f r o m t h e above e q u a t i o n s , v = --M.I. RC T h u s , t h e p o t e n t i a l i m p r e s s e d a c r o s s t h e o s c i l l o s c o p e i s p r o p o r t i o n a l t o t h e c u r r e n t f l o w i n g t h r o u g h t h e e a r t h e l e c t r o d e . The p e a k c u r r e n t c a n t h e n be f o u n d b y a n u m e r i c a l i n t e g r a t i o n o f t h e c u r r e n t wave f o r m shown i n F i g . 7 . 4 . 3 R e s t r i k i n g v o l t a g e g e n e r a t o r a n d t r i g g e r u n i t The r e s t r i k i n g v o l t a g e g e n e r a t o r and t r i g g e r u n i t a r e e a c h 9 i d e n t i c a l t o t h o s e u s e d b y C h u r c h i l l . 4 . 4 The p o t e n t i a l d i v i d e r f o r m e a s u r i n g r e s t r i k i n g v o l t a g e A n y p o t e n t i a l d i v i d e r c o n n e c t e d a c r o s s a s p a r k gap i s e q u i v a l e n t t o c o n n e c t i n g a n i m p e d a n c e i n p a r a l l e l w i t h t h e i m p e d a n c e o f t h e g a p . The c h o i c e o f p o t e n t i a l d i v i d e r , t h e r e f o r e w o u l d h a v e t o be one w h i c h i n t e r f e r e d as l i t t l e a s p o s s i b l e w i t h t h e i m p e d a n c e o f t h e g a p . A s p l i t c a p a c i t o r d i v i d e r ^ w i t h a l o w i n p u t c a p a c i t o r a s shown i n F i g . 8_was a d o p t e d . The r e s i s t a n c e s R, a r e t h e t e r m i n a t i n g r e s i s -t o r s . C o n n e c t i n g t h i s a c r o s s t h e gap i s e q u i v a l e n t t o c o n n e c t i n g a c a p a c i t o r C a c r o s s i t w h e r e C i s c l ( ° 2 + C 3 + C 4 ) / ( ° 1 + C 2 + C 3 + C 4 ) a n d , r e f e r r i n g t o t h e n u m e r i c a l v a l u e s g i v e n i n F i g . 8 , t h i s i s n e g l i g i b l e compared w i t h t h e c a p a c i t a n c e o f t h e g a p . F i g . 9 shows t h e b e h a v i o u r o f t h e p o t e n t i a l d i v i d e r when b e i n g c a l i b r a t e d . By d i r e c t m e a s u r e m e n t , t h e d i v i s i o n r a t i o i s 1/189 t o w i t h i n a 10% a c c u r a c y a n d t h e maximum r i p p l e o u t p u t i s 3%. 21 22 a. b. c. Input voltage V, sensitivity 20 volt/cm. Output voltage v, sensitivity 0.1 volt/cm. Ripple voltage, sensitivity 0.05 volt/cm. 23 4 - 5 The p r o b e f o r m e a s u r i n g r a d i a l d e p e n d e n c e o f r e c o v e r y A s k e t c h o f t h e p r o b e i s e x h i b i t e d i n F i g . 1 0 . The 3/16" d i a m e t e r t u n g s t e n e l e c t r o d e s AA w e r e h e l d f i r m l y i n b y f i x i n g n u t s B , i n c y l i n d r i c a l i n s u l a t i n g p e r s p e x s l u g s C , w h i c h i n t u r n w e r e h e l d b y t h e i r o n r o d s ' D E s c r e w e d i n t o t h e r e c t a n g u l a r p e r s p e x s h i e l d G, w h i c h was t a p p e d on o p p o s i t e f a c e s t o a l l o w D E , t a p p e d a t e a c h e n d , t o be s c r e w e d i n . J i s a p e r s p e x r o d w h i c h c o u l d s l i p t h r o u g h a h o l e i n t h e w i n d o w o f t h e s p a r k c h a m b e r . The 0 - r i n g i n t h e s h i e l d KK s e r v e s t h e p u r p o s e o f s e a l i n g t h e c h a m b e r . The h i g h v o l t a g e and e a r t h l e a d s w e r e l e d t h r o u g h t h e same w i n d o w v i a s c r e w s , on w h i c h s t a n d a r d vacuum t e c h n i q u e s h a d b e e n o b s e r v e d . 4 . 6 S p a r k chamber a n d t e s t gap The o v e r a l l v i e w o f t h e s p a r k chamber a n d t e s t gap i s shown i n F i g . 1 1 . 4 . 7 The d a m p i n g r e s i s t o r A c u r r e n t wave f o r m w h i c h w o u l d r i s e f r o m z e r o t o a maximum a n d t h e n d r o p b a c k t o z e r o i n a f i n i t e t i m e was d e s i r a b l e t o p r o v i d e a r e p r o d u c i b l e and d e f i n i t e gap c o n d i t i o n a t ' t h e commencement o f r e c o v e r y . T h i s a l l o w s a n i n v e s t i g a t i o n o f r e c o v e r y t o be made c l o s e t o c u r r e n t z e r o . I f a c r i t i c a l l y damped c u r r e n t i s u s e d t h e n t h i s c o n d i t i o n c a n n o t be r e a l i z e d s i n c e t h e c u r r e n t w i l l a p p r o a c h z e r o a s s y m t o t i c a l l y . F o r t h i s r e a s o n a 6" d i a m e t e r s i l i c o n c a r b i d e n o n l i n e a r r e s i s t o r was u s e d . The b e h a v i o u r o f t h e c u r r e n t f l o w i n g t h r o u g h i t i s described" 1 "" 1 " i n t e r m s o f t h e v o l t a g e d r o p a c r o s s i t b y V = klP Perspex c B= Iron Tungsten E A A Perspex Tungsten B E D Iron Perspex Perspex 4=8 =§ K Fig. 10. The probe 25 F i g . 1 1 . T h e spark chamber and electrode system 26 where k and 3; are constants. For appropriate parameters the required current wave form w i l l be obtained. However these r e s i s t o r s had the great drawback of having a rather l i m i t e d l i f e . They shattered with-out f a i l a f t e r at most sixteen hundred shots. The current trace, at the instant of shattering, i s shown i n F i g . 7. Photographs of the dl/dt wave forms taken a f t e r every f i f t y shots showed that the character-i s t i c behaviour of the r e s i s t o r does not depend on age. CHAPTER .5 PRELIMINARY EXPERIMENTS 5 . 1 D e p e n d e n c e o f r e i g n i t i o n v o l t a g e s o f t h e t e s t gap on c e r t a i n p a r a m e t e r s The r e i g n i t i o n v o l t a g e as a f m i c t i o n o f t i m e a f t e r i n i t i a t i o n o f t h e m a i n d i s c h a r g e c u r r e n t h a s b e e n m e a s u r e d f o r t h e f o l l o w i n g p a r a m e t e r s : c u r r e n t m a g n i t u d e , c u r r e n t wave f o r m , a n d gas p r e s s u r e . The e f f e c t o f c u r r e n t m a g n i t u d e i s c l e a r l y shown b y c o m p a r i s o n o f F i g . 13 w i t h F i g . 16 w h i c h d i s p l a y r e s p e c t i v e l y t h e r e c o v e r i e s a f t e r i n i t i a t i o n o f a n g l e c u r r e n t p u l s e s o f 4 k A a n d 40 kA w h e r e t h e o u t s t a n d i n g f e a t u r e i s t h e much s l o w e r r e c o v e r y o f t h e gap f o r t h e s m a l l e r c u r r e n t . B o t h f i g u r e s t o o , d i s p l a y t h e s t r o n g e f f e c t o f gas p r e s s u r e , a l t h o u g h t h e n o r m a l i z e d r e c o v e r y c u r v e s shown r e s p e c t i v e l y i n F i g . 14 and F i g . 17 a r e a l m o s t c o i n c i d e n t . Gas t e m p e r a t u r e s f o r b o t h c a s e s a r e d e r i v e d f r o m t h e P a s c h i n c u r v e f o r t h e t e s t gap i n F i g . 12 a n d a r e shown i n F i g s . 15 and 18 r e s p e c t i v e l y . The e f f e c t o f a n o s c i l l a t o r y c u r r e n t wave f o r m o n t h e r e c o v e r y i s shown i n F i g . 19 w h e r e t h e i n t e r e s t i n g f e a t u r e i s a f l a t p o r t i o n o n t h e r e c o v e r y c u r v e s h o w i n g a p e r i o d o f t i m e d u r i n g w h i c h t h e gap i s n o t r e c o v e r i n g . H e r e t o o , i s d i s p l a y e d t h e f a c t t h a t a r e d u c e d p r e s s u r e l o w e r s t h e r e i g n i t i o n v o l t a g e a l t h o u g h t h e n o r m a l i z e d c u r v e s ( F i g . 2 0 ) a r e a l s o a l m o s t c o i n c i d e n t . The t e m p e r a t u r e s d e r i v e d f r o m t h e s e r e c o v e r y c u r v e s a r e shown i n F i g . 2 1 . 27 9 •B "7 6 '5 4 2 1 TEST GAP. \ inch diameter electrodes 3 Electrode material: Tungsten Gas: 'Hydrogen Gap Length: 5 MM 2 I I l I I I I I 5 — 4 _ 1 — PROBE. 3/16 inch diameter electrodes Electrode material: Tungsten Gas: Hydrogen Gap Length: 1 cm. I l I I 1 I I F i g . 12. Breakdown Voltage vs. Pressure to 0 0 29 T a b l e I R e i g n i t i o n v o l t a g e s , p e r c e n t a g e b r e a k d o w n , and d e r i v e d t e m p e r a t u r e s a t d i f f e r e n t p r e s s u r e s i n h y d r o g e n f o r a 4 kA u n i d i r e c t i o n a l s u r g e c u r r e n t Gas p r e s s u r e S p a r k i n g v o l t a g e D e l a y a f t e r R e i g n i t i o n P e r c e n t a g e D e r i v e d mm. H g . V s ( k V ) . i n i t i a t i o n o f v o l t a g e v b r e a k d o w n t e m p e r -surge , c u r r e n t ( k V ) v o l t a g e a t u r e i n m i l l i s e c . 100 I _ d e g . K V s 4 . 5 8 . 8 95 350 9 4 . 0 8 . 5 92 366 760 9 . 2 5 2 5 . 0 7.6^. 82 425 • 1 0 . 4 6 . 0 65 595 7 . 5 5 . 1 55 768 6 . 1 3 . 3 36 2500 6 . 1 7 . 4 96 350 9 4 . 0 6 . 8 87 378 600 . 7 . 7 5 1 9 . 0 5 . 4 70 556 1 0 . 4 4 . 4 57 788 . 7 . 5 3 . 3 42 1990 8 . 0 5 . 8 93 350 400 6 . 2 5 9 4 . 0 5 . 0 80 443 1 9 . 0 3 . 8 61 740 1 0 . 4 3 . 3 53 1440 30 4 inch diameter tungsten electrodes. Electrode separation: 0.5 cm. 9 ~ 2 ~ 1 -O l I I I I M i l l _ J I I I I I I I 1 5 10 50 100 Time after spark i n i t i a t i o n ' i n m i l l i sec. Fig. 13. Test gap recovery i n hydrogen as a function of pressure 31 20 10 I I I I I I I I I I I I I I I I 1 5 10 50 100 Time after spark i n i t i a t i o n i n millisec. Fig. 14. Normalized reignition characteristics for test gap 32 "3 0) •P u ft a -P looo-800-600" \ inch diameter tungsten electrodes Electrode separation: 0.5 cm. Current: 2 kA 2 non linear resistors Working voltage: 20 kV. • 760 mm. X 600 mm. 0 400 mm. 400-200' JL 1 5 10 15 20 Time after spark i n i t i a t i o n i n millisec. 15. Derived gas temperature for test gap i n hydrogen 25 Fig. 33 T a b l e I I R e i g n i t i o n v o l t a g e s , p e r c e n t a g e b r e a k d o w n a n d d e r i v e d t e m p e r a t u r e s a t d i f f e r e n t p r e s s u r e s i n h y d r o g e n f o r a 40 kA u n i d i r e c t i o n a l s u r g e c u r r e n t Gas p r e s s u r e S p a r k i n g v o l t a g e D e l a y a f t e r R e i g n i t i o n P e r c e n t a g e D e r i v e d mm. H g . v s i n i t i a t i o n o f s u r g e c u r r e n t i n m i l l i s e c . v o l t a g e v b r e a k d o w n v o l t a g e 100 y_ v s t e m p e r -a t u r e d e g . K 0.44 3.3 36 2640 0.52 3.8 41 1320 0.69 4.8 52 798 0.89 . 5.8 63 545 760 9.25 1.4 6.3 68- 580 3.4 7.3 79 440 - 6.0 7.5 83' 399 10.4 8.0 87 396 34.0 8.3 90 380 100.0 8.8 95 350 0.44 2.8 36 2100 0.52 3.5 45 1260 0.69 4.5 58 756 0.89 4.8 62 675 600 7.75 1.4 5.3 .68 572 .3.4 5.5 71 525 6.0 6.0 . 77 472 10.4 6.5 84 420 34.0 6.8 88 . 394 100.0 7.3 94 350 0.44 2.8 47 1280 0.52 2 . 8 47 1280 0.69 3.8 63 640 400 6.0 1.4 •4.3 71 500 2.5 4.5 75 460 6.0 4.5 75 460 34.0 5.0 83 390 100.0 5.3 88 350 > CD bO ai +-> H O > fl o •H •P • r l fl SO •rt CO « • 760 mm. X 600•mm. O 400 mm. • I I I 1 I 0.4 J L \ inch diameter tungsten electrodes Electrode separation: 0.5 cm. Current: 40 kA 1 non linear resistor Working voltage: 20 kV. . I I I I I I I I I I I I I I I 10 100 Fig. 16. Time after spark i n i t i a t i o n i n millisec. Test gap recovery i n hydrogen as a function of pressure 4*. 100 90 80 70 60 50 40 -X © 760 mm. 600 mm. 400 mm. 30 I I I I J_L -| inch diameter tungsten electrodes Electrode separation: 0.5 cm. Current: 40 kA. 1 non linear resistor Working voltage: 20 kV. J I I I J _ J I M i l l 0.4 Fig. 17. 10 100 Time after spark i n i t i a t i o n i n millisec. Normalized reignition characteristics for hydrogen •j inch diameter tungsten electrodes Electrode separation: 0.5 cm. Current: 40 kA. 1 non linear resistor Working voltage: 20 kV. f 760 mm. X 600 mm. © 400 mm. — X —© 1 2 3 4 5 . 6 Time after spark i n i t i a t i o n i n millisec. Fig. 18. Derived gas temperature of test gap as a function of pressure 37 Table III Reignition voltages, percentage breakdown and derived temperatures at different pressures i n hydrogen for an oscillatory surge current with a period of 13 IA. sec. Gas pressure Sparking voltage Delay after Reignition Percentage Derived mm.' Hg. i n i t i a t i o n of surge current in millisec. voltage v breakdown voltage 100 Y_ V temper-ature deg. K 0.44 2.0 22 2650 0.89 - 2,8 30 2380 1.9 3.5 38 1590 3.4 4.0 43 1190 6.0 4.0 43 1190 760 9.25 9.1 4.3 47 800 14.0 5.0 54 790 20.0 6.5 70 530 34.0 7.8 84 400 100.6 8.8 ' 95 350 0.44 1.8 23 2260 0.89 2.5 32 2260 1.9 3.3 43 1560 3.4 3.8 49 1130 600 7.75 6.0 3.8 49 1130 9,1 4.0 52 1020 14.0 5.0 65 680 20.0 5.8 75 500 34.0 7.0 90 410 100.0 7.8 100 350 > so n +-> H O > o •H -P •H fl SO •ri Ci « •|: inch diameter tungsten electrodes Electrode separation: 0.5 cm. Current: 88 kA. Oscillatory current Working voltage: 20 kV. • 760 mm. X 600 mm. I I I I I I I I I 1 I I I 0.4 10 100 Fig. 19. Time after spark i n i t i a t i o n i n millisec. Test gap recovery i n hydrogen as a function of pressure oo 100 i 90 80 70 60 \ inch diameter tungsten electrodes Electrode separation: 0.5 cm. Current: 88 kA. Oscillatory current Working voltage: 20 kV. # 760 mm. y. 600 mm. J I I I I I I I 10 100 Time after spark i n i t i a t i o n i n millisec. Fig. 20 Normalized reignition characteristics for hydrogen 3000 \ inch diameter tungsten electrodes • Electrode separation: 0.5 cm. 0 5 10 15 20 25 30 35 Time after spark i n i t i a t i o n i n millisec. Fig. 21. Derived gas temperature for test gap 41 A l l the relevant experimental details of the work with the uni-directional 4 kA, 40 kA pulses and with the oscillatory current wave form are given respectively i n Tables I, II, III . CHAPTER 6 MEASUREMENT OF RADIAL RECOVERY 6.1 Reignition voltage of the test gap The reignition voltages of the test gap were measured as a function of the time after the i n i t i a t i o n of the unidirectional 40 kA main dis-charge current i n hydrogen at atmospheric pressure using the minimum breakdown criterion. The experimental data are given i n Table IV and the recovery curve i s given as shown i n Fig. 22. The reignition voltages of the probe were measured as a function of the time after the i n i t i a t i o n of the surge current in the test gap at different radial distances from the test gap a l l i n hydrogen at atmospheric pressure. The experimental data are given i n Table V and the results for each radial distance are-plotted i n Figs. 23 - 27. Fig. 28 shows a l l the recovery curves, while Fig. 29 describes the radial temperature variation. Comparison of the recovery curves for the test gap and probe at 2 cm. distance indicates that the recovering channel of the test gap i s of 2 cm. diameter. The dips i n the recovery curves at 4 cm., 5 cm., and 6 cm. could be explained i n terms of a diffusion process of hot particles from the test gap. 42 43 Table IV Minimum reignition voltages of the test gap Gas pressure Sparking voltage Delay after Minimum Derived mm. Hg. V s (kV) i n i t i a t i o n of reignition temperatures surge current voltage deg. K. in millisec. (Kl Vi) 0.44 3.5 1320 0.56 ' 4.5 770 0.74 ...5.5 550 0.95 6.0 440 760 9.25 1.2 6.5 385 1.56 7.0 370 2.1 7.3 366 3.4 7.3 366 10.4 .7.5 350 44 > • i SO a -P rH O > fl o •H P •ri fl bO •rl CO 10 ^ inch diameter tungsten electrodes Electrode separation: 0.5 cm. Current: 40 kA. 1 non linear resistor Working voltage: 20 kV. Atmospheric pressure J M i l l 0.1 1.0 Time after spark i n i t i a t i o n i n millisec. Fig. 22. Test gap recovery i n hydrogen 10 45 Table V Minimum reignition voltages of probe at different radial distances from test gap Probe distance Delay after i n i t i a t i o n Minimum reignition Derived gas from test gap of main surge current voltage (K-L Vj) temperatures cm. in millisec. deg. K ,• 0.33 2.5 1400 0.44 3.8 730 0.58 5.0 520 2 0.76 5.8 470 0.95 6.3 420 1.56 6.8 380 3.4 7.3 360 10.4 • 7.5 350 0.2 4.0 670 0.26 4.0 670 0.33 5.0 500 3 0.44 6.0 410 0.58 6.5 400 1.2 6.5 400 3.4 7.0 350 10.4 7.0 350 0.26 7.3 350 0.44 ' 7.0 368 0.56 6.7 370 0.74 4.5 600 4 0.92 4.7 625 1.2 6.0 406 1.56 7.0 375 2.1 7.2 365 3.4 7.3 360 10.4 •7.5- 350 0.1 8.0 350 0.2 7.5 375 0.34 6.5 440 5 0.57 .6.5 440 0.74 6.5 440. 1.2 6.5 440 3.4 7.0 400 10.4 7.0 400 0.1 8.0 350 0.2 8.0 350 0.34 7.0 400 0.44 7.0 400 6 0.56 7.0 400 0.74 7.5 380 1.56 7.3 383 3.4 8.0 350 10.4 .8.0 350 46 10 > CD &0 cti 4-> H O > o •H •P •H •rt CD 05 3/16 inch diameter tungsten electrodes Electrode separation: 10 mm. Atmospheric pressure Probe distance from test gap: 2 cm. J L M i l l J I M i l l 0 0.1 1.0 10 Time after spark i n i t i a t i o n i n millisec. Fig. 23. Reignition characteristic for. probe i n hydrogen 47 10 > xi i <a fac oJ •P H o r> fl O •H •P •H •H ru Pi 3/16 inch diameter tungsten electrodes Electrode separation: 1 cm. Atmospheric pressure Probe distance from test gap: 3 cm. 1 I I I I I I I 1 J 1 I I I I I 0.1 1.0 10 Time after spark i n i t i a t i o n i n millisec. Fig. 24. Reignition characteristic for probe i n hydrogen 48 10 6-3/16 inch diameter tungsten electrodes Electrode separation: 1 cm. Atmospheric pressure Probe distance from test gap: 4 cm. J I I I I I I I J I I L o 0.1 1.0 10 Time after spark i n i t i a t i o n i n millisec. Fig. 25. Reignition characteristic for probe i n hydrogen 49 3/16 inch diameter tungsten electrodes Electrode separation: 1 cm. •Atmospheric pressure Probe distance from test gap: 5 cm. J I I L J I I I I 0.1 1.0 10 Time after spark i n i t i a t i o n i n millisec. Fig. 26. Reignition characteristic for probe i n hydrogen 50 10 > I o W) oj •P H O t> S O •H -P •H a •H CD 1 -0 3/16 inch diameter tungsten electrodes Electrode separation: 1 cm. Atmospheric pressure Probe distance from test gap: 6 cm. J I I I I I I I J I M i l l 0.1 1.0 10 Time after spark i n i t i a t i o n i n millisec. Fig. 27. Reignition characteristic for probe i n hydrogen 51 10 > v ai P rH O t> fl O •ri P •ri fl hO •H (U Pi • 3/16 inch diameter tungsten electrodes Electrode separation: 1 cm. Atmospheric pressure Probe distances from test gap X 2 cm. O 3 cm. A 4 cm. O 5 cm. V 6 cm. • Test gap _L J I L _L_L J I I I I I I 0 . 1 1 . 0 . 10 Time after spark i n i t i a t i o n i n millisec. 'Fig. 28. Reignition characteristics for probe i n hydrogen 52 1200 1000 ~ 800 600 400 200 3/16 inch diameter tungsten electrodes Electrode separation: 1 cm. Gas: hydrogen ' Atmospheric pressure Time a f t e r spark i n i t i a t i o n • 0.33 m i l l i s e c . X 0.44 m i l l i s e c . O 0.58 m i l l i s e c . D 0.74 m i l l i s e c . & 1.0 m i l l i s e c . _L 2 3 4 5 Distance of probe from t e s t gap i n cm. F i g . 29. Derived gas temperatures CHAPTER 7 CONCLUSIONS 7.1 Conclusions - ' "The following conclusions may now be drawn. 1. A l l recovery curves are exponential i n form, their shape depending both on radial distance from the test gap and on the time after i n i t i a t i o n of current flow. 2. The recovering channel i s nearly 2.cm. i n diameter. 3. The passage of the surge current brings into being what seems to be a diffusion of electrified particles through the gas and the assumption of their existence offers an explanation for the be-haviour of the recovery curves. 53 BIBLIOGRAPHY 1. Allibone, T.E., and Meek, J.M., Proc. Roy. Soc. A 166, 97, (1938). 2. Townsend, J.S., "Electrons i n gases", London: Hutchinson, (1947). 3. Fisher, L.H., El e c t r i c a l Engineering 69, 613, (1950). 4. Bellaschi, P.L., El e c t r i c a l Engineering 53, 86, (1934). 5. McCann, G.D., and Clark, J.J., Trans. Amer. Inst. Elec. Engrs. 62, 45, (1943). 6. McCann, G.D., Connor, J.E., and E l l i s , H.M., Trans. Amer. Inst. Elec. Engrs. 69, 616, (1950). 7. Edels, H., and Crawford, F.W., Brit. J. Appl. Phys., 7 (10), 380, (1956) 8. Edels, H., and Crawford, F.W., Proc. Inst. Elec. Engrs. 107, 202, (1960) 9. Churchill, R.J., Journal of Electronics and Control 11, 1, 17, (1961). 10. Burch, F.P., Phil. Mag., 13, 760, (1932). 11. Ashworth, F., Needham, W., and Sellars, R.W., J.I.E.E. 93, 385, (1946). 54 

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