THE 11 T NbTi SUPERCONDUCTING MAGNETS COOLED WITH SUPERFLUID HELIUM

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THE 11 T NbTi SUPERCONDUCTING MAGNETS COOLED WITH SUPERFLUID HELIUM

S. Han, Z. Feng, C. Lin, S. Song, K. Li

To cite this version:

S. Han, Z. Feng, C. Lin, S. Song, K. Li. THE 11 T NbTi SUPERCONDUCTING MAGNETS

COOLED WITH SUPERFLUID HELIUM. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-

807-C1-811. �10.1051/jphyscol:19841165�. �jpa-00223639�

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JOURNAL

DE

PHYSIQUE

ColIoque C1, supplCment a u

n o

1, Tome 45, janvier 1984 page CI-807

THE

1 1

T NbTi SUPERCONDUCTING MAGNETS COOLED WITH SUPERFLUID HELIUM

S . Han, Z . Feng, C.Y. Lin, S . S . Song and K.W. Li

I n s t i t u t e of EZectricaZ Engineering,Aeademia S i n i c a , B e i j i n g , China

RESUME

Cet a r t i c l e concerne l e s problsmes d ' o b t e n t i o n d'hklium s u p e r f l u i d e (He 11) e t l e s c a r a c t d r i s t i q u e s de bobines s u p r a c o n d u c t r i c e s de Nb T i r e f r o i d i e s h He 11. Deux d i f - f b r e n t s t y p e s d'He I1 (He 11 s a t u r k e t He I1 p r k s s u r i s k ) e t l e u r mkthode d ' o b t e n t i o n s o n t p r s s e n t k s . Les r k s u l t a t s d e s t e s t s s u r de p e t i t e s bobines de Nb T i r e f r o i d i e s 2 He I1 s o n t donn8s. En u t i l i s a n t He I1 s a t u r k nous pouvons d b c r o i t r e l a tempkrature du b a i n 2 1,8K e t nous avons ob- t e n u un champ magnktique de 1 1 T dans une bobine supra- c o n d u c t r i c e en Nb T i de 10 nun de d i a m s t r e i n t k r i e u r .

ABSTRACT

This a r t i c l e concerns t h e problems of o b t a i n s u p e r f l u i d Helium (He

a

) and t h e c h a r e c t r i s t i c s of NbTi supercond- u c t i n g C o i l s cooled with H e x

.

Two d i f f e r e n t kinds of He

II

( s a t u r a t e d He

IL

and p r e s s u r i z e d He 1L ) and t h e i r producing method a r e discussed. The r e s u l t s o f t e s t i n g NbTi small c o i l s cooled with HeE a r e given. Using a s a t u r a t e d Hell system we can decrease t h e bath temperature t o about 1.8k and have obtained a magnetic f i e l d of 11T i n a l O m m bore of a NbTi superconducting c o i l .

INTRODUCTION

The magnetic f i e l d o f a superconducting c o i l cooled with H e x can be h i g h e r t h a n t h a t with He I. The r e a s o n s a r e a s f o l l o w s ; F i r s t , temperature o f He i s lower t h a n t h a t of He I , t h e r e f o r e , t h e c r i t i c a l parameters of superconductors a t H e x

,

such a s c r i - t i c a l f i e l d and c r i t i c a l c u r r e n t d e n s i t y g e t h i g h e r , Secondly, t h e c h a r e c t e r i s t i c of h e a t t r a n s m i s s i o n of H e x i s b e t t e r t h a n t h a t o f He I ; t h e r e f o r e , t h e superconducting s t a b i l i t y of a superconducting c o i l cooled with Hell i s much b e t t e r t h a n t h a t with He I . A l l of t h e s e promote t h e r e s e a r c h workson He I1 t e c h n i q u e and on superconducting magnets cooled with Hell

.

I n our I n s t i t u t e t h e r e s e a r c h works on H e x t e c h n i q u e and on superconducting magnets cooled with HeII have been developed s i n c e 1981. Our r e s e a r c h works d e a l with t h e following a s p e c t s ;

*

The system o r equipment of producing s u p e r f l u i d Helium i n c l u d i n g s a t u r a t e d Hell and p r e s s u r i z e d He

a .

*

The c r i t i c a l parameters of NbTi superconductors a t He I

.

+ The superconducting s t a b i l i t y of NbTi superconducting c o i l cooled with H e x

.

*

The e s t a b l i s h m e n t o f an experimental NbTi superconducting magnet cooled with press- u r i z e d He

.

Some of t h e s e works have been f i n i s h e d and t h e o t h e r s a r e being continued. I n t h i s paper i t w i l l be described i n d e t a i l .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841165

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CI-808 JOURNAL

DE

PHYSIQUE

SYSTEM OF PRODUCING SATURATED H e r

Although t h e s a t u r a t e d H e E i s n o t c o n v e n i e n t t o o p e r a t e , it i s s t i l l t h e b a s e o f p r e s s u r i z e d He IT

.

Then we u s e d o u b l e b a t h t e c h n i q u e t o produce t h e p r e s s u r i z e d H e l l . We must u s e s a t u r a t e d H e l l as a c o o l a n t t o c o o l t h e p r e s s u r i z e d H e l l

.

s o , we i n t e n d t o d e v e l o p e t h e s a t u r a t e d He 1 t e c h n i q u e first.

I n o u r l a b o r a t o r y , we have a l r e a d y d e s i g n e d and i n s t a l l e d a s a t u r a t e d H e 1 system.Us- i n g t h i s system we have g o t 1.8k s a t u r a t e d He 3

,

and d i d a s e r i e s o f NbTi c o i l t e s t s . The s c h e m a t i c diagram o f t h i s system i s shown i n f i g 1.

I n t h i s system t h e r e a r e two major p a r t s , a dewar c o n t a i n e r and a mechanical pump. The dewar c o n t a i n e r is c o n n e c t e d t o t h e mechanical pump t h r o u g h a n e l e c t r i c h e a t e r w i t h a bypass. The pumping g a s goes from pump t o g a s c o n t a i n e r t h r o u g h a n o i l f i l t e r . When t h e l i q u i d He i s drawn o u t hy a pump; a t b e g i n n i n g a l o t o f l i q u i d He i s i n t e n s e l y g a s i f i e d . A t t h i s t i m e , we must u s e a n e l e c t r i c h e a t e r t o h e a t t h e c o o l i n g He g a s t o p r e v e n t t h e o i l i n pump from s o l i d i f y i n g . When t h e t e m p e r a t u r e o f t h e l i q u i d He decr- e a s e s t o a b o u t h p o i n t , t h e amount o f pumping g a s d e c r e a s e s a b r u p t l y . I n o r d e r t o o b t a i n t h e lower t e m p e r a t u r e o f s a t u r a t e d H e l l

,

we must u s e a b y p a s s t o e n l a r g e t h e pumping g a s p a t h . The o i l f i l t e r i s u s e d t o p r e v e n t t h e whole system from o i l p o l l u t - i o n .

B e f o r e t e s t i n g t h e NbTi c o i l , we do a s e r i e s o f o p e r a t i n g t e s t s a b o u t t h i s s a t u r a t e d HeIL system, The r e s u l t s o f t e s t a r e shown i n f i g 2. From it we c a n s e e two c u r v e s t h a t d e s c r i b e t h e t e m p e r a t u r e change and t h e q u a n t i t y change o f r e s i d u a l l i q u i d He d u r i n g t h e p r o c e s s o f d e c r e a s i n g p r e s s u r e , B e f o r e r e a c h i n g t h e A p o i n t , t h e p r o c e s s o f d e c r e a s i n g p r e s s u r e changes a b r u p t l y , l a t e r it changes more smoothly. The pumping speed and t h e d i a m e t e r of g a s p i p e , d i r e c t l y a f f e c t t h e end t e m p e r a t u r e o f B e P

,

The l a r g e r a r e t h e pumping s p e e d and t h e d i a m e t e r of g a s p i p e , t h e l o w e r i s t h e end temp- e r a t u r e . Using t h i s system, we have a l r e a d y g o t a t e m p e r a t u r e of a b o u t 1.8k o f s a t u r - a t e d He 1

.

EQUIPMENT OF PRESSURIZED Hex

The p r e s s u r i z e d Hell is more u s e f u l and more a v a i l a b l e t o s u p e r c o n d u c t i n g magnets t h a n t h e s a t u r a t e d He 71

,

w h i l e former i s d i f f i c u l t t o produce. The major a d v a n t a g e s o f p r e s s u r i z e d Hell a r e a s f o l l o w s : i t i s e a s y t o complement l i q u i d Helium; i t i s conve- n i e n t t o c o n t r o l t h k b a t h t e m p e r a t u r e o f He

IL

; it h a s h i g h e r e l e c t r i c breakdown str- e n g t h . Only f o r t h e development o f p r e s s u r i z e d H e l l t e c h n i g u e c a n w e w i d e l y u s e He Z t o c o o l s u p e r c o n d u c t i n g magnet.

We d e s i g n e d a n e q u i p m e n t which c a n produce p r e s s u r i z e d H e

II .

The p r i n c i p l e o f i t i s based on t h e s o c a l l e d double b a t h t e c h n i q u e . There a r e two b a t h e s i n t h i s equipment.

One i s t h e s a t u r a t e d HeII c o n t a i n e r , t h e o t h e r i s t h e p r e s s u r i z e d He It c o n t a i n e r . The former is l o c a t e d i n s i d e o f t h e l a t e r . The s a t u r a t e d He SL c o n t a i n e r is c o n n e c t e d t o a mechanical pump forming a s a t u r a t e d H e x system. The s a t u r a t e d Hell is u s e d t o c o o l t h e p r e s s u r i z e d He

II

; s o , t h e s a t u r a t e d H e l l c o n t a i n e r must have good h e a t t r a n s m i s s - i o n c h a r e c t e r i s t i c . The p r e s s u r i z e d HelI c o n t a i n e r is t h e major p a r t o f t h i s eqipment.

An i n s u l a t i n g s e p a r a t o r d i v i d e s it i n t o two p a r t s . There a r e v e r y t h i n p a t h e s a t t h e i n s u l a t i n g s e p a r a t o r which l i n k up t h e two p a r t s . The s a t u r a t e d HeIL c o n t a i n e r is l o c - a t e d below t h e i n s u l a t i n g s e p a r a t o r t o c o o l t h e l i q u i d helium. So, t h e l i q u i d Helium below t h e i n s u l a t i n g s e p a r a t o r c a n b e changed t o p r e s s u r i z e d H e x

,

Its t e m p e r a t u r e i s l e s s t h a n 2 k , The l i q u i d He above t h e i n s u l a t i n g s e p a r a t o r i s s t i l l He I. Its temper- a t u r e i s 4.2k and i t s p r e s s u r e i s a t t h e a t m o s p h e r i c p r e s s u r e . Only u s i n g t h e i n s u l a t - i n g s e p a r a t o r c a n we make t h e Hell t o b e a t one a t m o s p h e r i c p r e s s u r e . There i s a temp- e r a t u r e g r a d i e n t from 4.2k t o l e s s t h a n 2k a t t h e i n s u l a t i n g s e p a r a t o r .

The s c h e m a t i c diagram o f t h i s equipment is shown i n f i g 3 . we a r e p r e p a r i n g and a d j u - s t i n g t h i s equipment now.

NbTi SUPERCONDUCTING COILS COOLED WITH H e x

I t is common now t o o p e r a t e NbTi s u p e r c o n d u c t i n g c o i l s a t 4.2k t o o b t a i n 8T magnetic f i e l d . I f we o p e r a t e it a t a b o u t 2k, we c a n g e t a f i e l d s t r e n g t h o f 10T o r h i g h e r . T h i s i s v e r y s i g n i f i c a n t .

Because t h e double b a t h t e c h n i q u e h a s been d i s c o v e r e d , we c a n e a s i l y g e t p r e s s u r i z e d H e x which i s c o n v e n i e n t t o c o o l i n g s u p e r c o n d u c t i n g c o i l s t o l e s s t h a n 2k t e m p e r a t u r e .

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So we are interested in using Hell to cool NbTi superconducting for getting more than 10T magnetic field.

Our research works on this area consists of three aspects: Measuring the critical characteristics of NbTi (and NbTiTa) superconductors at Hell

.

Studying the superconducting stability of NbTi superconducting coils cooled with H e x

.

Constructing and operating a practical, experimental NbTi superconducting magnet cooled with pressurized HelI, At first, we'simply use saturated H e 3 to do some research works. Up to now we have got some significant results. Using a NbTi coil cooled with saturated H e 1 we have got an11Tmagnetic field in lOmm bore. Other research works are in progress now in our laboratory.

(1) Tests of small NbTi superconducting coils at saturated Hex

In order to obtain the critical characteristics of NbTi superconductors, we designed three small NbTi superconducting coils which have different magnet constants (B/I) and tested them at He I (4.2k) and at saturated He 15 (1.9k). The parameters of NbTi sup- erconductor and three small NbTi coils are given in table 1. Each of these coils was tested at 4.2k and 1.9k. We measure the quench point of each coil at two different temperature. The results of tests are given in table 2 and fig 4. From the results of test we can see that the magnetic field of a NbTi coil operating at 1.9k is higher than at 4.2k by about 2.5T.

Connecting three quench point of NbTi superconducting coils at the same temperature we can get two I-B characteristic curves. One is obtained at 4.2k, the otherareobta-

ined at 1.9k. Of course, the I-B curve obtained by this way is different from the practical critical characteristic curve of a NbTi superconductor Ic-B. However, the testing NbTi coils are small; their degradation are also small. So their quench characteristic curve I-B is approximate to the critical_characteristic curve of a superconductor Ic-B. We can use this I-B curve as a reference for design of magnet.

Table 1 Parameters of superconductor and coils

Parameters of Superconductor Parameters of NbTi coils

Diameter of conductor coil I coil 2 coil 3

(mm) 0.5 inner dia. 2al (mm) 10 10 10

Cu/NbTi 1.5 outer dia. 2a2 (mm) 97 115 127

Nb Content of NbTi High 2b (mm) 30 52 58

53.5% filling coefficient of coil 0.67 0.65 0.67 coil constant k (T/A) 0.100 0.143 0.167 Table 2 Results of NbTi coils tests

B Ic Ic mag Jc NbTi B~ Bmax

(K) (A) 104(~/cm~ ) 10~(A/cm~ ) (T) (T)

coils 1 4.2 72.4 2.47 9.04 7.28 7.39

1.9 95.0 3.25 11.87 9.56 9.70

coils 2 4.2 57.0 1.88 7.27 8.15 8.31

1.9 75.0 2.48 9.59 10.73. 10.94

coils 3 4.2 50.0 1.69 6.35 8.35 8.38

1.9 65.9 2.23 8.39 11.01 11.05

( 2 ) Tests of superconducting stability of NbTi coils

We are carrying out two kinds of experimental research works on superconducting stability of NbTi coils cooled with saturated H e n

.

One of them is a series of NbTi superconducting coils, which have different diameters from several tens to several hundre- ds mm will be tested; The other is a nearly one hundred mm bore NbTi superconducting coil with saveral heater in its winding will be constructed and its ability to resist the heat disturbance will be measured. These works are preparing now.

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JOURNAL DE PHYSIQUE

CONCLUSION

1. Using the decreasing pressure method, we can easily obtain the saturated He 11

.

^In

order to reaching a less than 2k temperature about 40% liguid Helium are pumped, 90%

among them are pumped beforeit point.

2. The tests of NbTi superconducting coils show that: the magnetic field of a NbTi superconducting coil at 1.9k can be higher than that at 4.2k by about 2.5T. A 11T magnetic field has already obtained by use of a lOmm bore NbTi superconducting coil cooled with saturated Hell

.

3. Saturated H e 1 system is simple. We can use it to do some research works and to cool small NbTi superconducting coils for getting higher magnetic field. But for the practical large scale NbTi superconducting magnets we must use pressurized He11 to cool them. The reasons are that; the operation of saturated H e x is not convenient for complement of liquid He; control of temperature. Because of the development of double bath technique we'can use it to produce pressurized H e 1

.

REFERENCES

1. J.L. Augeres, et al. " 7 0 0 m m d i a m e t e r c r o s t a t s o p e r a t i n g a t 1.8K andatmospheric pressure" Cryogenics, Sept, 1980.

2. G. Claudet, et al. "Proc, 5th ICEC 1974. P. 265

Fig. 1 Scheme of the system of saturated He I1

Fig. 2 Scheme of the process of decresing pressure

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Fig. 3 Schematic diagram of pressurized He I1 equipment

Fig. 4 I-B curves of Nb-46.5Ti superconductor at 4.2k and 1.9k