ADVANCED SUPERCONDUCTING MATERIALS

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ADVANCED SUPERCONDUCTING MATERIALS

R. Flükiger

To cite this version:

R. Flükiger. ADVANCED SUPERCONDUCTING MATERIALS. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-365-C1-372. �10.1051/jphyscol:1984174�. �jpa-00223730�

ADVANCED SUPERCONDUCTING MATERIALS

R. Fliikiger

Kernforsehungszentrvm Karlsruhe, Institut fiir Teehnisahe Physik, D-7500 Karlsruhe, F.R.G.

Résumé - Les propriétés supraconductrices de différents matériaux sont discutées en vue de l'utilisation de ces derniers dans des aimants produisant de hauts champs magnétiques. Les densités de courant critique au-delà de 12T pour des con- ducteurs tels que NbN ou PbMOgSo sont comparés à celles des conducteurs actuels les plus avancés, a base de Nb,Sn avec différentes additions. Différents aspects du renforcement mécanique de conducteurs à hauts champs, rendu nécessai- re par les importantes forces de Lorentz (p.ex. dans des aimants pour la fusion) sont discutés.

A b s t r a c t - The superconducting p r o p e r t i e s o f various m a t e r i a l s are reviewed i n view o f t h e i r use i n high f i e l d magnets. The c r i t i c a l c u r r e n t d e n s i t i e s above 12T o f conductors based on NbN o r PbHOgS,, are compared to those o f the most advanced p r a t i c a l conductors based on a l l o y e d Nb^Sn. D i f f e r e n t aspects o f the mechanical reinforcement o f high f i e l d conductors, rendered necessary by the strong Lorentz forces ( e . g . i n f u s i o n magnets), are discussed.

I - INTRODUCTION

Although no new high Tc material was found after the discovery of the rhombohedral com- pound PbMofiSo (Tc = 15K) nearly 10 years ago / l / , there is no stringent or definitive argument : excluding the existence of new ternary or quaternary materials with Tc va-

lues close to 20K (or even higher). A possible limitation towards higher Tc values ari- ses from theoretical arguments considering the strong electron-phonon coupling, which suggest a "saturation" of >\, the electron-phonon interaction parameter / 2 / . At present, the search for new superconductors has been extended to ternary, quaternary and even multinary compounds, including also nonmetallic elements as sulfur, carbon, nitrogen, oxygen and others / 3 / .

The wealth of the actual research in the tie Id of superconductivity shows a pronounced trend toward applications, dictated by industrial requirements. The most successful ap- plication for industrial superconductors is actually doubtless represented by the full- body NMR magnets for clinical diagnosis, based on NbTi. For this purpose, mono- or mul- tifilamentary NbTi wires must satisfy severe homogeneity requirements. They are produ- ced by classical procedures, but do not need further improvements, the produced fields being always below 2T. There are other fields where the use of advanced superconductors is required: a) high field magnets, either for laboratory use or for future fusion ap- plications, and b) superconducting microelectronics (Josephson computer). The present survey will be restricted to high field conductors, with a particular emphasis on emer- ging materials as (Zr,Hf)V2, PbMogSg and NbN. A particular aspect of A15 conductors will also be treated, i.e. the behavior of J in mechanically reinforced wires, able to retain the strong Lorentz forces acting in fusion magnets.

II - COMPETING SUPERCONDUCTING MATERIALS

The materials which could hypothetically be used in superconducting wires (Table I) can be subdivided in three classes, following their values of the upper critical field, Bc„(0) * 16T, 16T =»Bc 2(0) *» 30T and Bc 2(0) * 30T (No Bc 2 values could be found in the literature for the compound Y ^ T h 3C-| 5g , synthetized under high pressure/30/).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984174

C 1-366 JOURNAL DE PHYSIQUE

Compound Struc- S t a t e Radia- Stress Thermal Tc

g

t u r e o f t h k t i o n Sensi- E q u i l i -

Sample Damage t i v i t y brium (I() ( m ~ / a t g ~ ~ ) (T)

NbTi A2 M No No Yes 10.1 14 4

Nb-Ta-Ti A2 M No No Yes 9 .O 16 5

VTI A2 M No No Yes 7 12 6

V-Ta-Ti A2 M No No Yes 6.5 16 7

ZrV2 Laves B No No

(Zr,Hf)V2 (cub.) M

Nb3Sn A1 5 B Yes Yes

B B M

Nb Sn + A15 M Yes Yes

adai t i o n s

V3Ga A1 5 B Yes Yes

- - -

Yes 8.7 8

Yes 9.7 21 (4.2K) 8,9

Yes 18.0 13.2(60%c) 10

-3O(c) 11

1 3 . 0 ( t ) 12

20+26(b) 11,13

Yes up t o 25(4.2K) 14,42

18.2

Yes 15.9 24 24 15

N ~ N + ) B 1 Q

B

(cub.) S

S v s S

PbMo6S8 Rhombo- B

hedral M

Yes Yes NO 23

22.5 20.8 7.58

Yes Yes No 19.1

18.4 9.0 18.0 . . . .

Yes Yes No 20.7

20.0 8.75

NO Yes No 15.7 2.04

15.8

Yes ? Yes 15 5

13.4

Tab& I . Supmcondu&Lng p m p d u 0 4 v w h u n ivLteme&&..Lc compounch a d t m

dim-

n e n t p h e p w n moden:B = bulk, M = muRti&&menta/ry, S = np&med, C = coevupouted, V = CVD, Q = quenched 6nom ;the L i q u i d n & t e and nehanodohmed.

The n w L t i v i t y t o k i g h w f n g y , i m a W n O h d i c a t e d , cuweRe f i e oc- c w e n c e . 06 e q W b n i u m . ) NbN Lb n t u b ~ z e d by C on 0: T k i n nymbol Lb une.d h m e Son Nb(N,C,Ol.

The compounds NbTi, Nb-Ta-Ti, VTi and V-Ta-Ti are d u c t i l e , i n c o n t r a s t t o a l l o t h e r m a t e r i a l s l i s t e d i n Table I , b u t e x h i b i t r e l a t i v e l y low T values (Tc 4 10K). The h i - ghest values o f B 2 ( 0 ) f o r t h e t e r n a r i e s Nb-Ta-Ti and V-TS-T~ reach 16T /5,7/. The adequate o p e r a t i o h range f o r magnets based on these conductors i s thus 1.8K, where t h e cor,responding values o f B a r e c l o s e t o 15T.

M u l t i f i l a m e n t a r v w i r e s b a ~ z d on these m a t e r i a l s a r e prepared by a succession o f ex- t r u s i o n and w i r e drawing steps w i t h o u t i n t e r m e d i a t e anneal ings. The h i g h e s t J values being obtained a f t e r t h e g r e a t e s t amount o f c o l d working. The r e c e n t progressCwith hy- d r o s t a t i c a l l y extruded NbTi /31/ ( i n s t e a d o f t h e o r d i n a r y h o t e x t r u s i o n ) confirms the b e n e f i c i a l e f f e c t of f u r t h e r c o l d working, which i s thought t o a c t on m i c r o s t r u c t u r a l p r o p e r t i e s .

This c l a s s o f m a t e r i a l s i n c l u d e s t h e t e r n a r y compound (Zr,Hf)V2 c r y s t a l l i z i n g i n the cubic Laves phase, C15, and the A15 type compounds Nb Sn ( w i t h and w i t h o u t a d d i t i o n s ) and V3Ga. I n b o t h cases, mu1 t i f i l a m e n t a r y wires a r e odtained by a d i f f u s i o n r e a c t i o n a t t h e end o f t h e p l a s t i c deformation, a t temperatures a t t h e v i c i n i t y o f 7 0 0 ~ ~ . A15 phases a t e u i l i b r i u m . A t 20T, V Ga s t i l l has h i g h e r J values than Nb3Sn, i n s p i -

t e of the impro!ements achieved i n d e l a s t years by i n t r o S u c i n g he

t

This value o f Jc a t a f i e l d Bo 1s g e n e r a l l y considered as t h e necessary c r i t e r i o n f o r reaching Bo i n a magnet wound w i t h t h e same conductor: The c o n s t r u c t i o n o f small labora- t o r y magnets reaching 19 o r even 20 T on the basis o f V Ga o r a l l o y e d NbgSn can thus p o s i t i v e l y be envisaged. The improvement o f Jc a t h i g h i i e l d s i n a l l o y e d Nb3Sn wires i s mainly c o r r e l a t e d t o an enhanced value o f Bc2, as a consequence o f t h e enhanced e l e c t r i - c a l r e s i s t i v i t y , g . A f u r t h e r progress c o u l d be reached by a c t i n g on the p i n n i n g mecha- nism, as suggested by t h e r e c e n t success o f the powdermetallurgical ECN technique/35/:

The h i g h Sn reserve a r i s i n g from NbSn2 leads here t o p a r t i c u l a r l y h i g h growing rates, thus causing a considerable enhancement o f Jc up t o 15T (see Fig. 1 ) .

Laves phases. The superconducting p r o p e r t i e s o f Laves phase compounds are r e l a t i v e l y i n s e n s i t i v e t o i r r a d i a t i o n w i t h h i g h energy p a r t i c l e s 1351, which c o u l d be a d e c i s i v e advantage o v e r A15 o r rhombohedra1 compounds i n view o f a p p l i c a t i o n s i n f u t u r e f u s i o n magnets. Two approaches f o r producing mu1 ti f i l a m e n t a r y (Zr,Hf)V w i r e s have been r e - ported. The f i r s t one i s based on t h e r e t r a n s f o r m a t i o n o f amorp6ous ( Z r 7 H f ) Vaq0 ribbons produced by r a p i d quenching /8/ i n t o t h e C15 phase by a subsequent adneB8 of 72 hours a t 600'~. I t i s thought t h a t t h e m e l t spinning process t o produce c o n t i - nuous d u c t i l e ribbons o f amorphous Zr-Hf-V would be promising.

The second approach f ~ r - ~ r o d u c i n ~ mu1 ti f i l a m e n t a r y (Zr,Hf)V conductors by a d i f f u s i o n r e a c t i o n has a1 ready been developped t o a l e v e l where an i n g u s t r i a l production o f l o n g l e n g t h s o f m a t e r i a l seems possible. By t h e composite process, Kuroda e t a1 ./9/ have prepared w i r e s c o n t a i n i n g 1634 (Hf,Zr)V f i l a m e n t s . Groups o f 7 o r 19 Zr-Hf rods con- t a i n i n g up t o 40 a t . % H f werg i n s e r t e d ?n a V

-

As shown i n Fig. 1, t h e o v e r a l l c r i t i c a l c u r r e n t d e n s i t i e s o f such wires are appreci- able: The w i r e c h a r a 6 t e r i z ~ d by Z r

-

-

Fig. 7

- O v W c h i t i c & cweQYLt d m - hLtiecl doh rnuRtid~umevLtcvry w i t e n b a e d o n d i 4 d u ~ ) t Q M . t mate-

-

me;tho& :

Nb Sn(ECN/35/) , Nb Sn ( b m n z e

- / i d / ) , NbtSn(ln ~ & / 5 3 / ) ,

U ~b S n ( h d m

3 ^* n/50/), V3Ga

(bhonze/54/ ), Nb ke(powdetr me- X U g y / 4 s / ) , (6b- 1 . 6 ~ ~ ) ,sn

( b m n z e / 3 2 / ) , (Nb-7Tal Sn

3 I I I ( b h o n z e / 3 4 / ) , ( z ~ - ~ o H &

l o *

&

BJT)

C1-368 JOURNAL DE PHYSIQUE

By o p t i m i z i n g t h e h e a t t r e a t m e n t c o n d i t i o n s , t h e p a c k i n g r a t i o s and t h e Hf c o n t e n t i n t h e r o d s , Inoue e t a l . /42/ r e c e n t l y showed t h a t ( Z r , H f ) V 2 w i r e s may e x h i b i t even h i g h e r Jc v a l u e s l y i n g v e r y c l o s e t o t h o s e o f u n a l l o y e d Nb3Sn w i r e s .

Even i n t h e case o f f u t u r e improvements on Jc f o r t h e w i r e s d i s c u s s e d i n t h e paragraphs A and B, t h e 1 im i t e d v a l u e o f Bc2(0) ( o r b e t t e r , Bc2(4.2K)) c o n s t i t u t e s a s t r i n g e n t l i m i t a t i o n . Other A15 t y p e compounds, as Nb3A1, Nb3Ga, Nb Ge, o r Nb (A1 ,Ge) a r e known t o e x h i b i t Bc-(0) v a l u e s above 307, w h i l e f o r ~ b f i and PbYo S v a j u e s above 50T have been r e p o r t e d ( s e e T a b l e I ) . I l n f o r t u n a t e l y , t h e s e h i g h B,;;:(

v a l u e s correspond e i t h e r t o b u l k o r non e q u i l i b r i u m s t a t e s o f t h e r e s p e c t i v e m a t e r i a l s and n o t t o t h e r e q u i r e d m u l t i f i l a m e n t a r y s t a t e . F o r each one o f t h e s e m a t e r i a l s , cha- r a c t e r i s t i c a l d i f f i c u l t i e s a r e encountered when p r e p a r i n g mu1 t i f il amentary w i r e s , b u t t h e r e i s no doubt t h a t i n t h e f u t u r e , one o r a n o t h e r w i l l be a v a i l a b l e i n t h e m u l t i - f i l a m e n t a r y c o n f i g u r a t i o n w i t h Bc2(0) v a l u e s approaching t h o s e 1 i s t e d i n Table I . N o n e q u i l i b r i u m A15 t y p e systems. I n t h e case o f A15 t y p e systems w i t h Bc2(0)>30T, two m a i n d i t t i c u l t i e s a r e encountered when comparing w i t h Nb3Sn o r V3Ga: ^{1 )} t h e s t o i - c h i o m e t r i c c o m p o s i t i o n i s m e t a s t a b l e ( f o r a r e v i e w o f t h e c o r r e s p o n d i n g phase d i a - grams see Ref. 41) and i i ) t h e d i f f u s i o n r e a c t i o n s t a r t i n g f r o m t h e Cu bronze i s n o t a p p l i c a b l e . Thus, a l l approaches t o p r e p a r e m u l t i f i l a m e n t a r y w i r e s based on m a t e r i a l s w i t h Bc (0)>30T w i l l have t o s t a r t l f r o m a n o n e q u i l i b r i u m s t a t e . The s t o i c h i o m e t r i c composl?ion can be reached by CVD (Nb3Ge / 1 8 / ) , c o e v a p o r a t i o n (Nb3Al /37/, Nb3Ge /16,13/), s p u t t e r i n g (Nb3Ge,/16/) o r quenching f r o m t h e l i q u i d s t a t e . The l a t t e r i s p a r t i c u l a r l y i n t e r e s t i n g , s i n c e i t a l l o w s t o r e t a i n t h e a l l o y i n a d u c t i l e s t a t e , e.g.

bcc /39,40,41/ o r amorphous /42/. I n t h e case o f Nb A!, t h e bcc phase was r e t a i n e d a t c o m p o s i t i o n s above 22 a t . % A l , e i t h e r by l i q u i d quenzhing /39/ o r by s p l a t c o o l i n g /40/.

A f t e r r e t r a n s f o r m a t i o n a t 900°C, Bevk e t a l . /40/ o b t a i n e d e x t r e m e l v h i g h Jc v a l u e s , 2.2 X l o 5 ~ / c m ~ a t 20 T, d e m o n s t r a t i n g t h e p o t e n t i a l f o r Nb3A1. A bcc

phase w i t h such h i g h A1 c o n t e n t s b e i n g n o t d u c t i l e enough f o r extended p l a s t i c d e f o r - m a t i o n needed f o r i n d u s t r i a l a p p l i c a t i o n s . A s i g n i f i c a n t improvement was r e c e n t l y r e a l i z e d b y Togano e t a l . /21/, who were a b l e t o produce amorphous Nb3(A1 ,Ge) r i b b o n s b y s p l a t c o o l i n g on a Cu s u r f a c e heated a t 6 0 0 ~ ~ . The ( s t i l l u n r e s o l v e d ) problem i s now t h e same as i n t h e amorphous (Zr,Hf)Vp r i b b o n s d e s c r i b e d above /8/: The p r o d u c t i o n o f l o n g l e n g t h s o f homogeneous amorphous r i b b o n s by m e l t s p i n n i n g , as a b a s i s f o r m u l t i f i l a m e n t a r y w i r e s . Another way f o r o b t a i n i n g A15 phase f i l a m e n t s i n a n o n e q u i l i - b r i u m s t a t e was d i s c o v e r e d by Ceresara e t a l . /43/ on Nb3A1. I f a l t e r n a t e l a y e r s o f Nb and A1 o f l e s s t h a n 1 um t h i c k n e s s a r e formed, a r e a c t i o n between 700 and 900°C l e a d s t o an A15 phase c o n t a i n i n g 22 a t . % A l . , i . e . 1.5 t o 2 a t . % A l above t h e e q u i l i b r i u m v a l u e /41/. The mechanism l e a d i n g t o t h i s f a v o u r a b l e s h i f t i n c o m p o s i t i o n i s n o t y e t understood, b u t a1 1 approaches i n f a b r i c a t i n g Nb3A1 w i r e s r e p o r t e d i n t h e meantime a r e based on t h i s process. A s p e c t a c u l a r one i s t h e p o w d e r m e t a l l u r g i c a l approach, f i r s t r e p o r t e d b y Larson e t a l . /44/: A m i x t u r e o f Nb and A1 powders i s " m e c h a n i c a l l y a l l o y e d "

by means o f h i g h energy b a l l m i l l i n g and subsequently drawn t o f i n e w i r e s and r e a c t e d . L a t e r on, t h e " c o l d p o w d e r m e t a l l u r g i c a l " p r o c e s s i n g , developped by t h e a u t h o r /45,51/

f o r p r o d u c i n g u l t r a f i n e NbgSn f i l a m e n t s was extended t o Nb A1 /46,47/ and was r e c e n t l y c o n s i d e r a b l y improved by Thieme e t a l . /48/ (see F i g . 1 ) . ? h i s method i s s t i l l under p r o g r e s s .

PbYo S The p o s s i b i l i t i e s o f p o w d e r m e t a l l u r g i c a l p r o c e s s i n g i n p r e p a r i n g supercon- m w m u l t i f i l a m e n t a r y w i r e s w i t h Bc (0)>30T a r e f a r f r o m b e i n g c o m p l e t e l y explored.

As i l l u s t r a t e d by t h e work o f Seeber e g al: 1281 on Pb*lo6S8 w i r e s , p r e p a r e d l u r i n g p r e - r e a c t e d powders o f t h e rhombohedra1 phase. T h i s method i s p o s s i b l e s i n c e t h e rhombo- h e d r a l Mo chalcogenides a r e r e l a t i v e l y s o f t , w i t h a V i c k e r s microhardness o f 150kg/mm2 /49/, which i s c o n s i d e r a b l y l o w e r t h a n t h a t o f NbjSn,

-

o f magnitude s m a l l e r t h a n t h e expected v a l u e i n PbMo6S8, due t o c r a c k s and i n s u f f i c i e n t compaction, as demonstrated b y Jc measurements on h o t pressed, b u l k PbYo6S8 samples w i t h g r a i n s i z e s <5 um ( s e e F i q . 2 ) . A tendency toward c o n s i d e r a b l y h i g h e r Jc v a l u e s f o r submicron g r a i n s i z e s can be r e c o g n i z e d ( g r a i n boundary p i n n i n g ) , l e a d i n g t o t h e e x t r a p o l a t e d v a l u e s r e p r e s e n t e d by t h e d o t t e d l i n e i n F i g . 2. I t appears, however, t h a t t h e p r o d u c t i o n o f PbMo S8 w i r e s w i t h submicron s i z e s i s p r e c i s e l y t h e main d i f - f i c u l t y . Indeed, n o t o n l y t t e rhombohedra1 Yo c h a l c o g e n i d e s a r e v e r y s t a b l e , w i t h m e l t i n g p o i n t s 1 5 0 0 ~ ~ /49/, b u t t h e tendency t o f o r m l a r g e g r a i n s i s e v i d e n t : s i z e s o f > 100 um can be e a s i l y o b t a i n e d as t h e p r o d u c t o f t h e s i n t e r i n g r e a c t i o n a t 1000°C.

The s t r o n g i n f l u e n c e o f t h e g r a i n s i z e on Jc i s c o n f i r m e d by t h e t o r k o Hamasaki e t

5

a l . /29/ on t h i n PbYo6S8 f i l m s w i t h 1 urn g r a i n s i z e : a b o u t 5 x 10 A/cm a t 8T and F i g . 2 .

Jc V A . BOdoh Rhe Lays 06 ~ u p m c c n - duc-ting r n a L m X i n w h u oh. & p a ptepmed by di4dmeM-t rndhodn:

(Nb,Xl 3Sn 132,341, Nb Ge 1381, Nb3 [At,Gel 1211, i l . t , H 6 1 V 2 191, 6 b ~ o S poiuim,~.

5 urn / 2 8 / , L a y a h , 1 urn 1894: NbN, n p L t m e d , 600 and 1000 f 1241, C V D 1261. The do-tted L i n e h e p a e n t s an ex-

~ ~ p ~ e a * & u p p m chit 60.t P ~ M O ~ S ~ 1281.

F.112----

5 10 15 20 25 30

B'JTI

4.2K were r e p o r t e d . Thus, t h e success of f u t u r e PbYo6S8 w i r e s w i l l depend on t h e a b i l i t y o f f u r t h e r r e d u c i n g t h e g r a i n s i z e . A v e r y s t i m u l a t i n g p o i n t i s t h e v e r y h i g h v a l u e o f Bc2 01,

-

Bodecreases v e r y i l o w l y i n t h e range between 8 and 14T. Thus, an i n c r e a s e o f J i n t h e l a y e r by a f a c t o r of 5 would be s u f f i c i e n t t o exceed t h e v a l u e s of t h e b e s t A15 w i r e s above 20T.

NbN. There a r e s i m i l a r i t i e s between w i r e s based on t h e L a v e s phase (Zr, Hf)V2 d i s c u s s e d

-

The h i q h e s t Bc2 v a l u e s a r e found i n t h i n f i l m s ( < 5000 i) d e p o s i t e d a t 450°C /24/:

These c o n d i t i o n s a r e a compromise between an e x t r e m e l y h i g h Bc /24 and a p i n n i n g a c t i v e columnar v o i d m i c r o s t r u c t u r e . Jc v a l u e s f o r 6 0 and 000 f i l m t h i c k n e s s e s

W ^{T i}

( F i g : 2) demonstrate t h e p o t e n t i a l o f NbN. F o r t h e 600 f i l m , a n e x t r a p o l a t i o n o f t h e c r ~ t i c a l d a t a near Tc y i e l d s B 2

-

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F i g . 3

Cmbon Libfie u6 6 . 5 urn d h e , € e ~ , coa- ted w-ith a I pm NbN Layen, and n

u e ~ y t h i n SLC intenlayex. Bah Length: 0.5 urn.

fl)i&ich ct at. /26/1.

a l o n g way t o go u p t o a p r a c t i c a l superconductor.

A f u r t h e r s t e p i n t h i s d i r e c t i o n i s t h e d e p o s i t i o n o f NbN on carbon f i b e r s by CVO and magnetron s p u t t e r i n g , by O i e t r i c h e t a1

.

l a y e r /26/. A Bc2 v a l u e o f 22T a t 4.2K was reached so f a r by these methods. A t y p i c a l J vs. B,,curve i s shown i n F i g . 2, w h i l e a carbon f i b e r , c o a t e d w i t h S i c and NbN, i s s6own i n F i g . 3. F u r t h e r p r o g r e s s i s expected from a l o w e r i n g o f t h e d e p o s i t i o n tem- p e r a t u r e .

111 - PRACTICAL CONDUCTORS SUBYITTED TO STRONG LORENTZ FORCES --

An i n h e r e n t problem o f l a r g e h i g h f i e l d magnets (e.g. f u s i o n magnets) i s t h e o c c u r r e n c e o f s t r o n g L o r e n t z forces, r e q u i r i n g a r e i n f o r c e m e n t o f t h e c o n d u c t o r b y h i g h s t r e n g t h m a t e r i a l s which can be i n c o r p o r a t e d e i t h e r p r i o r t o o r a f t e r t h e r e a c t i o n h e a t t r e a t - ment. I n t h e case o f c o l l e c t i v e h e a t i n g of t h e a l r e a d y assembled r e i n f o r c e d c o n d u c t o r ( r e a l i z e d f o r example i n t h e " c a b l e i n c o n d u i t " concept/57/), t h e l a r g e t h e r m a l expan- s i o n c o e f f i c i e n t of s t e e l , d = 16 X 1 0 6 ~ - 1 , i s expected t o e x e r t an enhanced precompres- s i o n on t h e s u p e r c o n d u c t i n g f i l a m e n t s , which have o r d i n a r i l y d v a l u e s between 5 and 7 X 106K-1. The q u e s t i o n whether and t o what e x t e n t J c i s i n f l u e n c e d b y t h e r e i n f o r c i n g s t r u c t u r e can be answered b y s t u d y i n g t h e c o r r e l a t i o n J c vs. &

.

i n u n r e i n f o r c e d w i r e s i s s h i f t e d t o v a l u e s c l o s e t o I%, and i i ) t h e r a t i o Jc/Jc de- from 0.3 t o 1 W o r r e i n f o r c e d w i r e s . T h e n e g a t i v e e f f e c t on Jc can be counter!alanced e i t h e r by a d d i t i o n s t o Nb3Sn o r b y u s i n g r e i n f o r c i n g m a t e r i a l s w i t h l o w e r & values, which b o t h l e a d t o an i n c r e a s e o f 6 2 . . I n t h e case o f a d d i t i o n s , B i s r a i s e d b y t h e enhancement o f t h e e l e c t r i c a l r e s i s E l v l t y , w h i l e l o w d m a t e r i a l s a f f e n u a t e t h e precom- p r e s s i o n on t h e f i l a m e n t s , which a l s o l e a d s t o h i g h e r Bc2 values. As shown i n F i g . 4,

-

-

x Khametr @o& dot iM,tmn&y t e h d o t c e d

s

-

o 2 1 Steel so no h n e n t m y Nb3Sn @ h e , &h did6me)rent com- b i M a t i o ~ n 06 o - t ~ ~ L d w n a t e & [high&) and

10 Mo (low 4C m a t e ~ i d l

.

.the vmy&g paecornptunion, B* chcuzgu

dmm 1620 21 T i~~eiikigetr e , t d . ? $ b / ) .

5

15 20

Bo (TI

me m u l t i f i l a m e n t a r y Nb3Sn wire, B*~ increases from 16 t o 21T as f u n c t i o n o f the s t e e l / Mo f r a c t i o n . For fusion a p p l i c a t i 8 n s , i t i s o f importance t h a t t h e precompression i n a r e i n f o r c e d conductor can be c o n t r o l l e d by simply changig the r a t i o between the h i g h d and t h e low oC r e i n f o r c i n g m a t e r i a l J

I V

-

Since no new h i g h Tc m a t e r i a l s c o u l d be found i n t h e l a s t decade, the development o f advanced superconductors i s now concentrated on t h e achievement o f h i g h J values a t h i g h magnetic f i e l d s on m a t e r i a l s a l r e a d y known. T h i s i s m o s t l y obtained by i n c r e a s i n g the upper c r i t i c a l f i e l d (by o p t i m i z a t i o n o f the r e a c t i o n heat treatment and/or by addi t i o n s t o t h e superconductor) o by r ducing the precompression a c t i n g on the f i l a m e n t s .

S

5 .

S e t t i n g a value o f J = 4 X 10 A/cm i n t h e superconducting l a y e r as a c r i t e r i o n , i t appears from F i g . 2 ghat superconducting magnets w i t h f i e l d s up t o 25T and more can be i n p r i n c i p l e expected i n t h e f u t u r e .

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