THE UTILIZATION OF FILAMENTARY Nb3Sn SUPERCONDUCTOR IN THE PRODUCTION OF A 15 TESLA SOLENOID OPERATING AT 4.2K

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THE UTILIZATION OF FILAMENTARY Nb3Sn SUPERCONDUCTOR IN THE PRODUCTION OF A

15 TESLA SOLENOID OPERATING AT 4.2K

N. Killoran, N. Kerley

To cite this version:

N. Killoran, N. Kerley. THE UTILIZATION OF FILAMENTARY Nb3Sn SUPERCONDUCTOR IN

THE PRODUCTION OF A 15 TESLA SOLENOID OPERATING AT 4.2K. Journal de Physique

Colloques, 1984, 45 (C1), pp.C1-87-C1-91. �10.1051/jphyscol:1984119�. �jpa-00223666�

JOURNAL DE PHYSIQUE

Colloque C l , supplément au n° 1, Tome *5, janvier 198* page Cl-87

THE UTILIZATION OF FILAMENTARY Nb-Sn SUPERCONDUCTOR IN THE PRODUCTION OF A 15 TESLA SOLENOID OPERATING AT 4.2K

N. Killoran and N.W. Kerley

Oxford Instruments Limited, Osney Mead, Oxford 0X2 ODX, U.K.

Résumé - Une grande attention a été apportée à la mise en oeuvre de conducteurs Nb3Sn à très haute densité de courant critique pour la réalisation de solênoides supraconducteurs . Ce n'est pourtant que récemment que de tels aimants commencent à bénéficier pleinement du potentiel offert par le Nb3Sn. On décrit le développement, la construction et les performances d'un aimant de ce type conçu pour produire 15 Tesla à 4,2K. A la lumière de ce travail, on envisage de nouveaux développements pour la production des champs magnétiques élevés.

Abstract - The problem of successfully exploiting the high criticalcurrent density of bronze route NbsSn in a supercon- ducting solenoid of conventional design has received much

attention. However, it is only recently that the full potential of Nb

Sn has been approached in such a magnet. We describe the development, construction and performance of one such magnet which is designed to achieve 15 Tesla at 4.2K. In addition, we

consider future developments in the production of high magnetic fields using superconducting wire in the light of this recent work.

1. INTRODUCTION

Previous attempts to achieve magnetic fields of 15T and above at 4. 2K solely with a combination of commercially available Nb

Sn and NbTi superconductors have often led to certain compromises in the final specification of the magnet. Such compromises include: restrictive internal bore diameter, the use of independently energised Nb3Sn

insert coils, the use of Nb

Sn tape (with its inherently unpredictable field/current ratios and high residual fields due to diamagnetic

effects), slow energisation rates and unreliable high field operation involving extensive magnet training. Obviously all these attributes are undesirable from the users point of view since they restrict the convenient use of a laboratory scale magnet both in swept and per- sistent current modes and the volume of homogeneous high field avail- able for experimentation. Consequently specifications for our 15T magnet were as follows:

(i) to achieve 15T at 4.2K with commercially available Nb

Sn and NbTi superconductor together with a useful internal bore diameter and volume of homogeneity

(il) to achieve stable and reliable operation under the electro- magnetic induced stress levels inherent in such a magnet ( H i ) to minimise training effects and risk of damage due to

energy dissipation under quench conditions

(iv) to construct a magnet that can be energised to full field

reasonably quickly, requires only series energisation of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984119

JOURNAL DE PHYSIQUE

c o n s t i t u e n t s o l e n o i d s and c a n t h e r e f o r e b e u s e d c o n v e n i e n t l y b o t h i n s w e p t a n d p e r s i s t e n t c u r r e n t modes

( V ) t o a c h i e v e 15T a t a n o p e r a t i n g c u r r e n t c o n s i s t e n t w i t h a c c e p t a b l e l i q u i d c r y o g e n l o s s r a t e s .

O b v i o u s l y t h e u s e o f m u l t i f i l a m e n t a r y c o p p e r s t a b i l i z e d ' b r o n z e r o u t e ' Nb3Sn s u p e r c o n d u c t i n g w i r e was c o n s i d e r e d t o b e a p r e - r e q u i s i t e i n a c h i e v i n g t h e o b j e c t i v e s l i s t e d a b o v e . The m e r i t s o f

' b r o n z e r o u t e ' Nb3Sn s u p e r c o n d u c t o r , a s u s e d i n magnet c o n s t r u c t i o n , are d i s c u s s e d i n d e t a i l b y IdcDonald a n d P r o c t o r / l / . A l t h o u g h t h e s h o r t s a m p l e c r i t i c a l c u r r e n t p r o p e r t i e s o f t h i s t y p e of Nb,Sn s u p e r - c o n d u c t o r a r e r e a s o n a b l y p r e d i c t a b l e , t h e s t r a i n e n h a n c e m e n t /

d e g r a d a t i o n o f t h e c r i t i c a l c u r r e n t d u e t o r e a c t i o n p r e s t r e s s a n d e l e c t r o m a g n e t i c stress p r e s e n t d u r i n g magnet e n e r g i s a t i o n s t i l l r e q u i r e a s s e s s m e n t .

T h i s d e v e l o p m e n t was s t i m u l a t e d by t h e r e q u i r e m e n t f o r a h i g h f i e l d s u p e r c o n d u c t i n g s o l e n o i d and c r y o s t a t s y s t e m f o r q u a n t i s e d H a l l e f f e c t / Z , m e a s u r e m e n t s by t h e E l e c t r i c a l a n d Time S t a n d a r d s S e c t i o n , N . R . C . , O t t a w a , Canada l e d by Dr I T e m p l e t o n . T h e i r b a s i c n e e d f o r a r e l i a b l e and c o n v e n i e n t l y q e r a t e d 15T s o l e n o i d t h a t w i l l accommodate a 1.5K-4.2K v a r i a b l e t e m p e r a t u r e i n s e r t and 3He i n s e r t ( a n d

u l t i m a t e l y a h e l i u m d i l u t i o n r e f r i g e r a t o r ) , s i n c e t h e q u a n t i s e d H a l l e f f e c t i s b e s t o b s e r v e d a t t h e h i g h e s t m a g n e t i c f i e l d s a n d l o w e s t t e m p e r a t u r e s , r e f l e c t s t h e d e v e l o p m e n t o b j e c t i v e s l i s t e d a b o v e . The e x p e r i m e n t , p e r f o r m e d o n a S i o r GaAs MOSFET, f a c i l i t a t e s t h e d i r e c t measurement o f h / e L ( w h e r e h i s P l a n c k ' s c o n s t a n t a n d e i s e l e c t r o n i c c h a r g e ) a n d h a s two i m p o r t a n t u s e s :

( i ) The MOSFET c a n b e u s e d a s a r e s i s t a n c e s t a n d a r d s i n c e h / e 2 h a s t h e d i m e n s i o n o f t h e ohm.

( i i ) t h e r a t i o h / e 2 d e t e r m i n e s t h e v a l u e o f t h e f i n e s t r u c t u r e c o n s t a n t , a , w h i c h o t h e r w i s e d e p e n d s o n l y upon t h e s p e e d o f l i g h t , t h e r e b y p r o v i d i n g a n o p p o r t u n i t y t o t e s t t h e f u n d a - m e n t a l t h e o r y o f q u a n t u m e l e c t r o d y n a m i c s .

2 . DESIGN AND CONSTRUCTION

The f i n a l s e t o f s p e c i f i c a t i o n f o r t h e magnet a n d a s s o c i a t e d c r y o - g e n i c s w e r e d e f i n e d as f o l l o w s :

maximum c e n t r a l m a g n e t i c f i e l d a t 4.2K

( * i t h t h e p o s s i b i l i t y o f e x c e e d i n g 15T a t 2.2K u s i n g a lambda p o i n t r e f r i g e r a t o r / S / ) .

o p e r a t i n g c u r r e n t f o r 15T 7 2A

clear b o r e d i a m e t e r 44 mm

( a c c o m m o d a t i n g a 1.5K-4.2K v a r i a b l e t e m p e r a t u r e i n s e r t w i t h a 33 mm d i a m t e r s a m p l e s p a c e a n d a 3 ~ e i n s e r t w i t h a 26 mm d i a m e t e r s a m p l e s p a c e ) .

h o m o g e n e i t y o f m a g n e t i c f i e l d a t t h e s a m p l e

c u r r e n t d e c a y i n p e r s i s t e n t mode

3 p a r t s i n

l o 4 o v e r 1 0 mm d s v

< l p a r t i n

To4 p e r h r

i n d u c t a n c e 480 H

s t o r e d e l e c t r o m a g n e t i c e n e r g y a t 15T : 1 . 2 5

106.J minimum e n e r g i s a t i o n r a t e

(OT-15T i n - 2 h r )

l i q u i d h e l i u m b o i l o f f r a t e , l e a d c u r r e n t = OA : 300 mR/hr l e a d c u r r e n t = 72A : 8 5 0 m a l h r The f i n a l d e s i g n c o n s i s t e d o f t h r e e c o n c e n t r i c s o l e n o i d s of Nb3Sn s u p e r c o n d u c t o r w i t h i n t h r e e c o n c e n t r i c s o l e n o i d s o f NbTi s u p e r c o n - d u c t o r . T h e g r a d i n g o f t h e s u p e r c o n d u c t i n g w i r e o n t h e c o n s t i t u e n t s o l e n o i d s was e v a l u a t e d o n t h e b a s i s o f s h o r t s a m p l e c r i t i c a l c u r r e n t d a t a g u a r a n t e e d by o u r w i r e s u p p l i e r s t o g e t h e r w i t h c r i t i c a l c u r r e n t a n d e l e c t r o m a g n e t i c s t r e s s r e l i a b i l i t y s t a n d a r d s e v a l u a t e d by

e x t e n s i v e i n h o u s e d e v e l o p m e n t . The g e n e r a l c o i l l a y o u t i s shown s c h e m a t i c a l l y i n f i g u r e 1.

Fig. 1 - Schematic view of t h e l5T TJb3:h/lhtTi solecoid. Dinlensions i n m.

A s mentioned above, t h e NbgSn s e c t i o n s were f a b r i c a t e d using 'bronze r o u t e ' super- conductor, t h e wind and r e a c t technique being employed. The Nb Sn s e c t i o n formers 3 consisted of copper bore tubes with s t a i n l e s s s t e e l flanges. Each wound s e c t i o n w a s p r e t r e a t e d and reacted under standard conditions t o produce Nb3in l a y e r gTowth by s o l i d s t a t e diffusion of t h e Sn fror.1 t h e bronze matrix i n t o t h e Nb filaments.

Each section was vacuum impregnated a f t e r reaction t o fom.1 a cchnposite matrix

which is both physically and cryogenically s t a b l e under t h e considerable Lorentz

f o r c e s generated during operation. The NbTi wire was wound onto dural formers,

each s e c t i o n again being vacuum impremated. A l l s e c t i o n s were series connected

using j o i n t s designed f o r moderate persistence, positioned a t t h e top of t h e m w e t .

A superconducting switch with a nonnal s t a t e r e s i s t a n c e of 100 ohms w a s connected

across t h e s e c t i o n s t o f a c i l i t a t e p e r s i s t e n t mode operation of t h e e n t i r e r m e t .

The magnet is suspended on a support s t r u c t u r e , which houses t h e quench protection

Cl-90 JOURNAL DE PHYSIQUE

r e s i s t o r s connected across each s e c t i o n of the magnet, within a simple bucket type c r y o s t a t . The main c u r r e n t and protection r e s i s t o r leads were o ~ t i m i z e d t o minimize l i q u i d helium b o i l o f f .

3. TEST RFSULTS AND DISCUSSION

A l l tests were p r f o m e d with t h e magnet imnersed i n a l i q u i d helium bath a t 4.2K.

Magnetic f i e l d nleasurement was accmplished using a NIR c a l i b r a t e d Hall probe positioned on t h e a x i s of t h e magnet bore. Figure 2 shows a schematic represent- a t i o n of t h e t r a i n i n g h i s t o r y of t h e magnet. The d e t e r i o r a t i o n i n performance apparent on energizations 3 and 4 w a s due t o a constrrlctional f a u l t which w a s remedied p r i o r t o subsequent energizations. It is worthy of note t h a t t h e magnet required only 6 t r a i n i n g quenches t o a t t a i n its f u l l 4.2K s p e c i f i c a t i o n . I n addition, t h e magnetic f i e l d was swept frcm OT t o 15T i n only 2hrs 13min during energization 9. The a x i a l m a g n e t i c f i e l d p r o f i l e and f i e l d decay i n p e r s i s t e n t mode a t 15T, a s measured using t h e Hall probe, were found t o correspond t o t h e i r respective design values. The m a g n e t is now capable of operating at a l e v e l corresponding t o t h e Nb3Sn and NbTi current d e n s i t i e s p l o t t e d i n f i g u r e 3, shown f o r ccmparison with t h e relevent s h o r t sanple current density curves. Further tests w i l l be necessary t o determine t h e e x t e n t t o which t h e current d e n s i t i e s r e l i a b l y approach t h e i r s h o r t sample values and hence, t o what extent t h e Nb3Sn wire performance is enhanced o r degraded by s t r a i n e f f e c t s . In addition, t h e degree of enhancement i n performance m a d e possible by operating t h e magnet a t reduced temperature ( 2 . X ) has still t o be investigated.

Fie;. 2 - Training h i s t o r y f o r t h e 15T Fig. 3 - Current d e n s i t i e s achieved f o r magnet showing t h e chronological sequence t h e PJb3Sn and NbTi superconductors used of quench events (Q) necessary t o acheive i n t h e magnet, shown with t h e relevent s t a b l e p e r s i s t e n t mode operation ( S ) a t s h o r t sample performance curves.

15T at 4.2IC. Energizations 8 and 9 correspond t o ms i n which progressively increased energization rates were used.

I t can be seen t h a t t h e magnet described above h a s acheived a l l our i n i t i a l

developnent objectives. W e are confident t h a t t h e design and method of construct-

ion employed has r e s u l t e d i n a magnet t h a t is convenient t o use and can r e l i a b l y

withstand quench conditions, should they occur. Further developPlent involving t h e

use of 'powder r o u t e ' Nb3Sn superconductor and t e r n a r y m a t e r i a l s based on Nb3Sn, is

now i n progress ?nd it is hoped t o r e a l i z e t o an even g r e a t e r extent t h e f u l l

p o t e n t i a l of high f i e l d superconductors i n magnet construction.

I'he authors would l i k e t o acknowledge the many helpful discussions with our colleagues a t Oxford I n s t m n t s .

(1) McDONAZD P. and PFtEIDR W. ,Proc. 8th I n t . Cryogenic Eng. Conf . ,Geneva 8(1980)509 (2) HUE1D C.,New S c i e n t i s t (Feb 1983)314

(3) BILTCLIFFE IUI.,IIANLFY P.,NcKINNON B.,ROWEAU P.,Cryogenics (Feb 1972).