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Design, construction and stabilisation of a large electromagnet 1950

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L £ -b ft 7 (1 So f\i D E S I G N , C O N S T R U C T I O N A N D S T A B I L I S A T I O N O F A L A R G E E L E C T R O M A G N E T b y D a v i d A n d r e w A a r o n s o n A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I 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 T H E D E G R E E O F M A S T E R O F A R T S i n P H Y S I C S • 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 S e p t e m b e r , 1 9 5 0 T H E U N I V E R S I T Y O F BRITISH C O L U M B I A VANCOUVER. CANADA D E P A R T M E N T O F P H Y S I C S October 12, 1950. Dr. L. W. Dunlap, L i b r a r i a n , University of B r i t i s h Columbia. Dear Dr. Dunlap: This w i l l c e r t i f y that the thesis of Mr. D. A. Aaronson has been c a r e f u l l y studied by the undersigned, and that the thesis meets the required standards and an abstract has been approved by the Department. Yours sincerely, G. M. Shrum Head of the Department GMS:1c G. G. Eichholz Assistant Professor of Physics THE UNIVERSITY OF BRITISH COLUMBIA V A N C O U V E R , C A N A D A /?/~<L <^-5 3 ? A B S T R A C T A s e v e n a n d o n e h a l f t o n e l e c t r o m a g n e t h a s " b e e n b u i l t p r i m a r i l y f o r b e a m a n a l y s i s i n c o n j u n c t i o n w i t h 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 e l e c t r o s t a t i c g e n e r a t o r . W i t h a c u r r e n t o f 4 9 a m p e r e s , a f i e l d i n e x c e s s o f 1 9 , 9 0 0 g a u s s o v e r a n a r e a o f 2 5 6 s q u a r e i n c h e s h a s b e e n o b t a i n e d a c r o s s a o n e i n c h a i r g a p . T h e h y s t e r e s i s l o o p i s s a t i s f a c t o r i l y s m a l l , b e i n g 0 . 2 a m p e r e s w i d e a t a m a g n e t i s i n g c u r r e n t o f 1 5 a m p e r e s . C u r r e n t s t a b i l i t y w i t h t h e m a g n e t i s i n g c u r r e n t v a r y - i n g f r o m z e r o t o 3 5 a m p e r e s h a s " b e e n m a i n t a i n e d t o a f e w p a r t s i n 1 0 , 0 0 0 o v e r p e r i o d s a s l o n g a s s e v e n h o u r s . F i e l d s t a b i l i t y h a s b e e n c h e c k e d u s i n g a p r o t o n r e s o n a n c e s i g n a l t o b e o n e p a r t i n 1 0 , 0 0 0 o v e r a s h o r t p e r i o d o f e i g h t m i n u t e s , a n d t o b e t h r e e p a r t s i n 1 0 , 0 0 0 o v e r t h e l o n g p e r i o d o f s e v e n h o u r s . U s i n g t h e s t a b i l i s i n g s y s t e m , t h e t i m e r e q u i r e d t o c h a n g e t h e f i e l d t o a n e w s e t t i n g i s l e s s t h a n t h r e e s e c o n d s . ACKNOWLEDGEMENT Acknowledgement i s g i v e n t o the N a t i o n a l Research C o u n c i l f o r t h e i r b u r s a r y , and t o the Defence Research Board f o r the grant i n support of t h i s research work. Thanks are due t o Dr. J . B. Warren of the Physics Department of the U n i v e r s i t y of B r i t i s h Columbia f o r the h e l p and guidance i n i n i t i a t i n g and c a r r y i n g out t h i s p r o j e c t . D. A. Aaronson S ept emb er, 1950 TABLE OF CONTENTS page Introduction 1 Chapter 1. The magnet (a) Functions of the magnet 5 (b) Pole shape 5 (c) Size 6 (d) S t a b i l i t y 7 (e) Design of magnet 8 (f) P a r t i c u l a r s of magnet construction 10 (g) The magnet c o i l s 11 (h) The water cooling system 12 (i ) The magnetic f i e l d 13 Chapter 2. The power supply f o r the magnet c o i l s (a) The generators 14 Chapter 3. The regulating system 16 (a) Operation 17 (b) Precision of the system 17 Chapter 4. The units of the regulator (a) Standard r e s i s t o r 20 (b) Standard reference voltage 21 (c) Brown converter and amplifier 22 (d) Current regulating tubes 23 (e) Power unit 24 (f) Protective devices 25 p a g e C h a p t e r 5 . O p e r a t i o n o f t h e c o n t r o l s y s t e m (a) G e n e r a l s w i t c h i n g 2 6 Ob) T h e m o t o r g e n e r a t o r s 2 7 ( c ) T h e R u b i c o n p o t e n t i o m e t e r 2 7 ( d ) M a n u a l - a u t o m a t i c s w i t c h a n d D . C . l e v e l c o n t r o l 2 8 ( e ) C u r r e n t a n d f i e l d s e t t i n g o f t h e m a g n e t 2 8 C h a p t e r 6 . E x p e r i m e n t a l r e s u l t s (a) D r i f t o f t h e m a g n e t c u r r e n t a n d m a g n e t i c f i e l d 3 1 ( b ) A m p l i f i e r a n d r e g u l a t i n g s y s t e m 3 2 ( c ) P e r f o r m a n c e o f t h e m a g n e t 3 4 ( d ) P r o t o n r e s o n a n c e m e a s u r e m e n t o f f i e l d d r i f t 3 5 ( e ) P r o t o n r e s o n a n c e R . P . h e a d 3 7 ( f ) S i m p l e p r o t o n r e s o n a n c e t h e o r y 3 7 ( g ) F u t u r e u s e o f p r o t o n r e s o n a n c e f o r s t a b i l i z a t i o n 3 8 A p p e n d i x 1 . M e a s u r e m e n t o f f i e l d d r i f t u s i n g t h e p r o t o n r e s o n a n c e ^ m e t h o d ( a ) C a l i b r a t i o n o f t h e o s c i l l o s c o p e 4 0 (b) S h o r t t i m e t e s t f o r 8 m i n u t e s 4 1 ( c ) S e v e n h o u r t e s t 4 2 Appendix 2. Thermal e f f e c t s i n the magnet (a) E f f e c t of temperature change on the magnetic f i e l d (h) Estimated time f o r magnet to cool Appendix 3. Adjustment of the control system necessary f o r optimum operation Appendix 4 . Use of f l i p - c o i l s and fluxmeter Bibliography I l l u s t r a t i o n s Figure Facing page 1. Perspective Sketch of Magnet 5 2. Sketch of Pancake C o i l 5 3. Sketch of Magnet 8 4. The Electromagnet 10 5. The Electromagnet 10 6. D. C. Generator Curves 14 7. Current Regulator Block Diagram 16 8. Amplifier and Power Supply C i r c u i t 22 9. Regulator Tube Chassis 23 10. Current Regulator; Wiring and power unit 24 11. The Control Panel 28 12. Hysteresis Loop and Magnetization Curve 34 13. F a l l - o f f of F i e l d 34 14. Proton Resonance R. F. Head C i r c u i t ;37 15. Proton Resonance R. F. Head 37 INTRODUCTION The primary consideration i n the design of a large electromagnet i s to get the largest f i e l d reasonably possible with an i r o n core ( upper l i m i t about 20,000 gauss ) over the pole area required f o r the l e a s t cost. The cost n a t u r a l l y depends on the pole faces and size of a i r gap which are chosen f o r the s p e c i f i c uses to which the magnet w i l l be put. However, the siz e of the magnet i s governed also by the type of c o i l s , e specially by t h e i r a b i l i t y to dis s i p a t e the heat generated i n them, as well as by the kind of i r o n and type of i r o n c i r c u i t used. These s e t t l e the window area required f o r the c o i l which i n turn dictates the dimensions of the i r o n of the magnet. Since the cost of the magnet increases with the siz e and weight, with due regard to the r e l a t i v e cost of copper and ir o n , the best design hinges on maximum weight economy. This economy depends on whether more i r o n and les s copper or more copper and l e s s i r o n can be used. Again, the leakage fa c t o r increases with increasing dimensions, e s p e c i a l l y 1, 2 of the a i r gap , therefore, the smaller the magnet, and a i r gap, the larger the percentage of usefu l f l u x . The minimum s i z e of the magnet i s governed by the saturation i n the i r o n used, which saturation usually - 2 - occurs f i r s t i n the i r o n j u s t behind the pole pieces. Preliminary calculations on the minimum path length and magnetomotive force f o r the a i r gap, with a minimum window area assumed from experience, give a tentative size f o r the magnet. Computations on the siz e of the c o i l needed may then be made by a few successive approximations, once the type of power supply has been decided upon. Then, i f the p a r t i c u l a r c o i l cannot radiate the power dissipated with a reasonable temperature r i s e , a larger c o i l , more turns, fewer amperes, but the same ampere-turns must be t r i e d . With t h i s , the calculations are repeated f o r the larger window area and length of ir o n path now needed. I t i s seen, then, that the best o v e r a l l design depends a great deal on adequate and e f f i c i e n t heat transfer from the c o i l s . 1 There i s a choice of a i r cooling, forced v e n t i l a t i o n ^ ) water or o i l cooling. D i f f e r e n t types of i n s u l a t i o n on the c o i l s also allow higher ambient working temperatures. Water cooling which i s the le a s t expensive and conserves most space and asbestos covering on the c o i l s were f i n a l l y chosen f o r t h i s electromagnet. A one quarter scale model was made up and i t s behaviour and f i e l d d i s t r i b u t i o n cheeked, but i t was not possible on such a model to check the c o i l design which -3- therefore was rather conservatively rated i n regard to heat d i s s i p a t i o n and working temperature. However sat- uration c h a r a c t e r i s t i c s were checked on the model using short pulses of current. Following t h i s , the seven and one h a l f ton electro- magnet was designed by members of the Physics s t a f f of the University of B r i t i s h Columbia a f t e r c a r e f u l consideration had been given to a l l the above fa c t o r s . I t was to supply a f i e l d of at l e a s t 15,000 gauss over an area of 16 inches square across an a i r gap of one inch and to do so at a t o t a l cost of about $5,000.00. The design made use of an ingeneous method of winding the c o i l s to conserve space and at the same time to provide adequate water cooling. The design was found to be quite conservative. A f i e l d of over 19,900 gauss was obtained with a magnetising current of close to 49 amperes and a power of about 11 kilowatts. The t o t a l cost was near the fi g u r e stated above. The magnet yoke and pole pieces were made by Messrs. C o l v i l l e of Glasgow f o r about $3,000.00; the c o i l s were made "by Canadian General E l e c t r i c f o r about $1,000.00. The S t a b i l i z a t i o n equipment has cost about $1,000.00# M Dr. J. B. Warren and Mr. F. Bowers c a r r i e d out most of the ca l c u l a t i o n s . Mr. T. Mouat assisted i n the engineering aspects of the magnet and c o i l s . In the f i r s t instance to achieve the energy r e s o l u t i o n required, i t was decided to s t a b i l i z e the magnetising current to the required p r e c i s i o n and check the v a r i a t i o n i n the magnetic f i e l d using a proton resonance method'. F i n a l l y i t i s intended to s t a b i l i z e the f i e l d i t s e l f using the proton resonance output. t_o •fate f>. S~. "5- Chapter l t The Magnet (a) Function of the magnet The magnet was designed to "bend a f i v e mev. "beam of protons and deuterons through a 90 degree angle f o r energy- resolution. ("b) Pole Shape I t s general.shape i s shown i n f i g . 1 . Various possible pole piece shapes were considered f o r t h i s pur- pose but i t seemed very desirable to be able to bend the i o n beam to the l e f t or to the r i g h t without s h i f t i n g the magnet so that two experiments might be set up together. Consequently the other most considered design, that of a single quarter c i r c l e pole piece with banana c o i l s wound round the poles with the whole magnet on a swivel, was given up. After examining possible shapes of pole taper needed to give the f i e l d a c t u a l l y required at the gap, and a round yoke on which to wind the poles, a square pole t i p was chosen. This gave a very conventional design which i s e a s i l y adapted f o r other experiments requiring a large H ̂  such as spectrograph applications or even f o r a cloud chamber; and provision was therefore made f o r -6- a l t e r i n g t h e g a p w i d t h a n d p o l e s h a p e , ( c ) S i z e C o n s i d e r a n i o n o f c h a r g e e, m a s s m , m o v i n g w i t h v e l o c i t y v , e n t e r i n g a m a g n e t i c f i e l d o f s t r e n g t h H . I t w i l l h e b e n t i n a n a r c o f r a d i u s p s u c h t h a t H e v - m v 2 H p - m v e I n a c o n s i s t e n t s e t o f u n i t s , c . g . s . , e . m . u . ; w i t h H i n e . m . u . a n d e i n e . s . u . w e h a v e , w h e r e e i s t h e v e l o c i t y o f l i g h t , H p - m v c v e I f t h e i o n a t t a i n e d t h e v e l o c i t y v b y b e i n g a c c e l - e r a t e d t h r o u g h a n e l e c t r i c f i e l d o f s t r e n g t h E , t h e n , 1/2 m c v 2 z e E r " v = / 2 e _ V m c E t h e r e f o r e H P a / 2 m c T h e v a l u e o f H 0 r e q u i r e d t o d e f l e c t i o n s a c c e l e r a t e d \ 4 t h r o u g h a n e l e c t r i c f i e l d o f f i v e m i l l i o n v o l t s i s : f o r p r o t o n s , 3 . 2 2 x 1 0 g a u s s c m . f o r deuterons 3.22 x 10 5 - 4.55 x 10 5 gauss cm., and r— 5 5 f o r t r i t o n s 7 3 x 3.22 x 10 = 5.6 x 10 gauss cm. Protons could "be deflected i n both d i r e c t i o n with a 5 f i e l d o f : 3.22 x 10 = 16,100 gauss i n a radius of 20 20 cms.. Deuterons i n one d i r e c t i o n with a f i e l d of : 5 4.55 x 10 - 15,200 gauss i n a radius of 30 30 cms.. Tritons i n one d i r e c t i o n only with a f i e l d o f: 5 5.6 x 10 - 16,000 gauss i n a radius of 35 35 cm.. Therefore, a square pole t i p of size 16 inches on a side, equal to 40.6 cm. on a side was decided on. This then set the size of the round pole pieces and thus the yoke cross section. (d) S t a b i l i t y Since t h i s molecular beam would be required f o r the 5 Van de Graaff e l e c t r o s t a t i c generator s t a b i l i z a t i o n , and also f o r energy r e s o l u t i o n of three kev. i n f i v e mev., we must require the A H to be equal to three parts i n H . — • 10,000; as from (2) p 4 H = / 2 m c 1/2 _E e and 4_H = j/2 4_E H E When current s t a b i l i z a t i o n i s used 4 I must be less I than A H because of the following f a c t o r s , a l l of the H . g order of a few parts i n 10 or less 1. The width of the a i r gap varies due to the thermal expansion of the i r o n and the magnetic force bending the i r o n 4' Ma q n et yoke and pole p i e c e s -8- 2. The incremental permeability of the i r o n and 6 hysteresis vary with the previous magnetic h i s t o r y . 3. There i s a small mechanical hysteresis i n the \ i r o n of the pole pieces, b o l t s and nuts. (e) Design of Magnet The design of the magnet and c o i l s followed i n t h i s manner: Take H p as 6 x 10 gauss cm. and consider a maximum H of 15,000 gauss across a one inch gap, allowing a beam siz e of one h a l f inch. The pole t i p s , to be square, w i l l be about 40 cm. on a side or 16 inches square; therefore the pole t i p area i s 256 square inches. Assume a leakage c o e f f i c i e n t of 1.8 : T o t a l l i n e s of force from-pole to pole H at centre of gap x pole t i p area Now choose the round diameter of 16/2 - 23- inches g i v i n g an area of the round poles of 2 tTD = 415 square inches 4 With t h i s and the leakage c o e f f i c i e n t , the i n t e n s i t y i n the i r o n of the round poles increases t o : 1.8 x 15.000 x 256 = 16,650 gauss ( 11.5)* Consider the yoke area to be 16 x 26 = 416 square inches -9- T h e n t h e m a g n e t o m o t i v e f o r c e r e q u i r e d , a s s u m i n g 8 0 o e r s t e d s p e r i n c h f o r 1 5 , 0 0 0 l i n e s / s q . c m . f o r a l o w c a r b o n s t e e l , a n d a n i r o n p a t h l e n g t h o f 1 2 3 i n c h e s , i s e q u a l t o H a i r 1 a i r - r - H i r o n 1 i r o n = 1 5 , 0 0 0 x 1 x 2 . 5 4 - h 8 0 x 1 2 3 x 2 . 5 4 = 3 8 , 1 0 0 + 2 5 , 0 0 0 = 6 3 , 1 0 0 e r g s T h e r e f o r e t h e t o t a l 4 TT N I = 6 3 , 1 0 0 1 0 a n d t h e a m p e r e t u r n s r e q u i r e d , N I = 6 3 . 1 0 0 x 1 0 - 5 0 , 3 0 0 " 4 a m p e r e t u r n s C o n s i d e r i n g a m a x i m u m c u r r e n t o f 4 5 a m p e r e s , t h e n t h e n u m b e r o f t u r n s w o u l d b e N = 5 0 . 3 0 0 = 1 1 1 8 t u r n s 4 5 C o p p e r t u b i n g , w h i c h c o u l d c a r r y t h e w a t e r a s w e l l a s t h e e l e c t r i c c u r r e n t , w a s p r e f e r r e d . T h e l o w e r l i m i t o n t h e b o r e d i a m e t e r o f t h i s t u b i n g w a s s e t b y i t s u s e a s a w a t e r c h a n n e l f o r c o o l i n g w h i l e t h e u p p e r l i m i t w a s s e t b y i t s a b i l i t y t o b e n d , a s v / e l l a s h a v i n g a s u i t a b l e v a l u e o f r e s i s t a n c e f o r t h e t u r n s r e q u i r e d . A f t e r s o m e t r i a l a n d e r r o r c a l c u l a t i o n s , t h e s i z e o f t h e c o p p e r t u b i n g w h i c h c o u l d b e r e a s o n a b l y e a s i l y w o u n d i n t h e p a n c a k e s h a p e v / a s d e c i d e d u p o n a s 3 / 1 6 1 1 o u t e r d i a m e t e r , 1 / 8 i n n e r d i a m e t e r . W i n d o w a r e a w o u l d a l l o w a n a v e r a g e c o i l d i a m e t e r o f 2 8 i n c h e s s o t h a t f o r t w o t i m e s 2 3 t u r n s p e r s e c t i o n , t h e l e n g t h o f c o p p e r , p e r p a n c a k e w o u l d b e 2 x 2 3 x ^ T T x 2 8 = 3 3 7 . 5 f e e t F i g . 4 . T h e E l e c t r o m a g n e t t o f a c e p a g e 10. f i g . 5 The E l e c t r o m a g n e t To f a c e page 10 -10- For 24 c o i l s i n series the e l e c t r i c a l resistance at 20 degrees C would be 4.48 ohms which at 50 degrees C would be increased to f i v e ohms1. Thus a D. C. supply at 45 amperes, of voltage 5 x 45 - 225 v o l t s would be required. The c o i l s would have to di s s i p a t e , at t h i s current, a maximum power of I 2 R = 45 2 x 5 = 10,120 watts (f) P a r t i c u l a r s of magnet construction The yoke and pole pieces were made of so f t s t e e l , the former of f i v e U sections bolted together, the l a t t e r of four* round and two square pieces bolted together. The specifications, of the s t e e l c a l l e d f o r a carbon content of less than 0.1% and manganese content of less than 0.4 % so that f o r a f i e l d strength of 15,000 gauss, the magneto- motive force required would be less than 80 ampere turns per inch. The only well machined parts were the pole t i p s , pole pieces and the inside surfaces of the yoke to which the pole pieces were attached. These were machined to give the one inch a i r gap a tolerance of plus or minus 0.010 inches. This a i r gap could be increased an ad d i t i o n a l ten inches by removing the inner f i v e inch sections of the pole pieces. The outside tolerance of the magnet was plus or minus 0.5 inches. T h e s q u a r e p o l e t i p s t w o i n c h e s " b y 1 6 i n c h e s s q u a r e h a d t w o t w o i n c h d i a m e t e r c i r c u l a r h o l e s c u t o u t a s s h o w n i n f i g u r e s 1 a n d 3 , w i t h l i k e p i e c e s o f s t e e l f i t t e d f o r f i n e f o c u s s i n g o f t h e " b e a m . T h e c r o s s s e c t i o n o f t h e y o k e w a s 1 6 i n c h e s x 2 6 i n c h e s w h i c h i s s u f f i c i e n t t o w i t h s t a n d a n a t t r a c t i v e f o r c e o f 3 0 t o n s " b e t w e e n t h e p o l e s . T h e t o t a l w e i g h t o f s t e e l w a s 7 . 5 s h o r t t o n s . T h e m a g n e t i s m o u n t e d o n r o l l e r s o n a s t e e l t r o l l e y w i t h r o l l e r h e a r i n g w h e e l s r u n n i n g o n s t e e l t r a c k s e m b e d d e d i n t h e f l o o r u n d e r t h e V a n d e G r a a f f g e n e r a t o r . I n c o n j u n c t i o n w i t h t h e r o l l e r s f o u r s c r e w s w e r e p r o v i d e d a t t h e b a s e f o r h o r i z o n t a l p o s i t i o n i n g a c r o s s t h e r a i l b e d , w h i l e f o u r m o r e w e r e p r o v i d e d f o r l e v e l l i n g ( f i g u r e s f o u r a n d f i v e ) . ( g ) T h e m a g n e t c o i l s T h e s p e c i a l d e s i g n o f t h e c o i l s i s i l l u s t r a t e d i n f i g . 2 T h e y w e r e w o u n d a s d o u b l e p a n c a k e s o f t w o t i m e s 2 3 t u r n s s o t h a t t h e e n d s c a m e o u t t a n g e n t i a l l y o n t h e o u t s i d e . T h e y w e r e m a d e o f a s b e s t o s c o v e r e d c o p p e r t u b i n g , 3 / 1 6 o f a n i n c h o u t s i d e d i a m e t e r , 1 / 8 o f a n i n c h i n s i d e d i a m e t e r s o t h a t t h e r e s i s t i v i t y w a s l e s s t h a n 8 x 1 0 " 7 o h m - i n c h e s a t 5 0 d e g r e e s c e n t i g r a d e . T h e i n s i d e d i a m e t e r o f t h e c o i l s w a s 2 3 i n c h e s a n d o u t s i d e d i a m e t e r w a s 3 3 i n c h e s , m a k i n g a t o t a l l e n g t h o f 3 3 8 f e e t . E a c h p a n c a k e w a s s p a c e d b y a 1 / 3 2 o f a n i n c h t e x t o l i t e i n s u l a t i n g r i n g . - 1 2 - T h e i n d i v i d u a l c o i l s w e r e c e m e n t e d t o g e t h e r w i t h g l y p t a l " b e f o r e " b e i n g b a k e d w i t h t h r e e c o a t s o f i n s u l a t i n g v a r n i s h . S h o r t c o p p e r s t r i p j o i n t h e c o i l s i n s e r i e s e l e c t r i c a l l y w h i l e t w o f o o t l e n g t h s o f 1 / 4 i n c h s a r a n t u b i n g j o i n t h e m h y d r a u l i c a l l y i n p a r a l l e l t o t h e c o o l i n g w a t e r m a n i f o l d s . ( h ) T h e w a t e r c o o l i n g s y s t e m A t a p r e s s u r e o f 3 7 l b s . / s q . i n . , 1 . 4 5 g a l l o n s o f w a t e r p e r m i n u t e f l o w i n g t h r o u g h 2 6 c o i l s i n p a r a l l e l k e p t t h e r i s e i n w a t e r t e m p e r a t u r e t o l e s s t h a n 2 5 d e g r e e s C a t a c u r r e n t o f 4 5 a m p e r e s . A p r e s s u r e s w i t c h w a s i n c o r p o r a t e d i n t h e i n t a k e m a n i f o l d a n d c o n n e c t e d i n t o t h e i n t e r l o c k s y s t e m . T h i s p r e v e n t e d p o w e r b e i n g a p p l i e d w h e n n o c o o l i n g w a t e r w a s i n t h e c o i l s . A l u c i t e c o v e r o n t h e o u t l e t m a n i f o l d a l l o w e d o b s e r v a t i o n o f w a t e r f l o w t h r o u g h e a c h c o i l . T h e t e m p e r a t u r e r i s e w a s o n l y t e n d e g r e e s i n a h a l f h o u r a t a c u r r e n t o f 3 5 a m p e r e s w h e n t h e w a t e r f l o w w a s s t o p p e d . T h e m a x i m u m c u r r e n t f l o w f o r a 2 5 d e g r e e C t e m p e r a t u r e r i s e , w h e n w a t e r c o o l e d w a s i n e x c e s s o f 60 a m p e r e s s h o w i n g t h a t t h e d e s i g n w a s q u i t e c o n s e r v a t i v e . - 1 3 - ( i ) T h e m a g n e t i c f i e l d T h e m a x i m u m f i e l d m e a s u r e d i n t h e g a p a t t h e c e n t r e a t a c u r r e n t o f 4 8 . 5 a m p e r e s w a s j u s t o v e r 1 9 , 9 0 0 g a u s s . T h e n u m b e r o f t u r n s w a s 2 x 2 3 x 2 6 = 1 , 1 8 6 a n d t h e r e f o r e t h e N I w a s 5 7 , 5 0 0 a m p e r e t u r n s . N o w 4 r r N I - 1 9 , 9 0 0 x 2 . 5 4 X x 1 2 3 x 2 . 5 4 1 0 t h e r e f o r e X = 7 2 . 2 0 0 - 5 0 . 5 0 0 3 1 2 = 2 1 . 7 0 0 = 6 9 . 5 a m p e r e t u r n s / i n c h 3 1 2 T h i s c o n f i r m s t h e d e s i g n f i g u r e t h a t t h e H i s l e s s t h a n 8 0 a m p e r e t u r n s p e r i n c h . T h i s p o i n t o f 1 9 , 9 0 0 g a u s s w a s n o t t h e l i m i t o f m a g n e t i z a t i o n " b u t i t w a s u p o n t h e k n e e o f t h e m a g n e t i z a t i o n c u r v e ( f i g u r e 1 2 ) . (0_ ro <w L V +0 u c :> 0 3o - o _ l Xo 10 0 1 1 1 I 1 0 o.t, 0.4- 0. fc 0-8 1 ° tl) G - e n e r c i t o r no series f i e l d , J" K w. it>a.d (X) " * " " " C.3 ) " * ' , Ser/es -f/e/d i ' n oppos!ti'tn } 5 \\w. load / n o p p o s i ± i o n y <e K*/. I o <* c ( . Ge-n e r g t o r s ' Curves Fig. <o to f Q C < ? p. j 4_, -14- Chapter 2. The Power Supply f o r the Magnet C o i l s (a) The generators Two compound-wound D. C. generators were made available to supply the ten kilwatts of power f o r which the magnet c o i l s were designed. One generator was rated at 5^ k i l o - watts at 44 amperes while, the other was rated at 6 kilowatts at 54 amperes. However, t h i s t o t a l power was not available i n the regulating system since the series f i e l d c o i l s were connected i n opposition to give a l i n e a r current-current c h a r a c t e r i s t i c to the machines when the shunt f i e l d c o i l s were separately excited. This was more suitable f o r smooth control of constant s e n s i t i v i t y but caused a loss of 3.5 kilowatts i n output power. ; Curves 3 and 4, figu r e 6 i l l u s t r a t e t h i s e f f e c t . Each generator was driven separately by a ten H. P. three phase A. C. motor using three V b e l t s on multiple pulleys. The two generators were connected i n series to d e l i v e r up to 200 - 250 v o l t s to the magnet c o i l s . Their shunt f i e l d -15- c o i l s w e r e a l s o e o n n e c t e d i n s e r i e s a n d s u p p l i e d s e p a r a t e l y w i t h u p t o o n e a m p e r e o f c u r r e n t b y t h e c o n t r o l s y s t e m . o Q 3 V C c » v3 """» 0 C O O I- _t o ux r U J c£ D < or o < o o CQ L 01 •p € V J *» •P • J *> O £ i. Q _ i <u C V •p XT « a : tro f q c e p. 16, - 1 6 - Char/ter 3. T h e R e g u l a t i n g S y s t e m T h e o b j e c t i v e o f t h e r e g u l a t i n g s y s t e m w a s t o g i v e a n e a s i l y v a r i e d b u t a c c u r a t e l y r e g u l a t e d m a g n e t i c f i e l d . T h i s h a s b e e n a c h i e v e d . T h e t y p e o f r e g u l a t o r c h o s e n u s i n g h i g h v a c u u m t u b e s , 7 , 8 , 9 , 1 0 w a s p r e f e r r e d o v e r o t h e r t y p e s b e c a u s e o f i t s e a s e o f o p e r a t i o n a n d a c c u r a c y . T h e o t h e r m o s t c o n s i d e r e d t y p e s w e r e t h e s a t u r a b l e - e o r e r e a c t o r w h i c h h a d a n a c c u r a c y o f o n l y o n e p a r t i n 1 , 0 0 0 a n d t h e p h o t o c e l l a n d g a l v a n o m e t e r 8 ' • - t y p e . T h e l a t t e r h a d a d e q u a t e a c c u r a c y b u t r e q u i r e d f i n e a d j u s t m e n t , m o r e c a r e a n d c o u l d l o s e c o n t r o l w i t h l a r g e c u r r e n t c h a n g e s . A b l o c k d i a g r a m o f t h e r e g u l a t i n g s y s t e m u s e d h e r e i s s h o w n i n f i g u r e 7 . A l l s w i t c h e s a n d m e t e r s w e r e m o u n t e d o n o n e r a c k , t h e r e g u l a t o r r a c k , a s p a r t o f t h e c o n t r o l c o n s o l e o f t h e V a n d e G r a a f f g e n e r a t o r . T h i s a l s o c o n t a i n e d t h e R u b i c o n p o t e n t i o m e t e r , s t a n d a r d c e l l , . . ^ g a l v a n o m e t e r , b a t t e r i e s , a m p l i f i e r , a n d t h e r e g u l a t o r t u b e s . A t a d i s t a n c e f r o m t h i s o f a b o u t 25 f e e t w a s t h e m a g n e t i t s e l f w i t h t h e m a n - g a n i n r e s i s t e r a n d s u r g e - p r o t e c t o r - r e c t i f i e r . L a s t l y , - 1 7 - t h e D . C . p o w e r u n i t , g e n e r a t o r s a n d m o t o r s w e r e s i t u a t e d i n a s m a l l a d j o i n i n g r o o m . ( a ) O p e r a t i o n T h e r e q u i r e d m a g n e t c u r r e n t w a s s e l e c t e d b y t h e s e t t i n g o f t h e R u b i c o n p o t e n t i o m e t e r w h o s e v o l t a g e ' b u c k e d ' t h a t d e v e l o p e d a c r o s s t h e m a n g a n i n r e s i s t o r i n s e r t e d i n t h e m a g n e t c u r r e n t l e a d s . T h e d i f f e r e n c e o f t h e s e t w o v o l t a g e , ' t h e e r r o r s i g n a l ' , a f t e r a m p l i f i c a t i o n , w a s u s e d t o c o n t r o l t h e f i e l d c u r r e n t o f t h e t w o g e n e r a t o r s t h r o u g h t h e r e g u l a t o r t u b e s . T h e m a g n e t c u r r e n t f a i t h f u l l y f o l l o w e d a n y c h a n g e i n s e t t i n g . O n c e t h e c u r r e n t w a s s e t , i t r e m a i n e d c o n s t a n t t h r o u g h o u t t h e w a r m - u p p e r i o d a n d t h r o u g h o u t t h e d a y s ' v a r i a t i o n s i n t e m p e r a t u r e . ( b ) P r e c i s i o n o f t h e S y s t e m T h e v a r a t i o n s i n m a g n e t c u r r e n t a r e t w o f o l d : 1 , S l o w d r i f t d u e t o w a r m i n g u p o f m a g n e t , c o i l s , g e n e r a t o r a n d o t h e r c o m p o n e n t s . 2. M o r e r a p i d v a r i a t i o n s d u e t o c o m m u t a t o r r i p p l e a n d o t h e r n o i s e p i c k - u p . T h e p r e c i s i o n o f t h e c u r r e n t c o n t r o l d e p e n d s o n t h e f o l l o w i n g p o i n t s : 1 . T h e v a l u e o f t h e m a n g a n i n r e s i s t o r m u s t b e s u c h t h a t t h e m i n i m u m v a r i a t i o n t h a t c a n b e t o l e r a t e d i n t h e m a g n e t c u r r e n t p r o v i d e s a n e r r o r s i g n a l a b o v e t h e i n p u t n o i s e l e v e l o f a m p l i f i e r . - 1 8 - 2 . T h e t e m p e r a t u r e v a r i a t i o n i n r e s i s t a n c e i n t h i s r e s i s t o r m u s t b e k e p t l e s s t h a n o n e " p a r t i n 1 0 , 0 0 0 . 3 . T h e i n p u t n o i s e l e v e l o f t h e a m p l i f i e r m u s t b e k e p t t o a m i n i m u m . 4 . A n a c c u r a t e l y c o n t r o l l e d D . C . r e f e r e n c e ( o r ' b u c k - i n g ' ) v o l t a g e m u s t b e a v a i l a b l e , w h o s e v a r i a t i o n i s a l s o l e s s t h a n o n e p a r t i n 1 0 , 0 0 0 5 . S u f f i c i e n t g a i n m u s t b e a v a i l a b l e i n t h e a m p l i f i e r a n d c o n t r o l l o o p t o a l l o w t h e s y s t e m t o f o l l o w u p a n y v a r i - a t i o n o n t h e i n p u t o r o u t p u t s o t h e e r r o r c u r r e n t d o e s n o t e x c e e d 1 / 1 0 , 0 0 0 p a r t o f t h e c u r r e n t p a s s i n g ; h o w e v e r t h e s y s t e m m u s t n o t o v e r s h o o t o r h u n t . I n o r d e r t o r e g u l a t e o v e r t h e c u r r e n t r a n g e f r o m f i v e t o 4 5 a m p e r e s , t o o n e p a r t i n 1 0 , 0 0 0 , a n d g i v e a 2 5 ji v o l t e r r o r s i g n a l , a r e s i s t o r o f a t l e a s t Pi R s 2 5 x 1 0 = 0 . 0 5 o h m s i s n e e d e d . 5 x 1 0 " 4 T h e u p p e r l i m i t o f t h e r e s i s t o r i s g o v e r n e d b y t h e a m o u n t o f h e a t i t m u s t d i s s i p a t e a t h i g h c u r r e n t s . I n t h i s c a s e , a t 4 5 a m p e r e s , I t m u s t d i s s i p a t e I 2 R s 1 0 1 w a t t s , w h i c h n e c e s s i t a t e s a d e q u a t e c o o l i n g t o k e e p i t s t e m p e r a t u r e a n d t h u s r e s i s t a n c e c o n s t a n t . B y u s i n g m a n g a n i n s h u n t s t r i p o f t e m p e r a t u r e e e e f f i - c i e n t o< = 0 . 0 0 0 0 2 p e r d e g r e e c e n t i g r a d e , i t i s r e q u i r e d t h a t : -19- A R / 1 ~R~ ^ 10,000 o now , R = R G ( 14- <K t) therefore A R = R G <* A t and A R = o< A t R o thereforeoCn. t < 1 , 10 4 4 t < 10~ 4 2 x 1 0 4 5 degrees C When the manganin r e s i s t o r the cooling water system i t was hour period i t s temperature d i d degrees C. The above condition f u l f i l l e d . was made up and attached to found that over an eight not vary more than 0.2 was therefore e a s i l y -20- Chapter 4. The Units of the Regulator (a) Standard Resistor A 0.05 ohm water cooled manganin'resistor was made up f o r the reasons given i n chaper three above. A 3fe foot length of one inch manganin 'shunt' was mounted i n a glass tube and connected to the magnet water cooling system. E l e c t r i c a l connections were made through heavy brass cylinders having separate p o t e n t i a l and current terminals. These cylinders were soldered to the ends of the manganin s t r i p . A f t e r annealing, the complete r e s i s t o r was sealed into the glass tube and mounted on the magnet frame as shown at the bottom of figur e 4. Care was taken to have i d e n t i c a l shielded copper leads both from t h i s r e s i s t o r and from the reference voltage source leading to the amplifier input. The temperature v a r i a t i o n i n the r e s i s t o r throughout one day, at currents up to 15 amperes was n e g l i g i b l e . Temperature of the cooling water available was found to vary from day to day by a degree or two but remained constant throughout the -21- day, to much l e s s than one degree centigrade as checked over a two week period. (b) Standard reference voltage The standard reference voltage was obtained from a modified Rubicon type B potentiometer, with i t s associated standard c e l l , galvanometer and battery supply. The modification consisted i n bringi n g out an extra terminal so that on the most used range, an uninterrupted voltage was available while the potentiometer was being checked f o r c a l i b r a t i o n . The voltage range of the potentiometer had also been extended to 6.4 v o l t s so as to handle currents i n t h i s regulating system up to 128 amperes i f that ever became necessary. The standard c e l l was c e r t i f i e d accurate to one part i n 100,000 at room temperature and the potentiometer to about 12 two parts i n 100,000 . The determining accuracy was there- fore the constancy of voltage of the battery supplying the potentiometer. After t r y i n g various sources of dry c e l l s and storage b a t t e r i e s , two heavy duty Exide lead and s u l f u r i c acid b a t t e r i e s were chosen f o r the f i n a l t e s t s . On the 50 ma. drain required, they dropped i n voltage about 0.5 JU v o l t s per minute a f t e r being connected continuously f o r 13 days. Superimposed on t h i s , however were small temperature f l u c t u a t i o n s . Primary c e l l s of zinc and carbon i n NaOH appeared to be  - 2 2 - the only ones having the necessary s t a b i l i t y and low temp- 13 14 erature and voltage d r i f t on low current dr a i n ' . A number of c e l l s of t h i s type, Ferbatco F B 4 had been ordered from Ferguson Battery Company, Slough, England, but have not yet been delivered. (c) Brown Converter and amplifier 15,16,17 A Brown Converter v i b r a t o r followed by a conventional three stage R-C coupled amplifier, feeding a l o c k - i h detector provided the D. C. a m p l i f i c a t i o n with high gain and s t a b i l i t y required to follow the slow d r i f t s i n current. An a d d i t i o n a l two stage A. C # amplifier was added i n p a r a l l e l to look a f t e r more r a p i d v a r i a t i o n s i n current. Both of these were then fed into one direct-coupled stage which was then d i r e c t l y coupled to the regulator tubes. Available voltage gain, at 60 cycles, i n the three stage A. C. section was over 300,000 and i n the two stage A. C. section, 5,000 ( at 1,000 cycles per second). The power supply f o r the whole amplifier was b u i l t on a separate chassis to keep the power l i n e frequency out of the system. The c i r c u i t diagram f o r the above i s shown i n f i g u r e 8 and T O follows that used at Chalk R i v e r * 0 # A 400 cycle Brown Converter had been decided upon f o r reasons of lower noise, le s s phase delay and greater e f f i c i e n - - cy. The noise of the 60 cycle type i s about two u v o l t s due to the capacitive coupling of the contacts on the v i b r a t i n g reed to — 7» - 2 3 - 1 5 , 1 6 , 1 7 . t h e e x c i t i n g c o i l . I n t h e 4 0 0 c y c l e o n e , t h e e x c i t i n g c o i l l e a d s a r e i s o l a t e d a n d b r o u g h t o u t a t t h e s i d e t h r o u g h a s h i e l d e d c a b l e c o n n e c t o r t o r e d u c e t h i s t y p e o f c o u p l i n g . T h e p h a s e s h i f t i n t h e a m p l i f i e r i s c o n s i d e r a b l y l e s s a t 4 0 0 c y c l e s t h a n a t 6 0 c y c l e s . L a s t l y , t h e g r e a t e r e f f i c i e n c y i s d u e t o t h e f a c t t h a t i n f o r m a t i o n o n v o l t a g e o r c u r r e n t f l u c t u a t i o n i s g a t h e r e d 4 0 0 t i m e s a s e c o n d i n s t e a d o f o n l y 6 0 t i m e s a s e c o n d . A s t h e 4 0 0 c y c l e v i b r a t o r d i d n o t a r r i v e u n t i l r e c e n t l y , a l l t e s t s h a v e b e e n c a r r i e d o u t w i t h t h e 6 0 c y c l e o n e . ( d ) C u r r e n t r e g u l a t i n g t u b e s E i g h t 6 A S 7 t u b e s i n p a r a l l e l c o n n e c t e d i n s e r i e s w i t h t h e g e n e r a t o r f i e l d s p r o v i d e d t h e c o n t r o l o f t h e m a g n e t c u r r e n t . T h e s e w e r e m o u n t e d o n s e p a r a t e * c h a s s i s t o g e t h e r w i t h a f i l a m e n t s u p p l y t r a n s f o r m e r a n d a s s o c i a t e d d e v i c e s a s s h o w n i n f i g u r e 9 . A t l o w c u r r e n t , t h e i n d i v i d u a l t u b e ' s i n t e r n a l r e s i s t a n c e v a r i e d b y a s m u c h a s 1 0 0 % b u t b e c a m e e q u a l i z e d a s t h e c u r r e n t i n c r e a s e d . F o r l o n g e s t l i f e , t h e t u b e s p a s s e d o n l y h a l f r a t e d c u r r e n t w i t h f u l l l o a d o n t h e g e n e r a t o r s . C o m p l e t e c h e c k o n t h e o p e r a t i o n o f e a c h t u b e w a s p r o - v i d e d b y a c i r c u i t o p e n i n g j a c k a s w e l l a s a ' s l o - b l o ' f u s e a n d n e o n b u l b . C a t h o d e c u r r e n t o f e a c h t u b e w a s r e a d b y a m i l l i a m m e t e r a n d j a c k - p l u g . T h e f u s e s , r a t e d a t \  - 2 4 - a m p e r e p r o t e c t e d t h e c i r c u i t a g a i n s t s h o r t e d t u b e s w h i c h w o u l d t h e n b e p o i n t e d o u t b y a g l o w i n g n e o n b u l b . A n o p e n c i r c u i t e d t u b e c o u l d b e f o u n d b y c h e c k i n g f o r c a t h o d e c u r r e n t . I n a d d i t i o n t o t h e a b o v e , a c u r r e n t o v e r l o a d r e l a y , a d j u s t a b l e f r o m 0 . 6 5 _ a m p e r e s t o 2 a m p e r e s , w a s i n s e r t e d i n t h e c o m m o n p l a t e l e a d o f t h e t u b e s . L a s t l y , a o n e m i l l i a m p e r e r e l a y , c o n n e c t e d t o t h e i n t e r l o c k s y s t e m , w a s i n s e r t e d i n t h e c o n t r o l g r i d c i r c u i t t o p r e v e n t t h e g r i d s f r o m g o i n g p o s i t i v e . T h e 1 , 0 0 0 o h m r e s i s t o r i n e a c h g r i d l e a d w a s a ' g r i d s t o p p e r ' t o d a m p o u t a n y p a r a s i t i c o s c i l l a t i o n s . F i n a l l y , a s m a l l a m o u n t o f n e g a t i v e f e e d b a c k a s w e l l a s b i a s w a s p r o v i d e d b y a 2 5 o h m p o w e r r e s i s t o r i n t h e c o m m o n c a t h o d e l e a d . * ( e ) P o w e r U n i t A 4 0 0 v o l t , o n e a m p e r e p o w e r s u p p l y f e d t h e 6 A S 7 t u b e s a n d g e n e r a t o r f i e l d s i n s e r i e s . T h i s u n i t , s h o w n i n f i g u r e 1 0 u s e d t w o G . E . F G 1 0 5 t h y r a t r o n s i n a f u l l w a v e c i r c u i t w i t h c h o k e i n p u t , f o l l o w e d b y a n a d d i t i o n a l L C f i l t e r s e c t i o n . T h e r i p p l e i s l e s s t h a n 0 . 2 % . T h e H . T . ( p l a t e ) t r a n s f o r m e r w a s a t h r e e K . V . A . o i l f i l l e d p o w e r - l i n e t r a n s f o r m e r w i t h i t s t w o 1 0 4 0 v o l t w i n d i n g s i n s e r i e s a s c e n t r e - t a p p e d s e c o n d a r y a n d i t s t w o 1 0 4 v o l t w i n d i n g s a l s o c o n n e c t e d i n s e r i e s f o r t h e p r i m a r y . -25- ( f ) P r o t e c t i v e d e v i c e s The complete system ( f i g u r e 10) was p r o t e c t e d by t h r e e i n t e r l o c k s and an o v e r l o a d r e l a y , as w e l l as the surge- p r o t e c t o r - r e c t i f i e r a c r o s s the magnet c o i l s themselves. H. T. o f the power u n i t was shut o f f i f the c o o l i n g - w a t e r p r e s s u r e f a i l e d , i f the r e g u l a t o r tubes drew g r i d c u r r e n t , or i f the power u n i t c a b i n e t doors were opened. The c u r r e n t o v e r l o a d r e l a y i n the g e n e r a t o r s ' f i e l d c i r c u i t as mentioned above and shown i n f i g u r e 9, would a l s o i n t e r r u p t power t o the magnet i n case o f i s h o r t c i r c u i t . L a s t l y , a s u r g e - p r o t e c t o r t h y r a t r o n r e c t i f i e r , mounted a t the magnet, was connected i n r e v e r s e p o l a r i t y a c r o s s the c o i l s t o d i s c h a r g e them q u i c k l y . T h i s r e c t i f i e r , an F 6 105, c o u l d pass a surge c u r r e n t o f 200 amperes a t 1,000 v o l t s . The surge t h a t c o u l d be expected as c a l c u l a t e d from a 5 ohm r e s i s t a n c e o f F G 105 i s not g r e a t e r t han 225 v o l t s . The f i l a m e n t o f t h i s t h y r a t r o n was connected i n the system such t h a t i t had to be on and warmed up b e f o r e power c o u l d be s u p p l i e d t o the magnet. T h i s i s d e s c r i b e d i n the next chapter. Chapter 5. Operation of the control system (a) General switching The switching arrangement of a l l the units is shown in figure 10. One toggle switch and one push button turn on a l l the electronic equipment while an additional push button i s needed for each motor generator. ( These may be controlled together i f desired with a small change i n wiring, by one set of push buttons. ) The main toggle switch S turns power on to a l l the thyratron heaters and Sola constant voltage transformer as well as a five minute time-delay-relay. The complete amplifier and a l l heaters but the surge-protector-rectifier are fed from the Sola transformer. The time-delay-relay allows the thyratron heaters to warm up before H. T. can be applied to the power unit. Thus, after five minutes, provided a l l interlock switches are closed, f i e l d excitation i s applied to the generators by pressing push button S . This closes relay S 4 which applies power to the plate transformer of the power * -27- u n i t . H. T. can be interrupted manually without r e - i n t r o - ducing the time-delay, by means of toggle switch Sg. A l l the equipment but the motor generators i s switched o f f by switch S^. (b) The motor generators ) The two motors are controlled i n d i v i d u a l l y by on-off push buttons which are provided with indicator lamps. The lamps and buttons are mounted on the switch panel. The motors may also be controlled independently by push buttons i n the motor-generator room. (c) The Rubicon -potentiometer The Rubicon potentiometer, used as the primary co n t r o l of the system, requires no extra switching i n checking i t s c a l i b r a t i o n when used on the main 1.6 v o l t range. By t h i s means, the accuracy of the current s e t t i n g may be kept as high as possible i n s p i t e of any variati o n s i n the potentio- meter battery supply, f o r magnet currents up to 32 amperes. For very small currents, or f o r currents above 32 amperes, c a l l i n g f o r the use of the 0.16 or 6.4 v o l t ranges, the control loop of the regulator system has to be opened temporarily and the magnet current l e f t p a r t i a l l y u n s t a b i l i z e d during the r e - c a l i b r a t i o n of the potentiometer. This i s done by f i r s t switching the 'manual-automatic * f i g . 1 1 . The Control Panel to face page 28 - 2 8 - s w i t c h o n t h e a m p l i f i e r t o ' m a n u a l 1 . C a l i b r a t i o n o r r e - c h e c k i n g o f t h e c a l i b r a t i o n i s t h e n c a r r i e d o u t b y t u r n i n g t h e E . M . F . - S . C . s w i t c h o n t h e R u b i c o n p o t e n t i o m e t e r t o S . C . ( s t a n d a r d c e l l ) p o s i t i o n . T h e r e g u l a t o r s y s t e m i s r e t u r n e d t o s e l f - s t a b i l i z a t i o n b y r e v e r s i n g t h e a b o v e s w i t c h i n g p r o c e d u r e . ( d ) M a n u a l - a u t o m a t i c s w i t c h a n d D . C . l e v e l c o n t r o l T h e ' m a n u a l - a u t o m a t i c ' s w i t c h c u t s o u t t h e D . C . a m p l i - f i e r p o r t i o n o f t h e c i r c u i t b u t l e a v e s t h e l o w g a i n A . C . l o o p s t i l l i n t h e c i r c u i t . W h i l e i n t h i s ' m a n u a l ! p o s i t i o n , t h e m a g n e t c u r r e n t m a y b e a l t e r e d o v e r t h e c o m p l e t e r a n g e , b y a d j u s t i n g t h e ' D . C l e v e l ' c o n t r o l k n o b o n t h e a m p l i f i e r . ( e ) C u r r e n t a n d f i e l d s e t t i n g o f t h e m a g n e t T h e m a g n e t i c f i e l d d e s i r e d i s o b t a i n e d b y r e f e r r i n g t o t h e l a r g e g r a p h o f t h e h y s t e r e s i s l o o p m o u n t e d a t t h e c o n t r o l p a n e l , a s s h o w n i n f i g u r e 1 1 . T h e m a g n e t c u r r e n t i s t h e n s e t t o t h e i n d i c a t e d v a l u e b y t u r n i n g t o t h e a p p r o p r i a t e s e t t i n g o n t h e R u b i c o n p o t e n t i o m e t e r . T h e a c t u a l m a g n e t c u r r e n t i s i n d i c a t e d b y t h e ' l a r g e m e t e r o n t h e r e g u l a t o r t u b e c h a s s i s a s s h o w n i n f i g u r e 1 0 . F o r e x a m p l e , w i t h t h e 0 . 0 5 o h m m a n g a n i n r e s i s t o r , a c u r r e n t o f 2 0 a m p e r e s w a s c h o s e n b y s e t t i n g t h e R u b i c o n p o t e n t i o m e t e r t o 1 . 0 0 0 0 v o l t s , a n d t h e n a d j u s t i n g t h e D . C . l e v e l c o n t r o l k n o b t o m a k e t h e -29- balance meter M read at centre. This gave a magnet f i e l d strength of 11,400 gauss. The add i t i o n a l meters on the con t r o l panel t e l l pre- c i s e l y what i s happening to the system. An understanding of t h e i r function and operation aids i n t r a c i n g any f a u l t s i n the system. Ammeter Mg reads the generators' shunt f i e l d current and should read between 0 and 1 ampere i n d i r e c t proportion to the magnet current. The terminals of the milliammeter M3 come out from the front of the panel to a jack plug which may be inserted i n the cathode c i r c u i t of each 6 A S 7 tube to check i t s operation. Ammeter as mentioned i n the preceding paragraph reads the magnet current d i r e c t l y and must follow the voltage s e t t i n g of the Rubicon potentiometer d i r e c t l y . The other three meters, on the amplifier chassis panel, t e l l the operation of the r e s t of the electronics of the system. Voltmeter M 4 when reading centre, indicates that the current s e t t i n g agrees exactly with the desired value as chosen by the Rubicon potentiometer s e t t i n g . A. C. voltmeter Mg reads the magnitude of the error s i g n a l being transmitted by the amplifier. For best operation under normal conditions, t h i s should read as close to zero as possible. This may be always kept reading zero f o r a l l magnet current settings by re-adjusting the' D. C. l e v e l i — 3 0 - c o n t r o l k n o b , w h i c h w i l l m a i n t a i n t h e b a l a n c e m e t e r a t c e n t r e a s w e l l . C o r r e c t s e t t i n g o f t h e c u r r e n t w i l l n o t b e o b t a i n e d u n l e s s b a l a n c e m e t e r M 4 i s s e t a t c e n t r e . V o l t m e t e r M g r e a d s t h e D . C . v o l t a g e a p p l i e d t o t h e g r i d s o f t h e 6 A S 7 r e g u l a t o r t u b e s . M e t e r M 4 a n d M g t o g e t h e r g i v e t h e p h a s e a n d m a g n i t u d e r e s p e c t i v e l y o f t h e e r r o r s i g n a l i n t h e s y s t e m a t a n y t i m e . I n i t i a l s e t t i n g u p o f t h e r e g u l a t i n g s y s t e m a n d a d d i t i o n - a l a d j u s t m e n t s a r e d i s c u s s e d i n a p p e n d i x , 3 . C h a p t e r 6 . • R v p ^ - p i m e n t a l R e s u l t s ( a ) D r i f t o f t h e m a g n e t c u r r e n t a n d m a g n e t i c f i e l d S l o w d r i f t s o f t h e m a g n e t c u r r e n t w e r e s a t i s f a c t o r i l y - s m a l l , o f t h e o r d e r o f a f e w p a r t s i n 1 0 , 0 0 0 o v e r a f e w m i n u t e s a n d u p t o s e v e n h o u r s a s t a k e n f r o m f i e l d m e a s u r e - m e n t s u s i n g f l i p - c o i l s a n d a f l u x m e t e r ( a p p e n d i x 4 ) . B y « L Q j 2 0 } 2 IL ^ u s i n g t h e h i g h l y a c c u r a t e p r o t o n r e s o n a n c e m e t h o d 2 2 , 2 3 , s l o w d r i f t s o f t h e f i e l d w e r e t h e n m e a s u r e d t o h e o n e p a r t i n 1 0 , 0 0 0 o v e r a n e i g h t m i n u t e p e r i o d , i n c r e a s i n g t o o n l y t h r e e p a r t s i n 1 0 , 0 0 0 o v e r t h e l o n g p e r i o d o f s e v e n h o u r s . P a r t " b u t n o t a l l o f t h i s c a n h e a c c o u n t e d f o r b y t h e r m a l c o n t r a c t i o n o f t h e i r o n d e c r e a s i n g t h e w i d t h o f t h e a i r g a p a s d i s c u s s e d i n a p p e n d i x 2 . T h e r e s t c a n m o r e t h a n b e a c c o u n t e d f o r b y a d e c r e a s e o f n e a r l y t w o p e r c e n t i n t h e r e s i s t a n c e o f t h e m a g n e t c o i l s f o r t h e f i v e d e g r e e t e m p e r a t u r e d r o p a t t h e c u r r e n t u s e d f o r t h i s t e s t . T h i s c o u l d j u s t i f y t h e a b o v e m e n t i o n e d a c c u r a c y o f t h e c u r r e n t d r i f t i n t h e r e g u l a t o r s i n c e o n t h e o t h e r h a n d , t h e g e n e r a t o r c o i l s w o u l d h a v e h e a t e d u p a b o u t 2 5 d e g r e e s C -32- at the same time. The corrected d r i f t , nevertheless, was a small "but a d e f i n i t e increase i n magnetic f i e l d . A small amount of commutator r i p p l e was observed but was reduced s a t i s f a c t o r i l y a f t e r large condensers were connected across the generator brushes. (b) Amplifier and regulating system The Brown converter and the amplifier and l o c k - i n detect- or provided D. C. amplification with a minimum inherent d r i f t . This resulted i n almost complete compensation f o r the thermal effects i n the components of the regulating system. The few parts i n ten thousand d r i f t observed, aft e r correct- ion f o r battery d r i f t and proton head o s c i l l a t i o n d r i f t are very i n s i g n i f i c a n t compared to the one to two per cent or greater changes i n resistance of the magnet and generator c o i l s due to cooling or warming. The gain and s t a b i l i t y of the system was investigated by t r y i n g d i f f e r e n t values f o r the manganin standard r e s i s t - or. With 0.05 ohms, the D. C. amplifier voltage gain could be advanced to 600 before low frequency o s c i l l a t i o n or 'hunting 1 of the system set i n . This showed up on a l l the meters and i n the magnet f i e l d as shown on the proton resonance 'scope. Various other values of resistance down to 0.0005 ohms were t r i e d allowing the gain of the amplifier to be increased to about 30,000, but with the s t a b i l i t y of -33- t h e s y s t e m d e c r e a s i n g t o o n l y a f e w p a r t s i n 1,000 o y e r s h o r t p e r i o d s . W i t h t h e l o w e s t v a l u e o f r e s i s t a n c e t h e n o i s e i n t h e i n p u t t o t h e a m p l i f i e r was a s l a r g e a s t h e e r r o r v o l t a g e . P a r t o f t h i s n o i s e was 60 c y c l e hum due 15,16 ,17 t o t h e 60 c y c l e B r o w n c o n v e r t e r . The r e s t o f t h i s was due t o t h e f i r s t t u b e c i r c u i t , m o s t p r o b a b l y f r o m t h e h e a t e r . The 400 c y c l e B r o w n c o n v e r t e r w i t h a s e p a r a t e s u p p l y o f 400 c y c l e p o w e r f e e d i n g i t a n d t h e l o c k - i n d e t e c t o r w o u l d a l l o w u s e o f a s m a l l e r m a n g a n i n r e s i s t o r o r r e s p o n s e t o a s m a l l e r e r r o r v o l t a g e . A s t a b i l i z e d 400 c y c l e o s c i l l a t o r a n d p o w e r a m p l i f i e r was b u i l t a n d t e s t e d b u t h a s n o t y e t b e e n i n c o r p o r a t e d i n t o t h e s y s t e m . B e s t s t a b i l i t y a n d a c c u r a c y was o b t a i n e d u s i n g t h e 0.05 m a n g a n i n r e s i s t o r . R e p e a t a b i l i t y o f t h e c u r r e n t s e t t i n g a n d t h e f i e l d was g o o d a n d o v e r s h o o t o n a p p l i c a t i o n o f a s t e p v o l t a g e t o t h e i n p u t was s m a l l . R e p e a t a b i l i t y o f t h e f i e l d o v e r a f i v e d a y p e r i o d a s m e a s u r e d w i t h a f l i p - c o i l a n d f l u x m e t e r was w i t h i n M o m e n t a r y o v e r - s h o o t o f t h e c u r r e n t o n a p p l i c a t i o n o f a s t e p - v o l t a g e was 4" ampere i n 20 a m p e r e s , a b o u t 1%. T h i s o v e r s h o o t was n o t a c o n t i n u o u s e r r o r , b u t t h i s o v e r s h o o t d i d i n c r e a s e i n v a l u e i f t h e v o l t a g e g a i n o f t h e a m p l i f i e r was d e c r e a s e d b e l o w a f e w h u n d r e d . K i [o$avss is it 14- I 1. |o i 6 - 1 - / -V - 7 7 t / r l l i , -TO -4o " J o "20 -'O / O 1 | | | io ^ 0 3o 4-0 5* - - 2 - 4 - - 6 - - 8 _ l e _ _ 14 - Ifc H v s t e r es is Loo p a n A M a < m e t u 4 t i ' « n C u r v e t-e. "face p. 3 4". K«l»jao5 down -tram centre to -f«ce p. 5 4-. -34- Th e time constant of the system i n response to ,a step-voltage was "between one and two seconds, which was roughly the time constant of the generators, but l e s s than that of the magnet. This was measured independently to be close to three seconds using a storage battery, meter" and stop-watch. I t was the time taken f o r the current to r i s e to 63% of i t s f i n a l value and to f a l l to 37% of I t s i n i t i a l value. (c) Performance of the magnet The inductance of the magnet was 13.9 henries as measured from i t s time constant of 3.02 seconds. The r e s i s - tance of the c o i l s at 20 degrees centigrade was 4.6 ohms. At a current of 48.5 amperes i t gave a f i e l d strength of 19,919 gauss as measured with a f l i p - c o i l and fluxmeter. The saturation curve has i t s knee at 18,000 gauss while the hysteresis loop i s only 0.2 amperes wide at a current of 15 amperes. These are superimposed i n fig u r e 12. The r e t e n t i v i t y , of the order of 70 to 100 gauss, was s u r p r i s - ingly small. Figure 13 shows the f a l l o f f of f i e l d i n the h o r i z o n t a l and v e r t i c a l d i r e c t i o n s . This shows that the leakage f l u x i s small which confirms the design described i n chapter 1. From the ca l c u l a t i o n s given i n chapter 1 under 'The magnetic f i e l d ' i t appeared that the H required f o r a f i e l d of nearly 20,000 gauss at the centre was about 70 ampere- -35- t u r n s p e r i n c h , 12 % b e t t e r t h a n w a s e x p e c t e d . T h e t e m p e r a t u r e r i s e o f t h e c o i l s a t 60 a m p e r e s w a s f a r l e s s t h a n a n t i c i p a t e d , o n l y 25 d e g r e e s c e n t i g r a d e . T h i s w a s m o s t l y d u e t o t h e f a c t t h a t t h e w a t e r p r e s s u r e a v a i l a b l e i n t h e b a s e m e n t o f t h e P h y s i c s b u i l d i n g w h e r e t h e m a g n e t i s l o c a t e d h a s b e e n h i g h , f r o m 35 p o u n d s t o 40 p o u n d s p e r s q u a r e i n c h a s a g a i n s t t h e 25 p o u n d s p e r s q u a r e i n c h a s s u m e d i n t h e d e s i g n . T h e m a g n e t , c o n t a i n i n g s e v e n a n d o n e h a l f t o n s o f i r o n a n d o v e r 500 l b s . o f c o p p e r , h a s a l a r g e t h e r m a l t i m e c o n s t a n t s u c h t h a t a t m e d i u m c u r r e n t s , u p t o 12 a m p e r e s t h e c o o l i n g w a s s u f f i c i e n t n o t o n l y t o k e e p t h e t e m p e r a t u r e r i s e d o w n b u t t o b r i n g t h e m a g n e t t e m p e r a t u r e g r a d u a l l y d o w n f r o m t h a t o f t h e r o o m t o t h e t e m p e r a t u r e o f t h e c o o l i n g w a t e r , s o m e f i v e t o s e v e n d e g r e e s c e n t i g r a d e l o v / e r . F r o m c a l c u l a t i o n s b a s e d o n 50 % e f f e c t i v e n e s s o f i r o n i n a b s o r b i n g h e a t , i t w o u l d t a k e a b o u t t w o h o u r s f o r t h e t e m p e r a t u r e o f t h e i r o n t o d r o p o n e d e g r e e c e n t i g r a d e a t a c u r r e n t o f 12 a m p e r e s , a n d w a t e r p r e s s u r e o f 37 p o u n d s p e r s q u a r e i n c h , ( a p p e n d i x 2) ( d ) P r o t o n r e s o n a n c e m e a s u r e m e n t o f f i e l d d r i f t A n R . F . h e a d o f a p r o t o n r e s o n a n c e m a g n e t o m e t e r o f d e s i g n f o l l o w i n g t h a t o f T . C o l l i n s w a s b u i l t u p t o m e a s u r e 24 t h e m a g n e t i c f i e l d d r i f t . I t w a s i n s e r t e d i n t h e o n e i n c h a i r g a p o f t h e m a g n e t a s s h o w n i n f i g u r e 5 s o t h a t a l - - 3 6 - though the search c o i l was i n a homogeneous part of the f i e l d the electron tubes were as far out of the magnetic f i e l d as possible. A sensitive short wave receiver together with a General Radio heterodyne frequency meter, an amplifier and' an oscilloscope were used i n conjunction with this head to carry out the measurements of f i e l d d r i f t . The measurements taken over a one day period are given i n appendix 1. The measurements of d r i f t were taken i n the following manner. The magnetic f i e l d was set with the current regulator to a value within the range of the oscillator for a proton sample i n the search c o i l . The frequency of the R. F. head oscillator was adjusted t i l l the proton resonance signal after amplification was observed near the centre of the trace on the cathode ray oscilloscope. The oscillograph trace was then calibrated in gauss as described i n appendix 1. By tuning the short wave receiver and heterodyne frequency meter to the R. F. frequency, i t could 5 be measured to one part i n 10 by listening for the zero beat. Readings of the oscillator frequency, heterodyne meter crystal calibration as well as the d r i f t i n the batteries supplying the Rubicon potentiometer were taken during the day and were Used i n correcting the d r i f t observed i n the motion of the proton resonance signal across the oscilloscope f i g . 1 5 . P r o t o n R e s o n a n c e R . P . H e a d . t o f a c e p a g e 3 7 . to f a c e p. 3 7. -37- screen. (e) Proton resonance R. F. head The c i r c u i t diagram, fiqure 14, shows the head to contain a 'weakly o s c i l l a t i n g detector' consisting of a p a i r of 6 A G 5 tubes i n push-pull arrangement. The o s c i l l a t i o n s are kept small "by feedback from the ad d i t i o n a l 6 A G 5 tube 24 . . . . . . . connected up as a low gain amplifier . Only one control, the two-gang variable a i r condenser, i s needed to change the frequency to any value i n the range from 20 to 42 megacycles per second. This condenser together with the search c o i l inside of which i s placed a 0.1 molar solu t i o n of MnS04, • forms the tank c i r c u i t which i s loosely coupled to the o s c i l l a t o r tubes. The complete R. F. head i s r i g i d l y mounted i n a heavy brass box with the search c o i l protruding as i l l u s t r a t e d i n f i g u r e 15. Two 40 turn double pancake wound sweep c o i l s are -cemented to the outside s h i e l d plates of the search c o i l f o r 'wobbling';.the.;.field. (f) Simple proton resonance theory Protons, of-spin I = , i , when i n a constant magnetic f i e l d H q w i l l orient themselves i n one of the two quantized energy states; the spin can be p a r a l l e l or a n t i p a r a l l e l to the f i e l d . I f a sample of protons i s placed i n a c o i l be- -38- tween the poles of an electromagnet with the axis of the c o i l at r i g h t angle to the poles, then t r a n s i t i o n s between the two states w i l l he induced; when the frequency of the si g n a l applied to the c o i l s a t i s f i e s the resonance condition' 1 0 2TrV« I V I H Q where V • U H I i s the r a t i o of the magnetic moment to the angular momentum of the nucleus, -j/ i s the frequency i n cycles per second and Ho i s the magnetic f i e l d i n t e n s i t y i n gauss at the nucleus. By using the most recent value, of V"and su b s t i t u t i n g i n the above equations* the simple r e l a t i o n i s obtained 24 25 fo r the magnetic f i e l d H 0 i n terms of the frequency * H Q(kilogauss) = 0.2348 (£ 0.0002) f ( i n mc/s) The accuracy of t h i s equation i s l i m i t e d by that i n the known value of Y . (g) Future use of iproton resonance f o r s t a b i l i z a t i o n Although up to the present the proton resonance s i g n a l has been used only to measure the f i e l d , i t i s intended to feed t h i s information derived from the magnetic f i e l d d i r e c t - l y into the current regulator so as to keep the f i e l d constant 25 and s t a b i l i z e d against a l l changes # The above type of R. F. head i s very suitable f o r t h i s when followed by a d i s - criminator or l o c k - i n detector, f o r i t has a large s i g n a l to -39- noise r a t i o , b e tter than ten to one, and has only -one main frequency c o n t r o l . Three or four R. F. heads may be necessary to cover the range of f i e l d s from a few kilogauss to 20 kilogauss as indicated i n the table below: frequency of o s c i l l a t o r H me/s kilogauss 8.52 2 17.04 4 42.6 10 85.2 20 The lower l i m i t of f i e l d measurements using t h i s method i s set by the weakness of proton signals below about 3,000 27 . gauss , while the upper l i m i t i s determined by the design of a stable variable high frequency o s c i l l a t o r with a separate tank c o i l , tunable up to 85 mc/e. - 4 0 - A p p e n d i x 1 . M e a s u r e m e n t o f f i e l d d r i f t u s i n g t h e p r o t o n r e s o n a n c e m e t h o d . ( a ) C a l i b r a t i o n o f t h e o s c i l l o s c o p e T h e o s c i l l o s c o p e X p l a t e s w e r e f e d f r o m t h e s a m e s o u r c e w h i c h p r o v i d e d t h e ' w o b b l i n g 1 c u r r e n t f o r t h s w e e p c o i l s a r o u n d t h e s e a r c h c o i l s o t h a t t w o r e s o n a n c e p e a k s w e r e o b s e r v e d e a c h c y c l e . T h e o s c i l l a t o r d i a l w a s m o v e d u p a n d d o w n a n d t h e f r e q u e n c y f o r w h i c h t h e r e s o n a n c e p e a k s w e r e a t t h e e x t r e m e r i g h t a n d e x t r e m e l e f t o f t h e s c r e e n n o t e d . F r o m t h i s a n d t h e r e l a t i o n g i v e n i n c h a p t e r 6 , t h e t r a c e o f t h e s c o p e w a s c a l i b r a t e d i n g a u s s . T h e r e s u l t s a r e s u m m a r i z e d a s f o l l o w s : O s c i l l a t o r d i a l H e t e r o d y n e m e t e r C r y s t a l c h e c k C o r r e c t e d r e a d i n g f r e q u e n c y m c / s c o r r e c t i o n m c / s f r e q u e n c y m c p e r s 7 4 . 8 1 4 . 9 3 9 5 - 0 . 0 4 4 2 x 1 4 . 8 9 5 5 = 2 9 . 7 9 1 7 4 . 6 1 4 . 8 4 3 0 - 0 . 0 4 4 2 : X 1 4 . 7 9 9 0 - 2 9 . 5 9 8 T h e f a c t o r t w o c a m e i n a s t h e s e c o n d h a r m o n i c o f t h e h e t e r o - d y n e f r e q u e n c y m e t e r w a s u s e d . T h e r e f o r e t h e w i d t h , r e p r e - s e n t e d b y t h e f o u r i n c h e s o f o s c i l l o s c o p e t r a c e = 2 3 4 . 8 x ( 2 9 . 7 9 1 0 - 2 9 . 5 9 8 0 ) - 4 1 - - 2 3 4 . 8 x 0 . 1 0 3 - 2 4 . 1 8 g a u s s a n d p e r 1 / 1 0 i n c h d i v i s i o n - 2 4 . 1 8 - 0 . 6 0 4 6 " 4 0 " •- a 0 . 6 0 g a u s s . ( b ) S h o r t t i m e t e s t f o r e i g h t m i n u t e s T h e e q u i p m e n t h a d " b e e n r u n n i n g f o r f i v e h o u r s w h e n t h i s t e s t w a s m a d e . T h e b a t t e r y d r i f t w a s n e g l i g i b l e . H o w e v e r t h e o s c i l l a t o r i n c r e a s e d 0 . 0 0 0 8 m c / s i n f r e q u e n c y f r o m t h e "til z e r o t o t h e e i g h t h m i n u t e w h i c h w o u l d s h o w u p a s a d r o p i n f i e l d o f 2 3 4 . 8 x 0 . 0 0 0 8 = 0 . 1 8 8 g a u s s - 0 . 2 g a u s s t h e o b s e r v e d f l u c t u a t i o n w a s f r o m x 0 . 6 = 0 . 3 g a u s s t o - 1 x 0 , 6 = - 0 . 6 g a u s s t h e r e f o r e c o r r e c t i n g f o r t h e 0 . 2 g a u s s d r o p , t h e f l u c t u a t i o n i n t h e f i e l d w a s f r o m -v- 0 . 3 t o - 0 . 4 g a u s s f o r e i g h t m i n u t e s T h e a v e r a g e m e a s u r e d f i e l d o v e r t h e e i g h t m i n u t e s w a s 2 3 4 . 8 x 2 9 . 6 5 = 6 , 9 6 2 g a u s s T h e r e f o r e f l u c t u a t i o n w a s . 7 = 1 p a r t I n 1 0 , 0 0 0 o f t h e 6 9 6 2 f i e l d o v e r a p e r i o d o f e i g h t m i n u t e s v T h e v a l u e o f t h e m a n g a n i n r e s i s t o r u s e d i n t h e c u r r e n t - 4 2 - r e g u l a t i n g s y s t e m w a s 0 . 0 4 9 9 o h m s . T h e v o l t a g e s e t t i n g o n t h e R u b i c o n p o t e n t i o m e t e r w a s 0 . 6 0 0 6 . T h e r e f o r e , t h e m a g n e t c u r r e n t w a s I = 0 . 6 0 0 6 - 1 2 . 0 3 2 0 . 0 4 9 9 = 1 2 . 0 3 a m p e r e s . ( c ) S e v e n h o u r t e s t T h e s m a l l v o l t a g e d i a l o n t h e R u b i c o n p o t e n t i o m e t e r w a s v a r i e d t o m o v e t h e r e s o n a n c e p e a k s f r o m o n e s i d e o f t h e o s c i l l o s c o p e s c r e e n t o t h e o t h e r i n o r d e r t o g e t t h e r e l a t i o n s h i p b e t w e e n s m a l l v o l t a g e c h a n g e s a n d s m a l l m a g n e t - i c f i e l d c h a n g e s . I t w a s f o u n d t h a t a c h a n g e o f 0 . 0 0 5 v o l t s c o r r e s p o n d e d t o a c h a n g e o f 2 4 . 1 8 g a u s s i n t h e f i e l d . T h e e q u i p m e n t h a d b e e n o n f o r o n e h o u r b e f o r e r e a d i n g s " w e r e t a k e n . T h e t e s t l a s t e d s i x h o u r s a n d 5 0 m i n u t e s . T h e r e s u l t s w e r e a s f o l l o w s : B a t t e r i e s d r o p p e d i n V o l t a g e 3 2 . 6 x 6 . 2 = 2 0 2 u v o l t s . ( - 2 0 2 = 0 . 4 9 3 u v o l t s / m i n . ) ( " 4 1 0 r ) T h i s d r o p i s e q u i v a l e n t t o d r o p i n f i e l d o f POP, y 1 0 " 6 x 2 4 . 1 8 s 0 . 9 8 g a u s s 5 x 1 0 - 3 A t t h e s a m e t i m e , t h e c o r r e c t e d o s c i l l a t o r f r e q u e n c y d r o p p e d f r o m 2 x 1 4 . 8 6 1 9 t o 2 x 1 4 . 8 5 8 0 = 0 . 0 0 7 8 m c / s w h i c h i s e q u i v a l e n t t o a d r o p i n f i e l d o f 2 3 4 . 8 x 0 . 0 0 7 8 « 1 . 8 3 g a u s s c o r r e c t i o n s t h e n a r e a f a l l o f 2 . 8 1 g a u s s : - 4 3 - T h e o b s e r v e d f a l l i n i f i e l d w a s 0 . 6 g a u s s s u b t r a c t i n g t h i s f r o m t h e a b o v e c o r r e c t i o n w e g e t 2 . 8 1 c o r r e c t e d r i s e i n f i e l d 2 . 2 1 g a u s s a t a n a v e r a g e f i e l d o f 6 , 9 6 2 g a u s s w h i c h i s a v a r i a t i o n o f 2 . 2 1 - t h r e e p a r t s i n 1 0 , 0 0 0 6 , 9 6 2 - 4 4 - Appendix 2. Thermal ef f e c t s i n the magnet (a) E f f e c t of temperature change on the magnetic f i e l d At the beginning of the long t e s t run the room tempera- ture was 22 degrees centigrade while the cooling water tem- perature at the intake was 15.2 degrees centigrade. At the end of the t e s t the cooling water had r i s e n to 17 degrees centigrade. I f we assume the magnet was cooled 5 degrees centigrade aft e r seven hours we may calculate the decrease i n a i r gap width due to the l i n e a r contraction of the s t e e l of the magnet. The net l i n e a r contraction w i l l be due to a one inch section of s t e e l . Taking ok = 10.5 x 10~ 6 per degree centigrade f o r s o f t s t e e l we f i n d from L e L 0 (1 -h* t) that A L - L Q o ( 4 t therefore -ol At 0 - 10.5 x 10" 6 x 5 - £ part i n 10 4 .. _ This w i l l a f f e c t the f l u x density i n the a i r gap d i r e c t l y -45- as H a i r = 4 T T N I - H i r o n l i r o n 1aiv a^AHair = - / 1 s 2 A l a i r 4 ^ H I - H i r o n I i r 0 n ] •air' therefore A HftiT. = - ./j l a i r T T "J a i r a i r This shows that a net decrease i n a i r gap width of % part i n 10,000 w i l l cause an increase i n f i e l d strength of the same amount. (b) Estimated time f o r magnet to cool This c a l c u l a t i o n i s not straightforward because of the unknown facto r s i n heat transfer from the copper c o i l s to the cooling water and also to the s t e e l of the magnet. However, i t can be estimated with the help of the empirical information given i n H. C. Roters 'Electromagnetic Devices'. The data i s as follows: Room temperature 22 degrees centigrade Cooling water intake temperature 15.2 " " Cooling water outlet temperature 17 " " (after 7 hours) Tot a l flow of cooling water 92.2 c.c./sec. Magnet current 12.03 amperes -46- Resistance of copper 4.6 ohms Weight of copper present 500 l b s . Weight of s t e e l of magnet 7-jjr tons. Heat capacity of copper 180 joules /lb./degree centigrade- Heat capacity of s t e e l 225 joules/lbs./degree centigrade The heat energy added by the e l e c t r i c current i s (12.03) 2 x 4.6 = 667 watts = 667 joules/see. The heat energy taken away by the cooling water i s 92.2 x 4.18 x 2 = 770 joules/sec. This leaves a net amount of heat energy of 103 joules/sec. which i s taken away from the copper and s t e e l and thus w i l l lower t h e i r temperature. Considering the copper alone, to lower i t s temperature one degree centigrade requires taking away from i t of 500 x 180 = 90,000 joules. Therefore, i f the copper i s considered f i r s t alone, the time taken to lower i t s temperature one degree centigrade i s 9Q«Q0Q = 14.6 minutes. 103 x 60 After some time, the s t e e l becomes between 45 % and 50 % e f f e c t i v e i n absorbing heat"1*. Therefore, assuming 50 % effectiveness, the heat energy taken away to lower i t s temperature by one degree centigrade i s -47-. £ x 7.5 x 2,000 x 225 = 1,690,000 joules. To t h i s , add that of the copper 90,000 joules g i v i n g a t o t a l of 1,780,000 joules. The time required to lower the whole magnet one degree centigrade once heat transfer had "been established between the copper and s t e e l would be 1.780.000 =4.8 hours 103 x 60 x 60 which could only account f o r a drop of about degrees centigrade i n seven hours. A complete analysis would have to take into account the thermal conductivity of the s t e e l of the magnet follow- ed by a s o l u t i o n of Fourier's heat equation f o r t h i s shape of s o l i d . I t can e a s i l y be seen that the s t e e l of the pole pieces would have to cool down before the yoke of the magnet could f e e l the e f f e c t of the cooling water. An assumption that the pole pieces formed \ of the weight of the magnet would change the time to cool them one degree to the r i g h t amount. This would change the time to cool the copper plus the s t e e l of the pole pieces only, to 1.4 hours f o r a one degree drop or 7 hours f o r a drop of 5 degrees centigrade. -48- Appendix 3. Adjustment to the c o n t r o l system necessary f o r optimum operation The procedure f o r s t a r t i n g the magnet i s as follows: 1. Push the main toggle switch S-^ up to apply power to the amplifier and a l l heaters. This l i g h t s the amber indicator "bulb. A f i v e minute time delay prevents operating the H . T. relay. 2. After waiting f i v e minutes, (or aft e r hearing the ' c l i c k ' of the time delay relay) push button S 4 whereupon the red jewel indicator shows, i n d i c a t i n g H . T. i s applied to the power unit. I f the red jewel does not l i g h t , then one or more of the i n t e r l o c k c i r c u i t s are open. These interlocks are: (a) A'-toggle switch on t h i s main panel marked ' H . T. interl o c k . This must be i n the 'on' p o s i t i o n . (b) The pressure switch at the intake manifold, the cooling system. Water must be turned on and most important the water outlet tap must also be turned on. (c) A one milliampere relay on the regulator tube -49- chassis. This relay opens i f the 6 A S 7 grids "become p o s i t i v e . (d) The push switch on the doors of the cabinet rack enclosing the power unit. This cabinet rack i s mounted i n the motor-generator room. Now, providing a l l the interlocks are closed, and depend- ing on the Rubicon potentiometer s e t t i n g and D. C. l e v e l c o n t r o l , the generator shunt f i e l d current meter 1% should read ( and the 6 A S 7 cathode current meter M3 i f plugged into one of the j a c k s ) . 3. Now the motor generators may be switched on by pushing the two black 'on' buttons, l i g h t i n g the i n d i c a t o r lamps. The magnet current meter should read. Current i s increased or decreased by moving the main voltage d i a l s on the Rubicon potentiometer. After each change of set t i n g , the D. C. l e v e l c o n t r o l knob should be adjusted so that balance meter M 4 reads centre. This should, at the same time cause error s i g n a l meter Mg to read minimum. I f i t does not do so, then the hum balancing potentiometer (screw adjustment) mounted at the back of the D. C. amplifier power supply chassis should be adjusted f o r minimum reading on t h i s meter with no signals coming i n (disconnected both shielded leads to the D. C. amplifier chassis input and ground the centre terminals). For preliminary adjustment and c a l i b r a t i o n of the -50- Rubicon potentiometer - see ' Operating d i r e c t i o n s p e c i a l Rubicon type "B" potentiometer cat. no. 2780, s e r i a l no. 53750.' from the Rubicon Company, Philadelphia, Pa., U.S.A. I f during the above adjustments, the shunt f i e l d current meter Mg drops to zero,'and the magnet current f a l l s while the meters on the amplifier panel, M 4 , M 5 and Mg, show o f f "balance readings, then the current overload relay ( figure 9) must have been tripped, opening the shunt f i e l d c i r c u i t of the generators. This may be reset by pushing the reset button on the current regulating tube chassis. I f i t w i l l not stay i n , then either there i s a short i n the c i r c u i t which would show up by a reading greater than one ampere i n Mg or else the rheostat adjustment on the relay i s set too low. This i s a screw d r i v e r adjustment at the back of the current regulating tube chassis. I t should be adjusted to t r i p a t j j u s t above one ampere of shunt f i e l d current. -51- Appendix 4. Use of f l i p - c o i l s and fluxmeter F i e l d strength measurements were made with s p e c i a l l y constructed f l i p - c o i l s and a Rawson E l e c t r i c a l Co. f l u x - meter. Four c o i l s wound on l u c i t e forms were made up to cover the range from 10 gauss to 20,000 gauss. Each c o i l was screwed t i g h t l y to a long, smooth "board two inches wide and shimmed with corrugated cardboard s t r i p s so as to s l i d e into and out of the one inch pole gap without turning. The "board was rubbed l i g h t l y with p a r a f f i n wax so that i t moved e a s i l y though f i t t i n g t i g h t l y . A square hardwood frame with holes spaced one inch and f i t t e d with l u c i t e plugs was mounted over the magnet pole pieces to hold the f l i p - c o i l s i n any p o s i t i o n i n the gap between the poles. Two of the f l i p - c o i l s were c a l i b r a t e d on a magnet using a proton resonance s i g n a l to measure the f i e l d . The other two c o i l s were then c a l i b r a t e d against these. B i b l i o g r a p h y 1 . R o t e r s , H . C . , " E l e c t r o m a g n e t i c D e v i c e s " , J o h n W i l e y a n d S o n s , N . Y . , 1 9 4 1 . 2 . S t a f f , M. I . T . , " M a g n e t i c C i r c u i t s a n d T r a n s f o r m e r s " , J o h n W i l e y a n d S o n s , N . Y . , 1 9 4 3 . 3 . B l a c k e t t , P . M . S . , P r o . R o y . S o c . 1 5 4 A ( 1 9 3 6 ) 4 . R o s e t t i , F . , " E l e m e n t s o f N u c l e a r P h y s i c s " , P r e n t i c e - H a l l I n c . , N . Y . , 1 9 4 8 . 5 . B a m e , S . J . a n d B a g g e t L . ffi., T h e R i c e I n s t i t u t e , H o u s t o n , T e x a s , P r o g r e s s R e p o r t N 6 o n r - 2 2 4 , T a s k o r d e r 1 N u c l e a r P h y s i c s , O c t . 1 , 1 9 4 9 . 6 . H a r n w e l l , G . P . , " P r i n c i p l e o f E l e c t r i c i t y a n d M a g n e t - i s m " , M c G r a w H i l l , N . Y . , 1 9 3 8 . 7 . C a r o , D . E . , P a r r y , J . K . , J . S . I . , 2 6 , 1 1 , 1 9 4 9 8 . L a w s o n , J . L . a n d T y l e r , A . W . , R e v . S c i . I n s t . 1 0 , 3 0 4 , ( 1 9 3 9 ) . 9 . R u t g e r s U n i v e r s i t y , D e p t . o f P h y s i c s , P r o g r e s s R e p o r t o n P r o j e c t n o . 4 1 . 1 0 . E l e c t r o n i c E n g i n e e r s o f t h e W e s t i n g h o u s e E l e c t r i c C o r p o r a t i o n " I n d u s t r i a l E l e c t r o n i c s R e f e r e n c e B o o k " , J o h n W i l e y a n d S o n s , 1 9 4 8 . 1 1 . D r i v e r H a r r i s C o . , " N i c h r o m e a n d o t h e r H i g h N i c k e l E l e c t r i c a l A l l o y s " . 1 2 . R u b i c o n C o . , P h i l a d e l p h i a , B u l l e t i n n o . 1 0 0 . - 5 3 - 1 3 . C o m m u n i c a t i o n f r o m F e r g u s o n B a t t e r y C o . . 1 4 . M e m o A E / c y e l / R L M , p r i v a t e c o m m u n i c a t i o n f r o m T . R . - E * , 1 9 4 6 . 1 5 . G r e e n w o o d I . , H o l d a m J . , M a c r a e D . / ' E l e c t r o n i c I n s t r u - m e n t s " , M . I . T . R a d i a t i o n S e r i e s V o l . 2 1 , M c G r a w H i l l N . Y . , 1 9 4 8 . 1 6 . M i n n e a p o l i s - H o n e y w e l l R e g u l a t i n g C o . , B r o w n I n s t r u m e n t D i v i s i o n , B u l l e t i n 1 5 - 1 6 , " B r o w n E l e c t r o n i k C o n t i n u o u s B a l a n c e P o t e n t i o m e t e r P y r o m e t e r " . 1 7 . M. I . T . R a d i a t i o n S e r i e s V o l u m e s . 1 7 , p a g e 4 9 8 1 8 , p a g e 4 0 2 1 . p a g e s 4 8 7 , 5 1 4 , 5 1 5 , 5 4 3 , 5 5 4 2 5 p a g e 1 0 8 1 8 . P r i v a t e C o m m u n i c a t i o n f r o m D r . E l l i o t t a n d D r . F e r g u s o n C h a l k R i v e r L a b o r a t o r i e s , N . R . C . 1 9 . B l o e m e r i b e r g e n , N . , " N u c l e a r M a g n e t i c R e l a c a t i o n " , T h e H a g u e , M a r i u s N i j h o f f , 1 9 4 8 . 2 0 . P u r e e l l , T o r r e y a n d P o u n d , P h y s . R e v . 6 9 , 3 7 , ( 1 9 4 6 ) . 2 1 . B l o c h , H a n s e n , P a c k a r d , P h y s . R e v . 6 9 , 1 2 7 , ( 1 9 4 6 ) . 2 2 . E l o e m e r i b e r g e n , P u r c e l l a n d P o u n d , P h y s . R e v . 7 3 , 6 7 9 , ( 1 9 4 8 ) . 2 3 . Anderson., H . L . , P h y s . R e v . 7 6 , 1 4 6 4 , ( 1 9 4 9 ) . 2 4 . C o l l i n s , T . L . P h . D . T h e s i s , U . B . C . , 1 9 5 0 . - 5 4 - 2 5 . P a c k a r d , M . E . , R e v . S c i . I n s t . , 1 9 , 4 3 5 , ( 1 9 4 8 ) . 2 6 . B l o c h , N i c o d e r a u s , S t a u b , P h y s . R e v . 7 4 , 1 G 2 5 , ( 1 9 4 8 ) . 2 7 . H o p k i n s , N. J . , R e v . S c i . I n s t . , 2 0 , 4 0 1 , ( 1 9 4 9 ) . .

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