EXPERIMENTAL STUDY OF THE STABILITY OF NbTi COMPOSITE CONDUCTOR UNDER THE INFLUENCE OF PULSED MAGNETIC FIELD

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Submitted on 1 Jan 1984

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EXPERIMENTAL STUDY OF THE STABILITY OF NbTi COMPOSITE CONDUCTOR UNDER THE

INFLUENCE OF PULSED MAGNETIC FIELD

Yi Changlian, Chang Hung, Yan Luguang, Chen Jinlin

To cite this version:

Yi Changlian, Chang Hung, Yan Luguang, Chen Jinlin. EXPERIMENTAL STUDY OF THE STABILITY OF NbTi COMPOSITE CONDUCTOR UNDER THE INFLUENCE OF PULSED MAGNETIC FIELD. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-495-C1-498.

�10.1051/jphyscol:19841101�. �jpa-00223569�

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J O U R N A L DE

PHYSIQUE

Colloque C1, suppldrnent au no 1, Tome 45, janvier 1984 page Cl-495

EXPERIMENTAL STUDY OF THE STABILITY OF NbTi COMPOSITE CONDUCTOR UNDER THE INFLUENCE OF PULSED MAGNETIC FIELD

Y i C h a n g l i a n , Chang Hung, Yan Luguang a n d Chen 3 i n l i n r I n s t i t u t e o f E l e c t r i c a l Engineering, Academia Sinica, China ' ~ n s t i t u t e o f PZasrna Physics, Academia Sinica, China

Resume

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Nous a v o n s c o n s t r u i t un a p p a r e i l l a g e e x p k r i m e n t a l d e s t i n e ?I Q t u - d i e r l ' i n f l u e n c e d ' u n champ m a g n 6 t i q u e p u l s 6 s u r l a s t a b i l i t e d ' u n c o n d u c - t e u r m u l t i f i l a m e n t a i r e c o m p o s i t e , t o r s a d 6 , d e NbTi. P l u s i e u r s Q c h a n t i l l o n s o n t e t 6 m e s u r 6 s . Nous p r k s e n t o n s n o s r 6 s u l t a t s .

A b s t r a c t

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To s t u d y t h e i n f l u e n c e o f p u l s e d m a g n e t i c f i e l d o n t h e s t a b i l i t y o f NbTi m u l t i f i l a m e n t a r y t w i s t e d c o m p o s i t e c o n d u c t o r , a n e x p e r i m e n t a l d e v i c e was c o n s t r u c t e d a n d s e v e r a l s a m p l e s h a v e b e e n s t u d i e d

.

The c o r r e s - p o n d i n g r e s u l t s a r e p r e s e n t e d .

1

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INTRODUCTION

F o r a m a g n e t i c l y c o n f i n e d f u s i o n r e a c t o r t h e s u p e r c o n d u c t i n g m a g n e t s a r e i n d i s p e n - s a b l e t o g e t a p o s i t i v e e n e r g y b a l a n c e . F o r t h e tokamak t o r o i d a l magnet, b e s i d e s i t s own d . c . f i e l d , t h e r e a r e a l s o p u l s e d f i e l d s o f t h e p o l o i d a l c o i l s a n d t h e p l a s m a c u r r e n t . So i t is i m p o r t a n t t o know t h e p u l s e d f i e l d i n f l u e n c e on t h e s t a b i l i t y o f t h e s u p e r c o n d u c t i n g magnets. F o r t h a t a s m a l l d e v i c e h a s b e e n b u i l t and t h e i n f l u e n c e o f t h e p u l s e d f i e l d a m p l i t u d e , d u r a t i o n a n d d i r e c t i o n h a s b e e n s t u d i e d o n s e v e r a l s a m p l e s made w i t h 0 0.5mm c o m p o s i t e w i r e .

2

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EXPERIMENTAL DEVICE AND METHOD

To s i m u l . a t e t h e tokamak o p e r a t i n g c o n d i t i o n s t h e d e v i c e ( F i g . 1 ) h a s a b a c k g r o u n d d . c . f i e l d ( Bb ) , a p u l s e d f i e l d ( Bp ) a n d a s a m p l e w i t h t r a n s p o r t c u r r e n t .

1

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B c o i l 2

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^Bb c o i l 3 - Sample P 4 - I e n i t r o n

5 - Memory o s c i l l o s c o p e 6 - P e n - r e c o r d e r

7

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M a g n e t i c f i e l d s e n s o r 8 - B power s u p p l y (150A) 9

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T r a n s p o r t c u r r e n t power b

s u p p l y (1000A) C - C a p a c i t o r bank

F i g . 1 Schema o f t h e e x p e r i m e n t a l d e v i c e

To s t u d y t h e i n f l u e n c e o f t h e d i r e c t i o n b e t w e e n t h e b a c k g r o u n d f i e l d Bb, t h e p u l s e d f i e l d Bp a n d t h e s a m p l e t r a n s p o r t c u r r e n t I, t h r e e t y p e s o f p u l s e d c o i l s a r e u s e d , i . e . : ( 1 ) s i n g l e s o l e n o i d : Bp is p a r a l l e l t o Bb and p e r p e n d i c u l a r t o I ; ( 2 ) a p a i r o f s o l e n o i d s : Bp, Bb a n d I a r e o r t h o g o n a l ; ( 3 ) t o r o i d a l c o i l w i t h t h e s a m p l e on t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841101

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CI-496 JOURNAL

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t o r o i d a l a x i s : Bp is p a r a l l e l t o I a n d p e r p e n d i c u l a r t o B b '

The s a m p l e s a r e made from 6m l o n g , 0.5mm O f d i a m e t e r a n d 1 6 3 filaments(24-25wm f i l a m e n t d i a m e t e r ) w i r e w i t h 15 mm t w i s t e d p i t c h a n d Cu/ s . c . = 1 . 4

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1.5. I n t h e f i r s t two c a s e s t h e y a r e wound b i f i l a r l y . To i n c r e a s e t h e i r m e c h a n i c a l r i g i d i t y a n d t h e a d i a b a - t i c c o n d i t i o n t h e s a m p l e s a r e i m p r e g n a t e d w i t h wax.

The b a c k g r o u n d f i e l d up t o 5.5 T i s p r o d u c e d by a u s u a l lOOmm b o r e s u p e r c o n d u c t i n g s o l e n o i d . The p u l s e d f i e l d i s p r o d u c e d by d i s c h a r g e o f a c a p a c i t o r bank w i t h i g n i t r o n ( F i g . 1 ) . Its wave form i s a t y p i c a l p o s i t i v e h a l f s i n u o i d . By c h a n g i n g t h e c a p a c i - t a n c e o f t h e b a n k , t h e p u l s e d d u r a t i o n c o u l d b e c h a n g e d f r o m 1 0 t o 42111s. The p u l s e d a m p l i t u d e up t o 1T a n d 3T is c o n t r o l l e d b y a d j u s t i n g t h e c h a r g i n g v o l t a g e o f t h e bank.

D u r i n g e x p e r i m e n t s t h e b a c k g r o u n d f i e l d Bb a n d t h e t r a n s p o r t c u r r e n t I a r e m a i n t a i n e d c o n s t a n t , t h e p u l s e d f i e l d Bp is i n c r e a s e d s t e p >y s t e p u n t i l t h e quench o c c u r s . The r e p r o d u c i b i l i t y o f t h e d a t a i s q u i t e good.

3 - EXPERIMENTAL RESULTS

The r e s u l t s o b t a i n e d from t h e e x p e r i m e n t s a r e t h e s a m p l e quench c u r r e n t Iq i n depen- d e n c e on t h e b a c k g r o u n d f i e l d Bb, t h e p u l s e d f i e l d a m p l i t u d e Bpm a n d d u r a t i o n T , on t h e d i r e c t i o n b e t w e e n Bb, Bp a n d I . They a r e p r e s e n t e d i n F l g . 2 - F i g . 5 i n t h e form o f a s e t c u r v e s u n d e r d i f f e r e n t c o n d i t i o n s . F i g . 2 shows t h e d e p e n d e n c e o f quench c u r r e n t I q on t h e p u l s e d f i e l d a m p l i t u d e Bpm a t c o n s t a n t b a c k g r o u n d f i e l d Bb a n d p u l s e d d u r a t i o n 1;f o r t h e t h e s i n g l e s o l e n o i d c a s e ( c a s e 1, Bp//Bb a n d B p l I ) . F i g . 3 a n d F i g . 4 a r e t h e same f o r t h e c a s e 2 ( a p a i r o f s o l e n o i d s , B p l B b l I ) a n d t h e c a s e 3 ( t o r o i d a l c o i l , B p / / I a n d BplBb ) r e s p e c t i v e l y . F i g . 5 p r e s e n t s t h e i n f l u e n c e o f p u l s e d d u r a t i o n o n t h e s a m p l e q u e n c h c u r r e n t .

F i g . 2 I = f ( B ). F i g . 3 I

q Pm q ⁼^f⁽^Bpm⁾

C a s e 1 : Bp//Bb, B$I. Case 2 : B _P

1

Bb

1

I

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From t h e e x p e r i m e n t a l r e s u l t s , it c a n be s e e n c l e a r l y t h a t :

( 1 ) The r e d u c t i o n o f q u e n c h c u r r e n t i n c r e a s e s w i t h t h e i n c r e a s e o f p u l s e d f i e l d a m p l i t u d e . B u t it i s a l m o s t i n d e p e n d e n t on t h e d i r e c t i o n b e t w e e n t h e b a c k g r o u n d f i e l d a n d t h e p u l s e d f i e l d s . Under t h e i d e n t i c a l b a c k g r o u n d f i e l d , p u l s e d f i e l d a m p l i t u d e a n d d u r a t i o n t h e quench c u r r e n t s f o r b o t h Bp//Bb a n d BpLBb a r e a l m o s t e q u a l ( c o m p a r e F i g . 2 w i t h F i g . 3 ).The more s e r i o u s r e d u c t i o n f o r BplBb a t Bb = 5.5 T may b e r e s u l t e d from t h e c o n d u c t o r movement d u e t o a l a r g e t o r q u e . I t is i n t e r e s t i n g t h a t c h a n g i n g t h e p u l s e d f i e l d d i r e c t i o n f o r Bp//Bb d o e s n o t i n f l u e n c e t h e q u e n c h c u r r e n t ( s e e t h e s o l i d a n d d o t t e d c u r v e s f o r Bb = 3.68 T i n F i g . 2 ) .

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F i g . 4 I = f ( B ) .

q pm

Case 3 : B p / / I . BplBb F i g . 5 The i n f l u e n c e o f

( 2 ) The d i r e c t i o n b e t w e e n p u l s e d f i e l d a n d t r a n s p o r t c u r r e n t h a s o b v i o u s i n f l u e n c e ( compare F i g . 4 w i t h F i g . 2 and 3 ) . The d u r a t i o n f o r B p l I is more s e r i o u s t h a n f o r Bp//I

.

( 3 ) The i n f l u e n c e o f t h e p u l s e d d u r a t i o n i s a l s o o b v i o u s . The r e d u c t i o n is larger f o r s h o r t e r p u l s e s ( s e e F i g . 5 ) .

A l l t h i s phenomena t o l d u s t h a t t h e i n f l u e n c e o f p u l s e d m a g n e t i c f i e l d o n t h e s t a b i - l i t y o f s u p e r c o n d u c t o r is m a i n l y r e l a t e d t o i t s h e a t l o s s e s .

4 - COMPARISON WITH THE CALCULATION OF LOSSES

I n o r d e r t o make c l e a r t h a t t h e h e a t l o s s e s o f t h e p u l s e d m a g n e t i c f i e l d a r e r e s p o n - s i b l e f o r t h e r e d u c t i o n o f quench c u r r e n t , a c a l c u l a t i o n h a s b e e n made o n t h e b a s e o f t h e l o s s e s f o r m u l a s /1/ f o r two o f o u r e x p e r i m e n t a l c o n d i t i o n s , ( 1 ) B p l I a n d

T =

17ms, ( 2 ) B p / / I

,

Bb = 3 . 6 8 T a n d 7 = 9ms. The r e s u l t s a r e p r e s e n t e d i n F i g . 6.

The c a l c u l a t i o n shows t h a t t h e l o s s e s g o up v e r y s h a r p l y w i t h t h e i n c r e a s e o f t h e p u l s e d f i e l d a m p l i t u d e . They a r e much l a r g e r f o r Bp

1

I t h a n f o r B p / / I . F o r Bp

1

I t h e d o m i n a n t t e r m i s t h e c o u p l i n g l o s s e s which a r e a p p r o x i m a t e l y i n v e r s e l y p r o p o r - t i o n a l t o t h e p u l s e d d u r a t i o n . A l l o f them a r e good i n q u a l i t a t i v e a g r e e m e n t w i t h o u r e x p e r i m e n t a l r e s u l t s .

From o t h e r s i d e i t is a l s o p o s s i b l e from t h e e x p e r i m e n t a l r e s u l t s t o c a l c u l a t e a minimum e n e r g y r e q u i r e d f o r t h e o b t a i n e d r e d u c t i o n o f quench c u r r e n t . T h i s is t h e e n e r g y n e e d e d t o r a i s e t h e c o n d u c t o r t e m p e r a t u r e a d i a b a t i c a l l y f r o m 4.2k t o some t e m p e r a t u r e T a t which t h e c o n d u c t o r c r i t i c a l c u r r e n t is e q u a l t o t h e q u e n c h c u r r e n t . The

?A

v a l u e c o u l d b e c a l c u l a t e d s i m p l y a s : Tq=Ts-(TB-4.2) ( I q / I c ) , w h e r e Ic is t h e c r i t i c a l c u r r e n t a t 4 . 2 k , TB is t h e c r i t i c a l t e m p e r a t u r e a t z e r o c u r r e n t a n d b a c k g r o u n d f i e l d e q u a l t o 8 . The minimum e n e r g y Qmin i s e q u a l t o t h e e n t h a l p y d i f f e r e n c e o f c o n d u c t o r b e t w e e n Tq a n d 4.2k. F o r NbTi t h e s p e c i f i c h e a t v a l u e s a r e t a k e n from /2/. The c a l c u l a t e d r e s u l t s a r e p r e s e n t e d i n F i g . 6 a s d o t t e d c u r v e s . From t h e c o m p a r i s o n b e t w e e n t h e c a l c u l a t e d l o s s e s Qloss and t h e c a l c u l a t e d minimum e n e r g y @,in, i t c a n b e s e e n t h a t b o t h h a v e t h e s i m i l a r t e n d e n c y , b u t l o s s e s are

much l a r g e r t h a n Qmin.

5 - CONCLUSION

( 1 ) The i n f l u e n c e o f t h e p u l s e d m a g n e t i c f i e l d on t h e s t a b i l i t y o f t h e s u p e r c o n d u c t o r i s e x p r e s s e d i n t h e r e d u c t i o n o f t h e quench c u r r e n t . The r e d u c t i o n i n c r e a s e s w i t h t h e i n c r e a s e o f t h e p u l s e d a m p l i t u d e a n d t h e r e d u c t i o n o f t h e p u l s e d d u r a t i o n . I t i s d e p e n d e n t o n t h e d i r e c t i o n b e t w e e n t h e p u l s e d f i e l d a n d t h e t r a n s p o r t c u r r e n t and

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Fig. 6 Qloss9 Qmin = f( Bpm )

almost independent on the direction between the pulsed and background fields for a round conductor. The reduction is much larger for BpL I than for Bp//I.

(2) The heat losses in conductor due to pulsed field seem to be the predominant factor responsible for the reduction of quench current.

(3) Calculated losses are much larger than the energy required for the measured quench current reduction. To get better quantitative agreement between them, some additional investigation is needed.

ACKNOWLEDGEMENT

Authors cordially thank Li FIuidong and Wang Yanju for helping the experiments.

REFERENCES

/I/ M.N.Wilson, C.R.Walter: Development of superconductors for fusion technology.

RL-76-038, P. 78 (1976).

/2/ S.A.Elrod, J.R.Miller, L.Dresner. "Advances in Cryogenic Engineering (Materials)"

vol. 28, P. 601 (1982).