DEVELOPMENT OF A FORCED COOLED D-SHAPED SUPERCONDUCTING COIL

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DEVELOPMENT OF A FORCED COOLED D-SHAPED SUPERCONDUCTING COIL

S. Itoh, T. Hamajima, Y. Wachi, A. Miura, M. Naganuma, N. Fujiwara, M.

Yamaguchi, H. Ohguma, H. Shiraki, S. Murase, et al.

To cite this version:

S. Itoh, T. Hamajima, Y. Wachi, A. Miura, M. Naganuma, et al.. DEVELOPMENT OF A FORCED

COOLED D-SHAPED SUPERCONDUCTING COIL. Journal de Physique Colloques, 1984, 45 (C1),

pp.C1-157-C1-161. �10.1051/jphyscol:1984133�. �jpa-00223688�

JOURNAL DE PHYSIQUE

Coltoque C l , s u p p l h e n t au n o 1, Tome 45, janvier 1984 page Cl-157

DEVELOPMENT OF A FORCED COOLED D-SHAPED SUPERCONDUCTING COIL S. Itoh, T.Hamajima,Y. Wachi, A. Miura, M. Naganuma, N. Fujiwara, M. Yamaguchi, H. Ohguma, H. Shiraki, S. Murase and H. Ichikawa

Toshiba Corporation, 2-4, Suehiro-cho, T s m i - k u , Yokohama, 230, Japan RBsurn6

Une bobine supraconductrice a refroidissement for&

dans 11h61ium supercritique a 6t6 fabriqu6e en vue de de- velopper une bobine 5 champ toroydal de 12T. Celle-ci est bobinge avec des conducteurs tubulaires et sera testBe 2

1 0 kA 2 un champ d'environ 1 0 T . Abstract

A forced cooled D-shaped superconducting test coil by supercritical helium was made for the purpose of developing 12T toroidal field coil. The coil is wound with cable-in- cond~it conductors and will be tested at 10 kA operating current at about 10T field.

1. Introduction

A fusion experimental reactor will require superconducting toroidal field coils which have a very large D-shape bore with its maximum field around 12T. A forced-cooling coil by supercritical helium is considered one of the probable candidates from the structural and cryogenic standpoint to meet the above requirement.

Supercritical helium flow is forced through the so-called cable-in- conduit conductor.

The cable-in-conduit conductor which has the capability of critical current 15 kA at the field of 12T was developed. The critical current was studied against various bending strains. The mechanical properties of conduit: 304L SS and 316L SS were studied on unwelded and welded part after or before the heat treatment of 700°C in 100 hours. The calculated stability margin for the designed conductor is 350 mJ/cc under the condition of 4 k inlet temperature and 3 g/s mass flow/l/. The coil manufacturing technologies were developed with dummy conductors prior to windtng a test coil. Any of above fundamental research and development showed promising results.

The test D-shape coil consists of 2 pancakes, each 6 turns and has the coil inner bore of 71.5 cm times 87.3 cm. The lower portion of the test coil is sandwitched by small background split coils. The operating current of the test coil is 10 kA and its maximum resultant field is 10T. The supercritical helium is supplied to the coil from a helium refrigerator.

2. Cable-in-Conduit Nb3Sn Superconductor 2-1 Characteristics

The cable conductor has advantage for the cryogenic stability

because of its large wetted perimeter/2/. The cable conductor is

capsulated in a conduit and then the Nb Sn reacts at 700°C and for

100 h, before winding the coil. The cazle-in-conduit conductor is

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984133

Cl-158 JOURNAL DE PHYSIQUE

d e s i g n e d t o s a t i s f y t h e s t a b i l i t y a t o p e r a t i n g c u r r e n t of 10 kA a t 12T on t h e h y d r a u l i c c o n d i t i o n of 1 MPa, 4 . 5 k a n d ' 0 . 0 1 k g / s s u p e r - c r i t i c a l helium. The c r o s s s e c t i o n of t h e conductor f a b r i c a t e d i s shown i n F i g . 1 , and t h e main c h a r a c t e r i s t i c s a r e l i s t e d i n Table 1.

The m a t e r i a l of t h e c o n d u i t , which mainly s u p p o r t s t h e mechanical f o r c e , i s made of SUS 316L c o n s i d e r i n g t h e mechanical s t r e n g t h b e f o r e and a f t e r t h e h e a t t r e a t m e n t t o produce NbgSn.

2-2 C r i t i c a l C u r r e n t

The reduced s i z e c o n d u c t o r s a r e u s e d f o r e s t i m a t i n g t h e c r i t i c a l c u r r e n t a t v a r i o u s bending s t r a i n s i n s t e a d of measuring t h e c r i t i c a l c u r r e n t of t h e f i n a l conductor a t 12T. The reduced s i z e conductor h a s 3x6 composites i n c o n d u i t and a r e r e a c t e d a t t h e same c o n d i t i o n a s t h e f i n a l c o n d u c t o r , where t h e v o i d f r a c t i o n i s changed from 30%

t o 50%. The composite used i s t h e same s i z e a s t h a t of t h e f i n a l c o n d u c t o r . The r e s u l t s a r e shown i n F i g . 2, where measured c r i t i c a l c u r r e n t of a composite i s p l o t t e d f o r t h e comparison. The c a b l e - i n - c o n d u i t conductor w i t h 50% void f r a c t i o n behaves l i k e a composite a g a i n s t t h e bending s t r a i n , w h i l e t h e c o n d u c t o r s w i t h void f r a c t i o n below 40% behave l i k e compacted composites. This comes from t h a t composites i n t h e c o n d u i t my be bound a s t h e v o i d f r a c t i o n d e c r e a s e s . O n t h e o t h e r hand, t h e c r i t i c a l c u r r e n t a t t h e 30% void f r a c t i o n d e t e r i o r a t e s even though no bending i s a p p l i e d . This may stem from rhe same r e a s o n a s t h e m o n o l i t h i c Nb3Sn conductor 131.

3. S t a b i l i t y of Forced Cooled C o i l

P r i o r t o c o n s t r u c t i o n of t h e f i n a l c o i l , a s m a l l c o i l w i t h 4 m l e n g t h was used f o r s t u d y i n g t h e s t a b i l i t y o f t h e c o i l cooled by t h e s u p e r c r i t i c a l helium. The conductor used has 3 x 3 ~ 6 (54) composites i n a c o n d u i t of 5.79 mm I . D . The composite has NbTi superconductor of 0.61 mm O.D. which i s t h e same as t h e r e a l c o n d u c t o r . High f r e q u e n c y h e a t i n g was employed t o g e n e r a t e t h e normal zone i n t h e c o i l . The h e a t i n g power t o l e a d t h e c o i l quench was measured a t t h e c o n d i t i o n s t h a t t h e mass flow r a t e , h e a t i n g l e n g t h and t h e o p e r a t i o n c u r r e n t a r e changed. The main r e s u l t s a r e a s f o l l o w s

( i ) h e a t i n g energy t o l e a d t h e c o i l quench i s approximately i n p r o p o r t i o n t o 2 . 2 t h power of mass flow r a t e , (ii) t h e quench energy f o r l o n g e r h e a t i n g l e n g t h i s s m a l l e r t h a n t h a t f o r s h o r t e r one.

4 . T e s t c o i l and f a c i l i t y

D-shaped Nb3Sn c o i l encased i n round s t a i n l e s s s t e e l c a s e was i n s t a l l e d i n t h e r m a l l y s h i e l d e d vacuum v e s s e l w i t h a p a i r of back- ground s p l i t s o l e n o i d s and 4.2 k l i q u i d helium b a t h c o n t a i n e d w i t h h a a t exchanger ( F i g . 3 ) . R e f r i g e r a t i o n and f o r c e d flow helium a r e s u p p l i e d by a TCF-100 S u l z e r B r o t h e r s r e f r i g e r a t o r which p o s s e s s e s s u p e r c r i t i c a l helium s u p p l y c i r c u i t and r e f r i g e r a t i o n c a p a c i t y of 200 W a t 4 . 6 k.

NbgSn c o i l i t s e l f was wound i n a double pancake 1 . 0 9 m high and 0.94 m wide u s i n g 45 m l e n g t h and 12 t u r n s o f c o n d u c t o r . Conductor was wound t i g h t l y and i n s u l a t e d w i t h r e s i n pre-impregnated g l a s s f i b e r t a p e and s t r e n g t h e n e d by i n t e r l e a v i n g s t a i n l e s s s t e e l t a p e 0 . 5 mm t h i c k a g a i n s t hoope s t r e s s . D-shaped c o i l was s e t i n round s t a i n l e s s c a s e u s i n g f i b e r g l a s s s p a c e r t o r e s i s t magnetic f o r c e and t o expose t h e c o o l i n g s u r f a c e of c o i l and c a s e f o r c o o l i n g . Background c o i l s clamped a c r o s s b o t t o n s e c t i o n of t h e t e s t c o i l g e n e r a t e 9T and superposed f i e l d of a b o u t 10T i s a t t a i n e d by e x c i t a - t i o n of t h e t e s t c o i l . I n d u c t i o n h e a t i n g c o i l was wounc! around t h e conductor of 113 l e n g t h of t h e m o s t i n s i d e t u r n of t h e h a l f pancake f o r r e c o v e r y c u r r e n t t e s t . D c o i l was encased i n c o i l c a s e and welded i n l i p s e a l a f t e r b o l t e d .

Subcooled f o r c e d flow helium p a s s e d through t h e h e a t exchanger

i n l i q u i d h e l i u m b a t h i s s u p p l i e d t o c o i l a t openings o f conductor c o n d u i t n e a r c o n n e c t i o n s between power l e a d s and c o n d u c t o r . Gas cooled l e a d s a r e c o o l e d by b o i l e d o f f a t m o s p h e r i c h e l i m i n l i q u i d helium b a t h . Power l e a d s l i q u i d h e l i u m b a t h and 9T background c o i l s were

assembled on t e s t c o i l w i t h o t h e r m i s c e l l a n e o u s p i p e s and i n s t a l l e d i n vacuum v e s s e l ( F i g . 4 ) . S i m p l i f i e d flow diagram of helium c o o l i n g system i s i l l u s t r a t e d s c h e m a t i c a l l y ( F i g . 5 ) .

S u p e r c r i t i c a l h e l i u m p a s s e d t h r o u g h NbgSn c o i l i s s u p p l i e d t o 4k b a s e , then r e t u r n s t o r e f r i g e r a t o r . On t h e o t h e r hand, low p r e s s u r e h e l i u m s t r e a m from r e f r i g e r a t o r i s branched away t h r e e s t r e a m s t o c o o l

t h e D c o i l , background c o i l s and l i q u i d helium b a t h , t h e n r e t u r n s t o r e f r i g e r a t o r a g a i n i n s i n g l e s t r e a m .

I n s t e a d y s t a t e o p e r a t i o n , w h i l e s u p e r c r i t i c a l s t r e a m i s s u p p l i e d t o D c o i l c o n t i n u o u s l y l i q u i d helium f o r 4 k b a t h and 9T background c o i l s i s s u p p l i e d p e r i o d i c a l l y from 1000 R dewar b e c a u s e o f i r n p o s s i b i l - i t y o f mixed mode o p e r a t i o n of l i q u e f a c t i o n and s u p e r c r i t i c a l helium s u p p l y . R e f r i g e r a t o r can produce s u p e r c r i t i c a l helium o f 16 atm max.

p r e s s u r e 20 g / s flow r a t e and 200 W r e f r i g e r a t i o n a t 4.8 k . Conclusion

The p r i n c i p a l fundamehtal t e c h n o l o g i e s r e q u i r e d f o r a 12T f o r c e d c o o l e d D-shaped s u p e r c o n d u c t i n g c o i l by s u p e r c r i t i c a l h e l i u m were developed. The D-shaped c o i l wound w i t h c a b l e - i n - c o n d u i t c o n d u c t o r s i s t o be t e s t e d a t 10 kA o p e r a t i n g c u r r e n t a t about 10T f i e l d .

R e f e r e n c e s

/l/ Y . Wachi, e t a l . , I E E E T r a n s . on Magn. Vol. MAG-19, No. 3 (1983) 320

/2/ J . W . Lue, J . R . M i l l e r and L . D r e s n e r , J. Appl. Phys. 2

(1980) 772

/ 3 / M . M . S t e e v e s and M . O . Hoening, I E E E T r a n s . on Magn. Vol.

MAG-19, No.3 (1983) 374

3 0 U R N A L DE PHYSIQUE

Table 1. Main conductor characteristics

Maximum Field 12 T Operating Current 10 KA Critical Current 1 5 K A at 12T Overall Dimensions 18.3 X 15.7 mm2 Conduit Thickness l mm Void Fraction 33 %

Cable Configuration 6 X 34 (486 composites) Composite Diameter 0.61 mm

Cu

l

Fig. l. Cross sect ion o f the conductor

l

4 0 0

-

6

-

E=IpV/cm at 7 T o n d 4 2 K

0 ^{I I I I I I I I}

0 0 2 0.4 0 6 0.8 1 0 1 2 1 4 1.6 18 2 Bend~ng stra~n. Eb (%l

Fig. 2. Critical current as a function

o f bending strain

Fig.3. Assembled test coil Fig. 4. Test facility and testfacility

/

L ~ q u i d hellurn dewor

Fig. 5. Schematic flow diagram o f cooling system