DEVELOPMENT OF INTERNALLY STRENGTHENED PREREACTED Nb3Sn SUPERCONDUCTORS

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DEVELOPMENT OF INTERNALLY STRENGTHENED PREREACTED Nb3Sn

SUPERCONDUCTORS

P. Turowski, A. Nyilas, M. Thöner, P. Sanger

To cite this version:

P. Turowski, A. Nyilas, M. Thöner, P. Sanger. DEVELOPMENT OF INTERNALLY STRENGTH-

ENED PREREACTED Nb3Sn SUPERCONDUCTORS. Journal de Physique Colloques, 1984, 45

(C1), pp.C1-399-C1-402. �10.1051/jphyscol:1984181�. �jpa-00223737�

JOURNAL DE PHYSIQUE

Colloque C l , supplément au n° 1, Tome 45, janvier 198* page Cl-399

DEVELOPMENT OF INTERNALLY STRENGTHENED PREREACTED Nb

Sn SUPERCONDUCTORS

P. Turowski, A. Nyilas, M. Thoner and P.A. Sanger*

Kernforschungszemtrum Karlsruhe, Institut fiir Technische Physik, P.O.B. 5640, D-7S00 Karlsruhe, F.R.G.

*Oxford-AIRCO Superconductors,.600 Milik Street, Carteret, N.J. 07008, U.S.A.

Résumé - Les critères d'élaboration pour des conducteurs de Nb

Sn prêrêagis et avec renforcement interne, ainsi que les propriétés supraconductrices à haut champ de conducteurs avec différents matériaux de renforcement sont présentés.

Abstract - Design criteria for a prereacted, internally strengthened Nb

Sn conductor and the superconducting properties in high fields of conductors with different reinforcing core materials are presented.

INTRODUCTION

A 10 T - 1.8 K superconducting solenoid with a clear bore of 390 mm is to be rated up in magnetic field by at least 2 Tesla by means of a Nb

Sn-insert coil. A reasonable operational current density of 6.5 • 10

A/cm

in the conductor leads to the following size of the insert coil: 300 mm i. dia., 380 mm o. dia. and a length of 400 mm. A pre- reacted Nb

Sn conductor must be used in the windings for forming cooling channels to warrant a sufficient electrical stability by LHe-ventilation. The Lorentz forces of

150N/mm~ in the coil require a mechanical reinforcement of the conductor. A compacted monolith configuration with a central steel core seemed to be a reasonable solution.

1),2)

THE CONDUCTOR CONFIGURATION

The geometric size of a prereacted Nb

Sn conductor is not only determined by the operational current, but the thickness of the conductor is determined by the strain occurring by bending during the winding of a coil. The Nb Sn conductor will be dam- aged irreversibly at strains exceeding 0.6 %, as shown by Kuckuck et al-" and Rupp^-*.

The tensionless state of a long length of such a conductor is a bent one, because the heat treatment has to be made on a spool. From this state, the conductor will be

straightened and rebent again and experiences strain and compression during the pro- cessing, so that the final geometric size is a compromise regarding all the steps of handling. Thus, for a final bending diameter of 300 mm and a heat treatment diameter of 600 mm, a conductor thickness of 3.5 mm seemed to be tolerable, in particular un-

der the aspect that the distance of the Nb

Sn fil- aments from the neutral axis is smaller than 1 .75 ram, which can be seen in Fig. 1 (a). The final strain £ between heat

Fig. 1:

The final size of the conductor (a);

the conductor in the first stage of compact- ion (b) .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984181

C1-400

JOURNAL

PHYSIQUE

treatment radius rl and coil radius r correspondsto the formula

= a[l/r - l/rl]

with the distance a between outermost2 f ilaments and the neutral axis. For the present conductor the expected strain value will be 0.46

Fig. 1 (a,b) show a cross section through the final compacted monolith of 3.5 x 5

nun2

and a slightly compacted version after the encapsulation into the outer copper

sleeve. The strands contain 64

copper and are protected by a Ta-diffusion barrier.

Because the strengthening material experiences also the heat treatment of the Nb,Sn, it must be a creep resistant material to keep sufficiently high yield strength. In the prototype conductors steel A 286, inconel, a molybdenum alloy, TZII, and from reason of comparison copper were used as core materials. They differ considerably in the thermal contraction coefficient a[OC-l] e.g. at room temperature 17.8 - ^for

A 236, 14.9 . for inconel, 4.9 - for TZPf and 16.7 - for copper. The steel A 206 was favoured, because the hardening heat treatment could be made in com- bination with the Nb,Sn heat treatment. The other core materials werc included in the investigations to study the iniluence of the different thermal contraction on the current density.

The composite conductor should fulfil the following requirements:

Cross section q

3.5 x 5 mm2

Critical current I,

¹⁷⁵⁰ A (0.5 ~ V / c m at 10 T and T

4.2 K) Yield strength 00.1

150 y / m 7

Cu/non-copper ratio Cu/bronze > 3 Twist in strand 50

Transposition in cable 100 mm

From empirical data of current density, a content of 20

Nb,Sn

bronze in the cross section should be sufficient. A subdivision of the remaining area into a cop- per and a steel part can only be made very crudely by simplifying assumptions. The mixing rule, assuming steel to behave elastically, copper and bronze being in the plastic regime and contributing to force carrying by GO

and 150 ~ / m r n ~ respec- tively, zives a steel fraction of 12

The composition of 20

Nb3Sn-bronze, 68

copper and 12 R core material could not be achieved in all the samples because all the core materials were not available in the right dimensions.

CRITICAL C U R E S T VALUES

3000 7 1- I

0 7 8 9 10 11

%[TI Fig. 2:

The critical currents I, versus magnetic field

of the Mb, Sn

conductor with different core materials.

The critical currents were measured in one lay- er "wind and react" coil sample over one turn at least, i.e. 29 cm,whereas the total number of turns was four. The I, values were defined for a voltage drop of 0.5 uV/cm and determined up to fields of 10.5 T. Because the bare con- ductor was well cooled by the liquid helium environment, the conductor was tested far into the current sharing regime where heat fluxes up to 7 . ^lo-' Watts/cm2 occurred.

Fig. 2 shows the critical current values for

the Nb,Sn conductors with the different streng-

thening cores. The conductor with the copper

core can serve as the basis for comparison,

because the composition of Nb,Sn, CuSn-bronze

and copper corresponds to the usual arrangement

in multifilamentary Mb,Sn conductors. Obvious-

ly the current densities decrease with increas-

ing thermal contraction coeff'cient of core

material as was e~pectcd.~).~f Also in the

slightly compacted state the compacted cable

showed the same reduction in lc as in case of

100

compaction comparing e.g. the A 286 core

conductor with the copper core conductor. The

current sharing regime in these Nb,Sn conduct-

ors is very much extended and covers between

a voltage drop of 0.5 uV/cm to 33 pV/cm an in-

crease of current of about 700 A. The copper

matrix contributes only about 70 A to the cur-

rent carrying capacity. Some typical data in

t h e c u r r e n t s h a r i n g regime a r e compiled i n Tab. I . The e x t e n d e d c u r r e n t s h a r i n g reg- ime i n d i c a t e s t h a t t h e Mb,Sn i n t h i s c o n d u c t o r c a n be o p e r a t e d s t e a d i l y beyond t h e c r i t i c a l c u r r e n t v a l u e i n a r e l a t i v e l y h i g h r e s i s t i v e s t a t e . The l o g a r i t h m i c i n c r e - ment a of t h e i n c r e a s e of r e s i s t i v i t y w i t h t r a n s p o r t c u r r e n t

po e x p [ a - ( I - I , ) ] ) h a s t h e same v a l u e 6.04 . ^[A-'1 f o r a l l t h e c o n f i g u r a t i o n s . I n comparing t h e c r i t i c a l c u r r e n t of s t r a i n i n f l u e n c e d c o n d u c c o r s i t might b e more c o n v e n i e n t t o u s e a n I, d e f i n i t i o n on t h e b a s e of t h e r e s i s t i v i t y of t h e s u p e r c o n d u c t o r i n t h e non-cop- p e r c r o s s s e c t i o n i n s t e a d of a v o l . t a g e / c m - d e f i n i t i o n , b e c a u s e t h a t i s l i k e w i s e c o r - r e l a t e d t o a n i n t e r n a l p h y s i c a l p r o p e r t y . Thus, a r e s i s t i v i t y v a l u e of p

1 0 - ~ ~ R c m o f t h e s u p e r c o n d u c t o r i n t h e b r o n z e c r o s s s e c t i o n , a v a l u e sometimes u s e d , would l e a v e unchanged t h e I, v a l u e i n t h e c a s e of t h e c o p p e r c o r e b u t d e c r e a s e t h e I, v a l u e from 275 A down t o 770 A i n c a s e of t h e

TXE CURXENT SHARIPTG STATE

DEPENDEPJCE OI.1 tlAGI\'ETIC FIELDS

PCS [Rcm]

4.66.10-lo 5 . 1 4 ' 1 0 - ~ ~ 5 . 7 6 . 1 0 - ' ~

7.0.10-~O

The h i g h f i e l d p a r t of t h e c o i l c o u l d be run i n t h e c u r r e n t s h a r i n g s t a t e i f t h e a v a i l a b l e c o o l i n g power would be s u f f i c i e n t . An e s t i m a t i o n of t h e l o s s e s t o b e ex- p e c t e d n e e d s t h e knowledge of c u r r e n t s h a r i n g b e h a v i o u r i n dependence on t h e m a g n e t i c f i e l d . 14easurements of t h e c u r r e n t s h a r i n g v o l t a g e a t c o n s t a n t c u r r e n t i n a d e c r e a s - i n g m a g n e t i c f i e l d g i v e i n a f i r s t a p p r o x i m a t i o n a n e x p o n e n t i a l r e l a t i o n s h i p f o r t h e r e s i s t i v i t y p

0, exp [-Y (B, - B) 1 w i t h a l o g a r i t h m i c decrement

1.98. From t h a t , a mean r e s i s t i v i t y f o r t h e t o t a l c o i l c a n be d e r i v e d t a k i n g i n t o a c c o u n t t h e k a b . I : The c r i t i c a l c u r r e n t

I,,

c u r r e n t Ics i n t h e c u r - r e n t s h a r i n g r e g i m e ,

t h e c u r r e n t i n t h e c o p p e r m a t r i x and t h e c o r r e s p o n d i n g r e s i s t i v i t y p i n t h e Nb,Sn/

b r o n z e c r o s s s e c t i o n a t 10.5 T.

f i e l d change o v e r t h e c o i l .

I,

0.5pV/

cm 1750 1500 1317 375 Core

Cu TZM I n c o n e l A 236

The l o s s e s of t h e i n s e r t c o i l mentioned b e f o r e would be 3.6 W a t t s i f i t o p e r a t e s a t a v o l t a g e of 0 . 5 pV/cm and a c u r r e n t l e v e l of 1350 A , i . e . a t c r i t i c a l c u r r e n t , i n t h e h i g h f i e l d w i n d i n g s . T h i s d e m o n s t r a t e s t h a t t h e c u r r e n t s h a r i n g regime c a n n o t be used i n a s t e a d y o p e r a t i o n mode.

TRIAL WINDINGS

About 5 m of d i f f e r e n t c o n d u c t o r samples were h e a t t r e a t e d on a 600

s p o o l a t 700° C f o r 120 h and from t h a t wound o n t o a sample c o i l o f 300

The f o u r w i n d i n g s were wound c i g h t t o t h e b o r e t u b e and i n s u l a t e d from t u r n t o t u r n by a cap- t o n f o i l . The o u t e r s i d e o i t h e c o n d u c t o r was w e t t e d by l i q u i d h e l i u m .

The i n v e s t i g a t i o n s were performed i n t h e 390

f o r e a t 4 . 2 K up t o

T. The L o r e n t z f o r c e s c a u s e t e n s i l e o r c o m p r e s s i v e f o r c e s on t h e c o n d u c t o r d e p e n d i n g on t h e d i r e c t i o n of sample c u r r e n t (a

= j

r ) . The com- p r e s s i v e f o r c e s i n p a r t i c u l a r a r e s u p p o r t e d by t h e b o r e t u b e s o t h a t t h e c o n d u c t o r h a s t o c a r r y no f o r c e s . F i r s t t h e c r i t i c a l c u r r e n t v a l u e s of t h e c o n d u c t o r s were de- t e r m i n e d i n t h e f o r c e f r e e mode and t h c n under t e n s i l e f o r c e s . P u r e b e n d i n g c a u s e d a r e d u c t i o n i n c r i t i c a l c u r r e n t between 10 and 15

compared t o v a l u e s g a i n e d from t h e

"wind and r e a c t " s a m p l e s . T h i s must be deduced from t h e t e n s i l e and c o m p r e s s i v e f o r - c e s o c c u r r i n g i n t h e c u r v e d c o n d u c t o r which do n o t compensate due t o t h e n o n - l i n e a r - i t y of t h e s t r e s s Even i n t w i s t e d and t r a n s p o s e d c o n d u c t o r s t h e r e i s no b a l a n c e by b y p a s s c u r r e n t s b e c a u s e of t h e r e s i s t i v i t y of t h e m a t r i x . I n any c a s e , bending w i l l l e a d t o a c e r t a i n r e d u c t i o n i n c r i t i c a l c u r r e n t .

The e f f e c t s of b e n d i n g and t e n s i l e l o a d on t h e c o n d u c t o r a r e g i v e n i n Tab. I1 f o r t h e i n c o n e l and TZM c o r e , where I,, I c b and Ic* a r e t h e c r i t i c a l c u r r e n t s i n t h e s h o r t sample, i n t h e c u r v e d c o n d u c t o r and u n d e r t e n s i l e l o a d , and

t h e t e n s i l e f o r c e a t

T and

ICU [A]

32.5 77.5 77.5 70.4 pc

[Rcm]

9 . 5 . 1 0 - ~ ' 1.11.10-'I 1 . 2 6 . 1 0 - ~ ~

1 . 9 . 1 0 - ~ '

I c s IA]

33pV/cm

2465

2233

2000

1542

C 1-402

JOURNAL DE PHYSIQUE

Fig. 3: Stress-strain characteristic of the

286 core conductor

0"

[ N / m 2 1

141 115

The Tab. I1 reveals that the core materials inconel and TZH have a different ef- fect on the current densities of Nb,Sn under force load. Under the bending tension both conductor configurations experience a reduction in current density, the TZM con- figuration a little bit more than the inconel version. Under tensile forces the cur- rent density of the inconel version increases whereas that of the TZM version decrea- ses. At the test the maximum load by running into the current sharing regime was 166

~ / m m ' with inconel and 137 N/m2with TZM. These maximum loads were cycled five-times

and by that the TZM version exhibited an aging effect in the sense that from the first run to the fifth the I,* value dropped from 1300

to 1675

with a tendency of approaching a constant value. The conductors were stressed up to the plastic regime and a remaining strain of about 0.2

could be measured by the change of conductor length. The behaviour of the TZM core conductor suggests a reduction of precompres- sion effect by the small. thermal contraction of molybdenum so that the original state is near the maximum value in the Ic/c characteristic and the tensile forces lead very soon into the decreasing branch, as was verified by measurements of Specking et a1.7;

A

rewinding of the 5 m length of the ilr

r-

cone1 core conductor showed an enhance-

ment of I,* up to 2133 A. That might be

300

an effect of the plastic deformation re-

icity at low temperature is negligible from a practical point of view.

I,* [A]

2050 1675

6-

IMPal

CONCLUSION Tab. 11:

The critical currents of different conductor configurations at differ- ent stages of load at

T.

-

The investigations have shown that strenghening considerably influences the critical current density of Nb3Sn conductors, due to its differential thermal contraction co- efficient. The results of the inconel respectively the A 286 core and of the TZH core suggest the possibility of compensating the thermal contraction effects by selected core materials. In the range of bending strain up to 0.6

the prereacted conductor can be wound and rewound without any damage. The final bending strain should be small to keep the loss in critical current in a tolerable limit of

Ic [A]

2117 2383 Core

Inconel TZtI

ducing the precompression.

stress-

- { strain characteristic of the A 2C6 core

REFERENCES

I) Spencer, C.R., Adam, E., Gregory, E., Adv. Cryog. Eng. 2, p. 815 (1982).

2) Spencer, C.R. et al., Proc. 8th Symp. Eng. Prob. Fusion Res. 1979, p. 265.

3) Kuckuck, H., Springer, E., Ziegler, G., Cryogenics 16, 350 (1976).

4) Rupp, G., J. Appl. Phys. 48, 3858 (1977).

5) Fliikiger, R., Drost, E., Goldacker, W., Specking, W., IEEE Trans. Mag., MAG-19, 1441 (1983).

6) Ekin, J.W., Cryogenics 3, 611 (1980).

7) Specking, W, Fliikiger, R., to be presented at MT-8, to be publ. in Journal de Phys.

Icb [A]

1267 2050

=J

-

100

0

: conductor is given in Fig. 3. The high

'

content of copper and copper-tin bronze

Compacted monolith

being in a weak state after the heat

3.5 x 5 mm

treatment leads to the very early indic-

A 286 core 12%

ation of plastic behaviour at c 2 0.1 Z.

At a virgin elongation of 0.3

the con-

~ w r n temperature

ductor obtained a remaining elongation

of 0.1 Z. The stress strain behaviour at room temperature can be considered

.L. -

as representative also for Lkle-temper-

ature because the enhancement in yield

a Iohl strength and average modulus of elast-