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THE QUENCH BEHAVIOUR OF SMALL SUPERCONDUCTIVE COILS
J. Cornelis, F. Biermans, N. Maene, A. van den Bosch
To cite this version:
J. Cornelis, F. Biermans, N. Maene, A. van den Bosch. THE QUENCH BEHAVIOUR OF SMALL SUPERCONDUCTIVE COILS. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-525-C1-528.
�10.1051/jphyscol:19841107�. �jpa-00223575�
J O U R N A L DE PHYSIQUE
Colloque C I , supplement a u n o 1, Tome 45, janvier 1984 page C l - 5 2 5
THE QUENCH BEHAVIOUR OF SMALL SUPERCONDUCTIVE C O I L S
J. Cornelis, F. Biermans, N. Maene and A. Van den Bosch
Materials Science Department, S. C . K . / C . E. N . , B-2400 Mol, Belgium
Resume
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Des e x p e r i e n c e s o n te t e
e f f e c t u e e s s u r des s o l e n o i d e s de p e t i t e Tiii&%ion, bobings ii l ' a i d e de f i l commercial ii f i l a m e n t s m u l t i p l e s . Ces s o l e n o i d e s supraconducteurs s o n t branches s u r une a l i m e n t a t i o n e n c o u r a n t c o n t i n u q u i c r o f t progressivement dans l e temps j u s q u ' i i l a t r a n s i t i o n ii l ' e t a t normal. Les donnees s u r l a c a r a c t e r i s t i q u e de l a charge e t s u r l a t r a n s i t i o n s o n t e n r e g i s t r e e s s u r un systeme d ' a c q u i s i t i o n de donnees ii cadence moderee. L e t r a n s i t o i r e e s t e n r e g i s t r e s u r un o s c i l l o s c o p e d i g i t a l ii memoire ii deux canaux, p u i s t r a n s f e r e v e r s l e systeme d ' a c q u i s i t i o n de donnees. L ' a n a l y s e des r e s u l t a t s f o u r n i t l e s parametres c a r a c t e r i s t i q u e s de l a t r a n s i t i o n 8 l ' e t a t c o n d u c t e u r .A b s t r a c t
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Quench experiments have been c a r r i e d o u t on s m a l l c o i l s , wound w i t h commercial NbTi mu1 ti fi l a m e n t w i r e . These s u p e r c o n d u c t i v e c o i l s a r e e n e r g i z e d by a DC power s u p p l y i n a mode t h a t i n c r e a s e s t h e sample c u r r e n t a t a c o n s t a n t r a t e u n t i l t h e quench o c c u r s . The d a t a on t h e l o a d l i n e and t h e quench a r e t a k e n by a d a t a a c q u i s i t i o n system a t a moderate r a t e . The t r a n s i e n t s i g n a l s a r e r e c o r d e d by a f a s t two-channel d i g i t a l s t o r a g e o s c i 11 oscope and r e g i s t e r e d a f t e r w a r d s by t h e d a t a a c q u i s i t i o n system. The a n a l y s i s o f these d a t a y i e l d s t h e s e l f - i n d u c t a n c e o f t h e c o i l , t h e quench parameters and t h e c a l o r i m e t r i c b e h a v i o u r .INTRODUCTION
A few y e a r s ago a s t u d y s t a r t e d i n o u r i n s t i t u t e f o r e v a l u a t i n g t h e importance o f d i f f e r e n t t e c h n i q u e s t o be used i n t h e f i e l d o f a p p l i e d s u p e r c o n d u c t i v i t y . The f i r s t aim was t h e c h a r a c t e r i z a t i o n o f s u p e r c o n d u c t i n g m a t e r i a l s . An apparatus has been s e t up /1/ f o r measuring c r i t i c a l c u r r e n t s on superconductors o f t e c h n o l o g i c a l i n t e r e s t . The system i s e q u a l l y w e l l s u i t e d f o r measuring quench c u r r e n t s on small supercon- d u c t i v e c o i l s . The d a t a a c q u i s i t i o n system as d e s c r i b e d i n r e f e r e n c e /1/ i s t o o slow t o y i e l d much i n f o r m a t i o n on t h e quench i t s e l f . The t r a n s i e n t signals a t t h e +quench, t h e r e f o r e , a r e now a l s o r e c o r d e d by a f a s t two-channel d i g i t a l s t o r a g e
o s c i l l o s c o p e . The aim o f t h e p r e s e n t c o n t r i b u t i o n i s t o r e p o r t on t h e quench expe- r i m e n t s , c a r r i e d o u t on s m a l l superconductive maqnet c o i l s .
EXPERIMENTAL SET-UP
A s e r i e s o f f o u r c o i l s have been wound, i n t h e E l e c t r o n i c s and I n s t r u m e n t a t i o n S e r v i c e , w i t h NbTi m u l t i f i l a m e n t w i r e , Niomax FM A61/60, p r o c u r e d f r o m I M I . The bare w i r e d i a m e t e r i s 0.60 mm. The copper-to-superconductor r a t i o i s 1.35. A l l c o i l s have been wound on a 16.6 mm d i a m e t e r s p i n d l e o v e r a Tength o f 39.5 mm. The number o f t u r n s p e r c o i l i s between 978 and 1410, t h e t o t a l superconductor l e n g t h p e r c o i l b e i n g between 80 and 136 m e t r e s . The c o i l s 2, 3 and 4 a r e wet wound w i t h d i f f e r e n t polymers. Each c o i l was measured i n l i q u i d h e l i u m a t normal p r e s s u r e .
The b a s i c c i r c u i t o f t h e quench measuring set-up i s g i v e n i n F i g u r e 1. The c o i l c u r r e n t i s generated by a Drusch DC power s u p p l y . Below an upper l i m i t , t h e supply
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841107
C1-526 JOURNAL DE PHYSIQUE
C o i l I
I .--- J
Dew ar
F i g . 1
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C i r c u i t o f t h e quench-current measuring set-up.f u n c t i o n s as a v o l t a g e - c u r r e n t c o n v e r t o r (VCC) which i s c o n t r o l l e d by a Drusch A33 s i g n a l g e n e r a t o r (SG). When t h e o u t p u t v o l t a g e reaches i t s upper l i m i t t h e power s u p p l y a c t s as a c o n s t a n t v o l t a g e d e v i c e . The c u r r e n t s a r e measured o v e r t h e r e f e - rence r e s i s t o r w i t h a d i g i t a l v o l t m e t e r . The t r a n s i e n t c u r r e n t s i g n a l E2 i s sampled over the leads b y channel 2 o f a T e k t r o n i x 468 d i g i t a l s t o r a g e o s c i l l o s c o p e . Channel 1 samples E l t h e v o l t a g e o f t h e c o i l , which v o l t a g e i s a l s o measured by a h i g h p r e c i s i o n v o l t m e t e r . The scope samples a t a r a t e o f thousand r e a d i n g s p e r second, t a k i n g a l t e r n a t i v e l y channel 1 and channel 2 .
EXPERIMENTAL RESULTS
The superconductive c o i l s a r e e n e r g i z e d by the power s u p p l y i n a mode t h a t i n c r e a s e s t h e sample c u r r e n t u n t i l t h e quench o c c u r s . D u r i n g t h e ramping up t h e s e l f i n d u c t a n - ce L = E l / ( d I / d t ) can be measured t o a f r a c t i o n o f a p e r c e n t . Not o n l y E l , I and the t i m e t a r e sampled b u t t h e t e n s i o n on t h e H a l l probe, which i s p o s i t i o n e d a t t h e c e n t r e o f t h e c o i l , i s a l s o measured a l l o w i n g t h e c a l c u l a t i o n o f t h e l o a d l i n e . Per- s u i n g t h e experiment t h e d a t a a c q u i s i t i o n system a l s o r e g i s t e r s t h e quench. A few data a r e g i v e n i n Table 1 f o r c o i l 4 f o r which L =24.9 mH.
Table 1 s e t
1 2 3
t ( s ) El ( V ) I ( A ) R(Q)
113.030 0.0408 179.35
113.320 2.8392 24.03 0.118
113.535 2.8742 6.10 0.471
: I
6 115.440 113.903 114.114 2.8740 2.8736 2.8727 6.15 6.25 6.52 0.467 0.459 0.4407 116.221 2.8724 6.62 0.433
Each s e t o f d a t a I ( A ) and E l ( V ) i s sampled a t a t i m e t ( s ) . A t y p i c a l sampling se- quence i s g i v e n b y t h e t i m e values o f s e t 1 t o 5. I t means t h a t t h e sampling r a t e i s somewhat l e s s t h a n 4 p e r second. The f i r s t s e t o f d a t a i s taken j u s t b e f o r e t h e quench. H a l f a second l a t e r t h e t r a n s i e n t i s o v e r . A t s e t 3 t h e s u p p l y a l r e a d y aperates as a c o n s t a n t v o l t a g e d e v i c e . The d i f f e r e n c e s i n t h e E l values between s e t 7 and 3 a r e due t o t h e v o l t a g e d r o p o v e r t h e l e a d s . A f t e r t h e t r a n s i e n t , t h e r e s i 5 - tance o f t h e c o i l , R = E1/I, i s equal t o 0.471 R. I t decreases w i t h t i m e .
The quench c u r r e n t i s remarkably r e p r o d u c i b l e . Up t o 19 quenches have been recorded i n t h e same c o n d i t i o n s , a t a nominal ramping r a t e o f 1000 A/10 min. The average o f t h e quench c u r r e n t , i s Iq = 179.29 A w i t h o = 0.15 A .
The d a t a a f t e r t h e quench show a s i m i l a r r e l i a b i l i t y . The average minimum c u r r e n t r i g h t a f t e r t h e quench, i s Im = 6.10 A w i t h u = 0.03 A . The c a l c u l a t e d average maximum c o i l r e s i s t a n c e , Rm = 0.471 R p r a c t i c a l l y has t h e same. r e l a t i v e p r e c i s i o n . F o r comparison the c o i l r e s i s t a n c e R10 = 0.145
R
i s g i v e n . I t has been measured, a f t e r h e l i u m b o i l o f f , d u r i n g t h e h e a t i n g up o f t h e c o i l , j u s t above t h e supercon- d u c t i v e t r a n s i t i o n temperature (Tc 10 K ) . The r e s i s t a n c e values R ~ T = 14.823 Rand R = 1.923 R have been measured a t room temperature (297 K) and a t l i q u i d n i - t r o g e h N temperature (77.3 K ) r e s p e c t i v e l y .
The v o l t a g e and c u r r e n t e v o l u t i o n on t h e c o i l a t t h e quench were r e c o r d e d w i t h a T e k t r o n i x 468 d i g i t a l s t o r a g e o s c i l l o s c o p e i n t h e p r e t r i g g e r mode a t a sampling r a t e which i s a b o u t two o r d e r s i n magnitude f a s t e r t h a n t h a t o f t h e d a t a a c q u i s i t i o n system used f o r t a k i n g t h e d a t a i n Table 1. The scope reads a l t e r n a t i v e l y t h e v o l - tage o v e r t h e c o i l and t h e v o l t a g e o v e r one o f t h e l e a d s . The d a t a a r e s t o r e d f r o m about 0 . 1 second b e f o r e t h e v o l t a g e r a i s e t o 0.4 s a f t e r t h a t s i g n a l . A f t e r t h e quench experiment t h e scope d a t a a r e t r a n s f e r r e d t o t h e d a t a a c q u i s i t i o n system.
The p r e c i s i o n o f t h e scope i s l i m i t e d t o 8 b i t s so t h a t t h e r e s o l u t i o n i s n o t b e t t e r t h a n 1 p e r c e n t o f t h e maximum s i g n a l .
F o r t h e c o i l c u r r e n t t h e s c a t t e r i s s t i l l more i m p o r t a n t as t h e maximum s i g n a l , which i s l e s s t h a n 0.24 V , c a r r i e s more t h a n 4 p e r c e n t n o i s e . The s c a t t e r has been reduced by a v e r a g i n g t h e d a t a o v e r 19 quenches, e l i m i n a t i n g t h o s e values which d e v i a t e f r o m t h e average by more t h a n t h r e e sigma. By t h e same ~ r o c e d u r e t h e v o l - tages o v e r t h e c o i l have been averaged t o o . The r e s u l t i s g i v e n i n F i g u r e 2.
T I M E ( s
>
F i g . 2
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C o i l c u r r e n t and v o l t a g e versus t i m e a t t h e quench.A f t e r t h e quench t h e r e m a i n i n g c u r r e n t causes J o u l e h e a t i n g i n t h e c o i l . T h i s h e a t i n g can be reduced by r e d u c i n g t h e o u t p u t v o l t a g e o f t h e power s u p p l y . The e x p e r i m e n t r e v e a l s t h a t t h e c o i l r e s i s t a n c e f o l l o w s r a t h e r q u i c k l y t o r e a c h a s t e a d y s t a t e . Some steady s t a t e d a t a a r e g i v e n i n Table 2.
Table 2
The quench c u r r e n t has been shown t o be v e r y r e p r o d u c i b l e f o r c o n s t a n t w o r k i n g con- d i t i o n s . Changing these, however, can change t h e quench c u r r e n t , as does t h e ramp r a t e . I n Table 3 nominal ramp r a t e s a r e r e p o r t e d t o g e t h e r w i t h t h e c o r r e s p o n d i n g quench c u r r e n t which has been measured on c o i l 4.
JOURNAL DE PHYSIQUE
Table 3
DISCUSSION
The c o i l r e s i s t a n c e above Tc i s l a r g e l y determined by t h e copper o f t h e w i r e / 2 / . The measured s e l f - i n d u c t a n c e L i s a c c u r a t e enough t o analyse t h e quench b e h a v i o u r . The quench b e h a v i o u r o f c o i l 4, as g i v e n i n F i g u r e 2, r e v e a l s t h a t a t t h e quench t h e v o l t a g e o v e r t h e c o i l r i s e s s t e e p l y f r o m 0.04 V t o 2.96 V i n 28 ms. The appea- rance o f t h e sharp peak i n t h e average o v e r t h e 19 quenches i n d i c a t e s t h a t t h e j i t t e r i n t h e t r i g g e r i n g i s n o t l a r g e r t h a n t h e t i m e r e s o l u t i o n o f t h e scope, 2 ms i n t h i s case. A t t h e peak t h e power s u p p l y s w i t c h e s f r o m t h e c o n t r o l l e d c u r r e n t mode i n t o t h e c o n s t a n t v o l t a g e mode. The c u r r e n t as shown i n F i g u r e 2, s t a r t s t o decrease a t t h e o n s e t o f t h e v o l t a g e i n c r e a s e . The i n c r e a s e i s due t o t h e r e s i s - tance i n c r e a s e o f t h e c o i l w i r e . The e q u a t i o n d e s c r i b i n g t h e e f f e c t i s
E l = .L ( d I / d t ) t R ( t ) . I . As L i s known t h i s e q u a t i o n y i e l d s t h e t i m e dependence o f R ( t ) . I n t h e f i r s t 70 ms o f t h e quench i t i n c r e a s e s f r o m 0 t o 0.41 R, which i s 88 % o f t h e maximum g i v e n i n Table 1. I n t h e same p e r i o d t h e c u r r e n t goes down f r o m i t s maximum by a f a c t o r o f two. The d a t a t a k e n w i t h t h e scope e v o l v e i n t o t h e values o b t a i n e d w i t h t h e s l o w e r p a r t o f t h e d a t a a c q u i s i t i o n system. F o r a homogeneous d i s t r i b u t i o n o f t h e r e s i s t a n c e o v e r t h e c o i l w i r e , R = 0.471 R corresponds w i t h a maximum c o i l temperature o f 48 K / 2 / . The decrease o f t h e c o i l r e s i s t a n c e w i t h time, a f t e r the quench (see Table 2 ) , i n d i c a t e s a c o o l i n g o f t h e c o i l w h i l e 18 w a t t h e a t i s generated i n s i d e i t . The r e d u c t i o n i n t h e c o i l r e s i s t a n c e r e s u l t s i n an i n c r e a s e o f t h e J o u l e h e a t i n g . The system e v e n t u a l l y reaches an e q u i l i b r i u m . I n Table 2 two such e q u i l i b r i a a r e r e p o r t e d , which were t a k e n a t lower E l v a l u e s . These were l i m i t e d so t h a t t h e power g e n e r a t i o n s were 2.6 and 1.79 w a t t r e s p e c t i v e l y . F o r these J o u l e h e a t i n g t h e c o i l r e s i s t a n c e i s s m a l l e r t h a n R10 = 0.145 R t h e l o w e s t r e s i s t a n c e o f t h e w i r e above Tc. Because t h e r e s i s t a n c e v e r s u s temperature c u r v e o f a supercon- d u c t i v e w i r e near Tc i s q u i t e w e l l approximated by a s t e p f u n c t i o n , one c a l c u l a t e s t h e f r a c t i o n s which a r e i n t h e non-superconducting s t a t e and which a r e denoted by F i n Table 2.
The s t u d y proves t h a t , among t h e c o i l s t e s t e d , c o i l 4, which can r e a c h 6 . 3 t e s l a i n l e s s t h a n one m i n u t e , has a good quench r e p r o d u c i b i l i t y . The d i g i t a l a v e r a g i n g o f t h e quench d a t a i"
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an a c c u r a t e d e s c r i p t i o n o f t h e quench b e h a v i o u r .REFERENCES
/1/ VAN DEN BOSCH, A., CORNELIS, J. and BIERMANS, F . , IEEE Trans. Mag. V o l . Mag-19 (1983) 930.
/2/ HALL, L.A., N.B.S., B o u l d e r , Colorado, U.S.A., T e c h n i c a l Note 365 (1968) 40.
