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Submitted on 1 Jan 1984
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STABILITY IMPROVEMENTS IN THE TRIUMF CYCLOTRON MAGNETIC FIELD TO ± 0.7ppm
K. Reiniger, H. Baumann, D. Dohan, D. Pearce
To cite this version:
K. Reiniger, H. Baumann, D. Dohan, D. Pearce. STABILITY IMPROVEMENTS IN THE TRIUMF CYCLOTRON MAGNETIC FIELD TO± 0.7ppm. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-225-C1-228. �10.1051/jphyscol:1984145�. �jpa-00223701�
STABILITY IMPROVEMENTS IN THE TRIUMF CYCLOTRON MAGNETIC FIELD TO
t 0.7ppm
K. R e i n i g e r , H. Baumannr, D. Dohan and D . Pearce
TRIUMF, University o f B.C., 4004 Wesbrook MaZZ, Vancouver V6P 2A3, Canada ' S I N , 5234 ViZZigen, SwitzerZand
Rksumk - L'aimant du c y c l o t r o n TRIUMF p r o d u i s a n t un champ maximum de 5 , 6 kG pour un c o u r a n t e x c i t a t e u r de18.000A a k t 6 rkcemment s t a b i l i s i : h 2 0,7 pprn, dans l e b u t d ' o b t e n i r une h a u t e r k s o l u t i o n de f a i s c e a u , avec deux couches skparkes h 500 MeV. Nous exposons l a mkthodologie employke pour a m e l i o r e r l e s performances de l ' a l i m e n t a t i o n de p u i s s a n c e de p l u s d'un o r d r e de gran- d e u r . Outre d e s boucles de c o u r a n t e t de t e n s i o n on a u t i l i s k d e s boucles a c t i v e s d e champ u t i l i s a n t 2 l a f o i s une bobine de d e t e c t i o n e t un systsme 2
RMN. Une a m k l i o r a t i o n u l t i m e d ' e n v i r o n un f a c t e u r 2 , de l a phase du f a i s c e a u a k t 6 obtenue h l ' a i d e de signaux de phase e x t r a i t s du f a i s c e a u .
A b s t r a c t - The TRIUMF c y c l o t r o n magnet with a h i l l f i e l d s t r e n g t h of 5.6 kG a t an e x c i t a t i o n c u r r e n t of 18,000 amperes h a s r e c e n t l y been s t a b i l i z e d t o
f 0.7 ppm with a view t o r u n n i n g high r e s o l u t i o n beam w i t h s e p a r a t e d t u r n s a t an energy of 500 MeV. The paper w i l l h i g h l i g h t t h e methodology involved i n improving t h e performance of t h e power supply by more t h a n an o r d e r of magnitude. I n a d d i t i o n t o c u r r e n t and v o l t a g e feedback l o o p s , a c t i v e f i e l d feedback from both a f l u x c o i l and an NMR system were used. F u r t h e r impro- vement by about a f a c t o r two t o beam phase s t a b i l i t y was achieved u s i n g beam phase s i g n a l s from t h e e x t r a c t e d beam.
INTRODUCTION
I n t h e f a l l of 1982, work was begun t o s t a b i l i z e t h e f i e l d o f t h e magnet of t h e TRIUMF c y c l o t r o n t o t h e o r d e r of a p a r t p e r m i l l i o n . A t t h e t i m e , f i e l d s t a b i l i t y was i n t h e o r d e r of one t o two p a r t s i n l o 5 comprising of c e r t a i n unexplained e x c u r s i o n s i n t h e magnetic f i e l d a s observed u s i n g an NMR system and monitoring of beam phase by means of an e x t e r n a l c a p a c i t i v e probe u s i n g t h e e x t r a c t e d beam. The p r a c t i c e a t t h a t time was t o t r a c k t h e f i e l d v a r i a t i o n with t h e RF frequency t o p r o v i d e s t a b l e beam o p e r a t i o n .
The TRIUMB c y c l o t r o n magnet i s e x c i t e d by a s e r i e s p a s s c u r r e n t r e g u l a t e d DC power supply w i t h an o u t p u t of 80 v o l t s DC and a c u r r e n t of 18,500 amperes. The c u r r e n t feedback element i s a w a t e r cooled even ohm s h u n t w i t h a c h a r a c t e r i s t i c v o l t a g e drop of 1 v o l t a t a c u r r e n t of 26,700 amperes, o r a nominal v o l t a g e of 0.69 v o l t s a t o p e r a t i n g c u r r e n t . As o r i g i n a l l y d e l i v e r e d t h e power supply had an i n h e r e n t s t a b i l i t y o f two t o t h r e e p a r t s i n l o 5 and work was done i n 1980 t o b r i n g t h e p e r - formance of t h e power supply t o an a p p a r e n t c u r r e n t s t a b i l i t y of a few p a r t s i n l o 6 .
An a t t e m p t was made t o i n t r o d u c e a c t i v e f i e l d feedback f o r f u t h e r f i e l d c o r r e c t i o n , b u t it was found t h a t t h e supply would break i n t o unexplained o s c i l l a t i o n s . The r e s u l t was t h e implementation of low l e v e l a c t i v e feedback f o r minor f i e l d c o r r e c - t i o n s below t h e l e v e l which would r e s u l t i n o s c i l l a t i o n s . The c o n t r o l l o o p a s shown i n f i g . 1 was implemented.
Field Coil
Fig. 1 O r i g i n a l C o n t r o l Loop Ref. V
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984145
JOURNAL DE PHYSIQUE
METHOD
B e f o r e embarking on a program of implementing changes t o t h e e x i s t i n g s u p p l y , i t was n e c e s s a r y t o b e t t e r u n d e r s t a n d t h e e x i s t i n g power s u p p l y b e h a v i o u r , s p e c i f i c a l l y w i t h r e s p e c t t o t h e o b s e r v e d i n s t a b i l i t i e s . An e x h a u s t i v e t r a n s f e r f u n c t i o n a n a l y s i s was p e r f o r m e d , which showed t h e c o n t r o l l o o p t o be s t a b l e . E x t e n s i v e measurements were t a k e n t o q u a n t i f y t h e performance of t h e power s u p p l y , r e l a t i n g c u r r e n t v a r i a t i o n s t o f i e l d v a r i a t i o n s , c u r r e n t s t a b i l i t y t o f i e l d s t a b i l i t y , r e f e r e n c e s t a b i l i t y , and t h e performance of t h e e x i s t i n g c o n t r o l l o o p a s a whole.
To a c h i e v e t h e d e s i r e d c u r r e n t s t a b i l i t y of one p a r t i n l o 6 , t h e r e a r e a number of p r e r e q u i s i t e s :
1 ) An e r r o r a m p l i f i e r c a p a b l e of d i s c r i m i n a t i n g changes i n o u t p u t c u r r e n t t o one p a r t i n lo6.
2 ) The r e f e r e n c e v o l t a g e s o u r c e and t h e c u r r e n t m e a s u r i n g s h u n t must b o t h be s t a b l e t o b e t t e r t h a n one p a r t i n lo6.
3 ) A s t a b l e c o n t r o l l o o p which d o e s n o t i t s e l f i n t r o d u c e c u r r e n t f l u c u a t i o n s i n e x c e s s of t h e r e q u i r e m e n t .
4 ) Sound and j u d i c i o u s c o n s t r u c t i o n t e c h n i q u e s w i t h o u t which a d d i t i o n a l s o u r c e s o f e r r o r and i n s t a b i l i t y a r e i n t r o d u c e d .
By m o n i t o r i n g t h e m a g n e t i c f i e l d v i a a f i e l d s e n s i n g c o i l and t h e power s u p p l y o u t p u t c u r r e n t s i m u l t a n e o u s l y , i t became e v i d e n t t h a t t h e r e e x i s t e d b o t h s p u r i o u s and p e r i o d i c changes i n t h e m a g n e t i c f i e l d which a p p e a r e d t o be t o t a l l y u n r e l a t e d t o o b s e r v e d c u r r e n t f l u c t u a t i o n s . It was a l s o n o t i c e d t h a t o u t p u t c u r r e n t v a r i e d depending on whether o r n o t t h e power s u p p l y d o o r s were open. It was a l s o p o s s i b l e t o i n t r o d u c e i n s t a b i l i t i e s i n o u t p u t c u r r e n t by moving w i r e s l e a d i n g from t h e s h u n t t o t h e e r r o r a m p l i f i e r and from t h e e r r o r a m p l i f i e r t o t h e m a s t e r / s l a v e a m p l i f i e r s l o c a t e d some 10 f e e t d i s t a n t . See f i g . 1. A d d i t i o n a l s h i e l d i n g on t h e s e w i r e s was a t t e m p t e d which somewhat d e c r e a s e d t h e t e n d e n c y towards i n s t a b i l i t y , however n o t enough t o be s a t i s f a c t o r y . It was d i s c o v e r e d t h a t t h e e r r o r a m p l i f i e r and t h e p r e a m p l i f i e r were t i e d t o s l i g h t l y d i f f e r e n t ground p o t e n t i a l s due t o t h e i r p h y s i c a l l o c a t i o n . A s a r e s u l t t h e r e was a c i r c u l a t i n g ground c u r r e n t which i n t r o d u c e d e r r o r s i n t h e s h u n t v o l t a g e which was used t o r e g u l a t e t h e o u t p u t c u r r e n t . T h i s problem was s o l v e d by t h e i n s e r t i o n of a d i f f e r e n t i a l a m p l i f i e r b u f f e r between t h e e r r o r
negating of effects previously seen in opening and closing the doors of the power supply cabinets.
Once it was established that the power supply was in fact an accurate and stable constant current source the variations in the magnetic field as measured by the NMR system and the flux sensing coil were still in excess of those to be predicted based on the actual current fluctuations observed. This lead us to believe that regardless of how precise the current regulation was, beyond a certain minimum threshold, we were dealing now with other phenomena which had to be compensated for by changing the output current as required to achieve the desired constant field.
As the basic overall criteria of system performance is the extracted beam phase stability, a great deal of effort was put into establishing the correlation of independent readings using beam phase, NMR system readout and flux sensing coil with the view of using one of these signals to fine tune the magnet current to establish the required stability in beam phase. NMR feedback and beam phase feedback have some inherent drawbacks. Beam phase which would be the ultimate fine tuning mechanism regretably is only present when beam i$ in fact extracted. N M R feedback has the disadvantage of progressively deteriorating signal as a function of radiation exposure. It is also only indicative of the field in one highly localized area, being highly sensitive to local trim coil changes and therefore not representative of the magnetic field throughout the cyclotron.
We therefore reintroduced active field feedback by integrating the signal from the field sensing coil and introducing this signal to the input summing amplifier to provide the appropriate current corrections.
RESULTS
The basic performance goals of the power supply based on the modified control loop were exceeded with observed stability of the magnetic field for periods exceeding 8 hours being + 0.7 parts per million as shown in fig. 2. Fig. 3 represents the modified control loop.
I I
5 10
T ( rnin )
F ig.2 Field Stability Comparison
JOURNAL DE PHYSIQUE
Field Coil
Ref. V
Differential Buffer
Integrator 1 amp
p-
v O u tV Buffer
Shunt
I out
L A - -
F i g . 3 Modified Control Loop as Implemented
