A COMPACT 5 kN-TEST FACILITY FOR SUPERCONDUCTING CONDUCTORS CARRYING UP TO 1.5 kA IN MAGNETIC FIELDS UP TO 14 T

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A COMPACT 5 kN-TEST FACILITY FOR

SUPERCONDUCTING CONDUCTORS CARRYING UP TO 1.5 kA IN MAGNETIC FIELDS UP TO 14 T

W. Specking, R. Flükiger

To cite this version:

W. Specking, R. Flükiger. A COMPACT 5 kN-TEST FACILITY FOR SUPERCONDUCTING CON-

DUCTORS CARRYING UP TO 1.5 kA IN MAGNETIC FIELDS UP TO 14 T. Journal de Physique

Colloques, 1984, 45 (C1), pp.C1-79-C1-82. �10.1051/jphyscol:1984117�. �jpa-00223644�

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A C O M P A C T 5 k N - T E S T FACILITY FOR S U P E R C O N D U C T I N G CONDUCTORS CARRYING UP T O 1,5 k A IN MAGNETIC FIELDS UP TO 14 T

W. Specking and R. F l t i k i g e r

Kernforsohungszentrum Karlsruhe, Institut far Technisehe Fhysik, P.O.B. 3640, V-7S00 Karlsruhe, F.R.G.

Résutnê - Un appareil permettant d'étudier l a v a r i a t i o n du courant c r i t i q u e le en fonction de l a tension uniaxiale (jusqu'à 5 kN) sur des conducteurs â des courants _< 1.5 kA et des champs magnétiques atteignant 14 T a été c o n s t r u i t . Quelques résultats actuels sur des conducteurs de Nb3Sn avec renforcement i n t é r i e u r son présentés.

A b s t r a c t - The v a r i a t i o n of t h e c r i t i c a l c u r r e n t , I

c

, i n dependence on t h e a p p l i e d s t r a i n , e , i s i m p o r t a n t f o r c h a r a c t e r i z i n g Nb3Sn s u p e r c o n d u c t i n g w i r e s . An apparatus f o r s t u d y i n g t h e s t r a i n s e n s i t i v i t y of I

c

up t o a magne- t i c f i e l d of 14 T under a mechanical load up t o 5 kN and an e l e c t r i c c u r r e n t up t o 1.5 kA i s d e s c r i b e d . Some actual r e s u l t s f o r i n t e r n a l l y r e i n f o r c e d I ^ S n conductors are p r e s e n t e d .

I - INTRODUCTION

For applications of Nb3Sn superconductors in large magnets the knowledge of critical current, I

c

, as a function of magnetic field, B, and applied strain, e, is essential because of the high Lorentz forces and the special characteristic of Nb3Sn under strain. In general, I

c

exhibits a maximum as a function of e / I , 2/. The maximum of the critical current occurs at a strain value, e

m

, which varies between zero and about 1 %, depending on the configuration and composition of the conductor /3 - 5/.

The ratio I

C o

/ I

C m

, where I

C o

represents the critical current at e = 0, lies between 0.1 and close to 1 and depends on both magnetic field and the conductor composition.

This behavior reflects the precompression on the Nb3Sn filaments by the matrix ma- terial caused by the difference in both thermal expansion coefficient, a , and Young's modulus of the various components during cooling from reaction to helium temperature. As a consequence, stress-induced modification of the crista! structure are observed /6/, which are finally responsible for the correlation of I

c

vs. s.

The aim of this paper is to present an apparatus for testing superconducting wires up to a load of 5 kN, a current of 1.5 kA in a magnetic field up to 14 T.

I I - APPARATUS

The main components of t h e t e s t f a c i l i t y are t h e c r y o s t a t , t h e t e n s i l e machine, the s u p e r c o n d u c t i n g magnet and t h e t e s t r i g ( F i g . 1 ) . The c r y o s t a t (Messer Griesheim) i s a normal LHe-bath c r y o s t a t w i t h a L H e - c o n t a i n e r o f 560 mm I. D. and a l e n g t h of 1450 mm and i s designed f o r two bar o v e r p r e s s u r e . The s p i n d l e d r i v e t e n s i l e machine ( S c h e n k - T r e b e l ) i s c o n s t r u c t e d f o r 10 kN and i s w o r k i n g i n load and s t r a i n c o n t r o l led mode. The Nb3Sn tape wound s u p e r c o n d u c t i n g magnet ( I n t e r m a g n e t i c General C o r p . ) i s of a s p l i t c o i l t y p e , which a l l o w s v a r i a b l e gaps of 0 , 10, 2 0 , and 30 mm. The main data f o r t h e 0 and 30 mm gap p o s i t i o n are shown i n Table 1.

The magnet i s mounted w i t h i t s a x i s i n a h o r i z o n t a l p o s i t i o n , t o a l l o w f o r v e r t i c a l access o f the t e s t r i g .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984117

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Cl-80 JOURNAL DE PHYSIQUE

Table 1 - Main data o f superconducting s p l i t c o i l magnet f o r 0 and 30 mm gap u o s i t i o n .

Maximum Central F i e l d a t 4.2 K ( T ) Maximum Current a t 4.2 K (A) Homogeneity a t 50 mm DSV (%)

Magnetic Energy (MJ) Clear Bore Diameter (mm) Length (mm)

Outer Diameter (mm)

Change Rate up t o blax. F i e l d :

- V i r g i n Run (Min.)

- Subsequent Run (Min.)

gap 0 mm gap 30 mm

As the c e n t r a l f i e l d o f a s p l i t c o i l decreases w i t h i n c r e a s i n g qap, the smallest gap o f 10 mm i s used f o r t h e 5 kN t e s t r i g , which allows a c e n t r a l f i e l d up t o 14.2 T.

The 5 kN t e s t r i g (Kernforschungszentrum Karlsruhe) provides the sample w i t h mecha- n i c a l l o a d and e l e c t r i c c u r r e n t from o u t s i d e o f t h e c r y o s t a t . I t c o n s i s t s mainly o f a tube, 0. D.: 70 mm, w i t h t h e p u l l r o d f o r 5 kN and t h e vapor cooled c u r r e n t leads f o r 1.5 kA i n s i d e , having a f l a n g e a t t h e t o p f o r s u p p o r t i n g the t e n s i l e machine and a "sword" ( F i g . 2) housing t h e sample a t t h e lower p a r t . The cross s e c t i o n o f the sword i s given by the dimensions o f t h e magnet gap (10 mm

X

70 mm). I n t h i s sec- t i o n a bothway l e a d f o r l o a d and c u r r e n t , d e t e c t i o n l i n e s and a s t r a i n measurement have t o be placed. The end o f the p u l l r o d t r a n s f e r s i n t o a movable p u l l frame, hous-

Tensile Mach~ne

PulI Rod

Test Rig

Cryostat

SC-Magnet

Fig. 1 - Schematic arrangement o f aouaratus

Pull Rod

Movable Pull Frame

SC Current Lead

\

F i g . 2 - "Sword" o f 5 kN t e s t r i g

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i t s e l f .

The current leads inside the sword are made from Nb Sn superconductor and copper braid. The ohmic losses i n t h i s area a t 1.5 kA and

?l

T amounts t o 2 Watt, of which 70 % are used f o r the contacts.

The length of the sample measures 200 mm, a t which 50 mm of each end i s soldered in a contact by a s p e m i l v e r solder (Fontargen, No. A 624). The f r e e length of 100 mm i s supported a t one side f o r balancing the Lorentz force, which reaches up t o 2

kN

i n the worst case. The contacts provide the sample as well with mechanical as with e l e c t r i c a l load. They are made from nonmagnetic s t a i n l e s s s t e e l and supplied with an i n l e t of copper around the sample. They can be removed f r o r the load frame

f o r soldering the sample.

The s t r a i n measurement of t h e sample occurs a t low temperature by means of a capa- c i t i v e probe. The probe and a parallel-sided metallic counter p l a t e form a plate capacitor, where the p1 a t e distance,

S ,

i s proportional t o i t s reactance (Eichhorn +

Hausmann, Fig. 2 ) . The advantages of a capacitive s t r a i n measurement are the inde- pendence of magnetic f i e l d , the negl i gible influence of the d i e l e c t r i c he1 i urn in com- parison t o a i r and the l i n e a r c h a r a c t e r i s t i c . A disadvantage i s the r e l a t i v e large dimension: For supplying the sample with 3 % s t r a i n (As = 3 mm) an outer diameter of the capacitive probe of 20 mm i s required. Thus the probe cannot be placed inside the 10 mm gap of the magnet but in the lower p a r t of the tube, and the counter p l a t e i s connected with a t h i n , powerless pipe t o the movable pull frame. This displace- ment, As, a t the capacitive probe, which i s proportional t o

E

of the sample, has t o be corrected. This correction considers a certain shear s t r a i n inside the solder connection, a deformation of the sample i t s e l f inside the contacts and s e t t l i n g ef- f e c t s of the contacts. Simultaneous s t r a i n measurements by s t r a i n gauges f o r a cer- tain s i z e of sample are in preparation.

I11 - MEASUREMENTS

After mounting the sample and cool i ng t h e magnet t o LHe-temperature the t e s t r i g can be inserted i n t o magnet gap. Then the t e n s i l e machine i s moved upon the top flange of the r i g . The c r i t i c a l current, I C Y i s measured under constant

E

and B.

Fig. 3 shows the importance of t h i s investigations a t three d i f f e r e n t l y , i n t e r n a l l y reinforced Nb3Sn conductors (Airco): Heat treatment: 720

O C

- 120 hrs i n argon, di- mensions: 3.5 mm

^X

5.0 mm. The composition i s shown i n Table 2.

Table 2 - Composition of d i f f e r e n t l y reinforced conductors ('TZM: 0.5 % Ti ,

0.07 % Zr, 0.05 % C , balance MO)

The amount of Ta, used as diffusion b a r r i e r , i s negligible in t h i s context. The d i f - ferent reinforcement materials, which should protect the Nb3Sn against mechanical overloading, influence the Ic-E-characterisitc d r a s t i c a l l y . The prestress on Nb3Sn f o r samples SS and Inconel i s much higher than f o r the Cu-sample.(The Cu-sample was measured a t

E =

O only.) Since both materials, s t a i n l e s s s t e e l and inconel, possess

Bronze + NbgSn Vol. %

20 20 2 0 20 Sample

SS Inconel TZM Cu

Reinforcement Copper

Vol. % 68.0 67.5 75.8 80.0

Materi a1 Stainless Steel

Inconel TZF?')

-

Vol. %

12 .O

3.5 4.2 -

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Cl-82

JOURNAL

DE PHYSIQUE

h i g h y i e l d s t r e n g t h s they remain e l a s t i c i n s p i t e the h i g h i n t e r n a l stresses i n t h e conductor, produced by the d i f f e r e n t thermal expansion between r e a c t i o n and LHe- temperature. The stresses due t o s t a i n l e s s s t e e l o r i n c o n e l are a d d i t i o n a l t o those o f t h e remaining m a t r i x (copper and bronze), thus r e s u l t i n g i n a h i g h e r degradation o f Ic a t

^E

= 0. ICO/ICm amounts t o 47 % f o r t h e SS and 70 % f o r t h e Inconel con- ductor, t h e c o r r e s p o n d ~ n g values o f

E,

being 0.74 and 0.53 %, r e s p e c t i v e l y . The reason f o r t h e s m a l l e r i n f l u e n c e o f i n c o n e l w i t h respect t o s t a i n l e s s s t e e l i s i t s lower volume f r a c t i o n and t h e s l i g h t l y lower a-value. The thermal c o n t r a c t i o n o f TZM (molybdenum) i s t h r e e times s m a l l e r than t h a t o f copper o r bronze. Therefore TZM i s b u i l d i n g up a counterforce a g a i n s t copper and bronze d u r i n g c o o l i n g down.

That would mean f o r t h e TZ??-sample, t h a t the i n t e r n a l stresses produced by copper- bronze and TZM reinforcement are almost i n balance, an a d d i t i o n a l e x t e r n a l s t r e s s l e a d i n g t o a degradation o f I , d i r e c t l y . F u r t h e r i n v e s t i g a t i o n s a t tnese conductors are a l s o made a t o u r i n s t i t u t e /7/.

F i g . 3

I, vs.

E

f o r d i f f e r e n t l y r e i n - f o r c e d Nb3Sn conductors ( A i r c o ) a t B = 11.3 T.

The autors l i k e t o thank K. H. R e i c h l i n g f o r assistance a t design and c o n s t r u c t i o n o f t h e t e s t r i g and s u p p l y i n g t h e measurements and P. Turowski f o r p r o v i d i n g the Ai,rco conductors.

REFERENCES

/l/ G. Rupp, IEEE Trans. Mag. MAG-17, 1099 (1981).

121 T. Luhman , Adv. Cryog. Eng. E T . 28, 639 (1982).

/3/ R. F l u k i g e r , E. Drost, W. ~ o l d a c k e x and W. Specking, IEEE Trans. Mag.

MAG-19, 1441 (1982).

/4/ J. WTEkin, H. Sekine, and K. Tachikawa, J . Appl. Phys. 52, 6252 (1981).

/5/ R. F l u k i g e r , W. Goldacker, W. Specking, L. P i n t s c h o v i u s , T . M i l l e r , and J. W. Ekin, Proc. ICIlC 9, Kobe (Japan), 11-14 Way 1982, p. 19.