670 IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-17, NO. 1, JANUARY 1981

Nb3Sn CONDUCTORS FOR 1 2 TESLA TOROIDAL FIELD COILS*

M . S. Walker, D. A . Damiano, W . C . Hamilton, R . E . Wilcox, B . A . Z e i t l i n , and R . 'VI. Stuart , Intermagnetics General Corporation; a n d J . P. Heinrich a n d C . L . Linkinhoker, General Electric Company

SUMMARY Several advanced mult i f i lamentary NbgSn conductors

were examined for var ious modes of cooling for the 1 2 Tesla ETF appl ica t ion . Of these , a pancake-wound 15,000 Ampere f l a t c a b l e o f t r i p l e t s o f l a r g e s t r a n d s in a perforated stainless steel channel was selected as the preferred candidate. This cable will be cold- end cryostable in a pool-boiling environment with a maximum heat t ransfer from the unoccluded strand sur- face of 0.24 Watts/cm2. The cons t ruc t ion of the con- ductor and co i l i s r e l a t ive ly s imp le , o f f e r ing po ten - t i a l economies in qual i ty assurance and production c o s t s .

I . INTRODUCTION

approaches for a Tokamak Engineering Test Facility, the Department of Energy in 1979 funded scoping s tud ie s fo r the design of t o r o i d a l f i e l d c o i l s t o a general se t of spec i f ica t ions ' . The overall objec- t i ves , spec i f i ca t ions , and r e su l t s o f one of these s tudies ' , performed by t h e General E l e c t r i c Company and Intermagnetics General Corporation, i s presented i n a companion paper3: Key elements of the scoping study were (1) ident i f icat ion of conductor and winding arrangements suited to various cooling modes, and ( 2 ) a n eva lua t ion of the su i tab i l i ty o f the var ious cooling/conductor modes for ETF. This paper presents the a l te rna t ive concepts tha t were examined, elements of the p rocess tha t have led t o t he s e l ec t ion o f a pool-boiling approach, and the analyses and experimen- tation supporting the choice of the selected conductor concept and i t s d e s i g n .

In order to ident i fy viable candidate conductor

11. THE CONDUCTOR DESIGN P R O B L E M

c o i l winding has indicated t h a t subs t ruc ture for the transmission of radial and lateral compressive loads will be required2y3. Evaluation of various structural approaches has led t o the select ion of a pancake winding of conductor t h a t i s e i t h e r s t e e l - r e i n f o r c e d or co-wound w i t h s teel reinforcement, based upon consi- derations mainly of cost and of the effect iveness of the s t ructure . Evaluat ion of winding s t ra in for Nb3Sn, which i s considered necessary for operation a t t h e 12 Tesla f ie ld level specif ied and i n a pre-reacted form fo r p rac t i ca l co i l cons t ruc t ion , fu r the r guided the design o f the s teel substructure t o a pancake stacking of turns, where each turn s t r u c t u r a l l y resembles a wide f l a t s t e e l box enclosing the e lec t r ica l ly-ac t ive par t o f the conductor . This s t ruc- tural concept was selected mainly independently of the choice of cooling mode and detai led descr ipt ion of the conductor i t se l f .

The design problem for conductor , insulat ion and cooling from th i s s t ruc tu ra l s t a r t i ng po in t can be br ie f ly s ta ted as fo l lows: a r e l a t i v e l y high winding current densi ty , 1 ,700 Amp/cm2, must be achieved in a ful ly cryostable fashion even t h o u g h subs tan t ia l mechanical s t ruc ture i s inc luded i n the w i n d i n g space to t ransmit the very large s imultaneous s t resses re- s u l t i n g from the 1 2 T operat ion: 1600 psi average radial compressive load a t t h e n o s e , 3,700 psi peak average lateral compressive load, (7,300 psi for one- coil-down), and 0 .13% t ens i l e s t r a in . The challenge i s t o i d e n t i f y a conductor/cooling scheme w i t h s u f f i -

*Research supported by the Office of Fusion Energy

A consideration of loads and s t resses wi th in the

Department of Energy through Subcontract No. 22Y-Oj881C with the Oak Ridge National Laboratory operated by Union Carbide Corp. for the Department of Energy under Contract W-7405-ENG-26.

c i en t capab i l i t y t o meet these requirements, b u t with su f f i c i en t s imp l i c i ty t o assure a n economically fabri- cable design within the time-scale of ETF.

111. CONDUCTOR AND COOLING CONCEPTS

considered in this scoping study: (1) a atmosphere pool bo i l ing , ( 2 ) 1 atmosphere subcooled superfluid bath4, ( 3 ) supercrit ical heat-induced f low in confined spaces5, ( 4 ) supercr i t ical forced f low w i t h continuous heat exchange to the bath; (5) "convent ional" super- cr i t ical forced-f low . These cooling approaches were ranked in the o rder o f p reference l i s ted above a f t e r comparison on a general basis. One atmosphere pool boi l ing was ranked f i r s t because o f i t s s i m p i i c i t y and inherent low cost , providing t h a t s u f f i c i e n t h e a t t r a n s - f e r can be achieved to meet the winding current density requirements. Subcooled superfluid bath cooing has the advantage of very h i g h sur face hea t t ransfer , u p t o 1.9 Watts/cm2, b u t i t r e s u l t s i n decreased refrigera- t i on coe f f i c i en t of performance, particularly if A C l o s ses a r e a c r i t i ca l par t o f the overa l l ETF design. There i s a l s o added c o s t and complication for the heat exchanger t o subcool the bath. Conventional force flow was ranked l a s t because pumping losses can be high and the plumbing necessary to avoid an excessive tempera- t u r e r i s e due t o t h e p u m p i n g and/or relieve excessive pressure i s complicated and costly, al though the use of supercr i t ica l helium permits increased strand sub- d iv is ion and the creat ion of surface for high heat t r a n s f e r . A n advantage i s t h a t the conductor forms the inner wall of a dewar with the conductor becoming a wel l - insulated, sol idly enclosed uni t . Forced flow with continuous heat exchanged t o the bath was ranked fourth because pumping lo s ses , r ad ia t ion l o s ses , and AC losses can be removed continuously. However, the cryogenic design i s complex, requiring b o t h p1,umbing for forced flow and a surrounding dewar. Heat ex- changed, heat-induced flow in confined spaces was ranked t h i r d , ahead of other forced flow concepts, be- cause thermal disturbances supply the pumping power f o r heat exchange where i t i s needed fo r s t ab i l i t y , w i th the other advantages of (3) being present. Selective openings t o the b a t h can be used t o r e l e a s e t h e p r e s s u r e build-up on quench, as wel l .

Conductor Concepts. To properly assess the con- s t r a i n t s on conductor design implied by these cooling approaches, candidate conductor designs were examined. Some of these are presented: (1) those for pool boi l ing or subcooled superfluid cooling are shown in Figure 1, and ( 2 ) those for heat-induced or forced flow in Figure 2 . The order of presentat ion of the f igures roughly indicates an evolution of the designs t o the prime concepts shown a t t h e bot tom of the Figures. Note t h a t the computed s t a b i l i t y margin for the heat- induced flow version of the concept shown in Figure 2 was found t o compare favorably with t b a t of the Westinghouse LCP conductor a t 8 Tesla r 7 . Overall cur- ren t dens i t ies a re h igher t h a n those achieved with the pool -boiling concepts.

The pool boiling conductor shown in Figure Id was chosen a s t he prime candidate for ETF, and the heat- induced flow version of the conductor shown in Figure 2d was chosen as the best al ternate, based upon the r e su l t s o f a 1 imited t rade-off analysis . The pool- boiling conductor was selected mainly because i t can achieve the required winding current density and be-

ments, QA, r e l i a b i l i t y , and cos t weighed heavi ly in the cause of i t s s i m p l i c i t y , with manufacturing require-

a l l cur ren t dens i ty , the heat-induced flow concept and select ion process . For more s t r i n g e n t demands on over-

Cooling Approaches. Five cooling approaches were

Selection of a Preferred Conductor/Cooling Concept.

0018-9464/81/0100-0670$00.75 @ 1981 IEEE

671

f ind the Von Mises equ iva len t s t r e s s . The maximum channel web bending s t r e s s due to the Lorentz load appears i n the 12 Tesla grade, taking i n t o account the e f f e c t s of vent i la t ing ho les by properly reducing the bending section width. Potential buckling of the web due t o s ide loads was assessed u s i n g e las t ic buckl ing theory for a p l a t e clamped on two edges and f r e e on two edges. The ana lys i s was performed f o r both normal and 12-coil operation and f o r one coil a t zero current with 11 c o i l s a t f u l l c u r r e n t . Design l i m i t s were in- creased t o t h e y i e l d s t r e n g t h f o r t h e l a t t e r c a s e , which is a f au l t cond i t ion . The r e s u l t s o f t h e s t r e s s ana lys i s a r e summarized i n Table 2 . Note t h a t reason- able design margins were achieved with the assumptions noted.

and superfluid bath concepts could be prime choices , assuming continued experimental verification of the potential for these approaches.

IV. THE SELECTED CONDUCTOR Description of t h e Conductor/Cooling Concept and

Manufacturing Approach. For th i s l imi t ed s tudy , t he c o i l was a rb i t r a r i l v d iv ided i n to 2 Tesla sections. The six grades achieve the required overall average winding current density of 1,700 Amps/cmz. Figure Id and Table 1 descr ibe the 12 T and 2 T grades. The w i d t h o f t he channel sides increases as one moves from t h e high f i e l d t o t h e low f ie ld grades to take u p t h e increase i n accumulated radial Lorentz force load. The s t rands and t r i p l e t s f o r ' t h e low f ie ld g rade a re smaller and occupy a smaller volume so t h a t t h e con- ductor i s thinner and higher i n current densi ty while r e t a in ing t he sam'e overall conductor w i d t h f o r an even pancake winding. The smaller conductor cross section a t low f i e l d s i s permitted because of higher cri t ical cur ren t in the Nb3Sn and lower magnetoresistance i n t h e s t a b i l i z e r . The reduct ion in s tab i l izer c ross section impacts coil protection, however, and would have t o be eva lua ted fur ther for an optimized coil design. Strand insulat ion, i f required to achieve low l o s s e s , i s assumed t o be ava i lab le from developments on the LCP program.

channel are conventional processes. I t i s a n t i c i p a t e d , however, that the es tabl ishment of the cabl ing tech- nique will be a key element in the development of this conductor. To our knowledge, a lightly-compacted core- l e s s cab le o f this type has been manufactured only once (a cable of subcables made by IGC for the Oak Ridge National Laboratory'). The basic conductor con- c e p t i s amenable t o e i t h e r (1) fabricat ion of the entire conductor as an integral uni t us ing the re- en t r an t cha rac t e r i s t i c of the channel t o hold t h e cable , as shown in Figure I d , or ( 2 ) co-winding of the s t e e l channel and c a b l e o f t r i p l e t s . For the former approach, the cable wil l be annealed, compacted i n t o the s tee l channel , and reacted i n full grade lengths on a large diameter spool t h a t serves both as a re- action spool a n d shipping and winding spool. The channel thus protects the conductor during handling and coil winding. Type 316 L N s t a i n l e s s s t e e l was chosen f o r t h e channel material, based upon i t s strength, toughness and weldabi l i ty . G-10 i n t e r t u r n insu la t ion wi l l be co-wound w i t h the conductor , i n s t r i p s which l i e between the s t r i ngs of holes in the channel and along the outer edges o f the channel, leaving the shor t l a te ra l spaces between the holes uninsulated to a v o i d the need fo r ax i a l r eg i s t r a t ion o f i n su la t ion and channel.

The cab le o f t r i p l e t s compr i s ing t he e l ec t r i ca l ly - act 'ive part of the conductor i s an open, large surface area, porous s t ructure with re la t ively large channels f o r bubble clearing, even wi th the requis i te degree of precompaction before reaction for good mechanical sup- por t . The f r e e movement of helium and clearance of bubbles within the pie eliminates the need f o r any helium replenishment or bubble clearing from the sides of the pie . Relat ively inexpensive sol id sheets o f G-10 have accordingly been se l ec t ed a s t he i n t e rp i e insu la t ion mater ia l . The design will accept ei ther . mono1 ithic (internal bronze) or composites of separately fabricated elements, including internal or external- bronze, as shown i n Figure 3. The external bronze approach, in which t i n is diffused into copper and Nb s t rands, a l though not as ful ly-developed, offers the potential for lower conductor cost . The reference design summarized in Table 1 assumes an internal bronze conductor s t rand of the sor t developed a t IGC, w i t h s o l i d Nb f i laments reacted in a bronze matrix.

S t resses in the Channel Insu la t ion . In o r d e r t o assess design margins, stresses were c a l c u l a t e d a t a l l c r i t i c a l p o i n t s i n the channel and then combined t o

Tr ip le t ing and the ex t rus ion and punching of t h e

IC, S t r a i n , and Conditions for Reaction of the m. For full-scale production of internal bronze, used as a reference for the design, the 2.88 mm dia- meter strands of the high field grade conductor will contain 2.5 micron diameter filaments. Scaling from t h e z e r o - s t r a i n c r i t i c a l c u r r e n t d e n s i t y a t 1'0 Tesla , 4.2 K measured i n a sample of IGC conductor having 2 .5 micron diameter fialments and filament spacing equiva len t to t h a t of the present design, a Jc of 4.8 x l o 4 Amps/cm2 i s expected in the bronze and f i l a - ment a r e a a t 1 2 T . Since the conductor will be wound on both a s t r a igh t l eg o f t he D-shaped co i l and on a minimum bend rad ius , a d i f fe rence in bend s t r a ins o f from (Cable Thickness/R,) = 0.00% t o (Cable Thickness/ Rmi ) = -0.65% will be experienced by tha t pa r t o f t he cabre a t t h e conductor edge d u r i n g winding. Ideally, t o maximize the c r i t i ca l cu r ren t dens i ty ove ra l l , t he m i n i m u m and maximum s t r a i n would be b iased to s t raddle t h e s t r a i n a t which t h e peak J occurs in the Nb3Sn. The b i a s ing s t r a in has been degermined by assuming t h a t the maximum c r i t i ca l cu r ren t occu r s a t an applied s t ra in of 0 .18%,as observed for IGC internal bronze conductor with 0.87 micron filaments15. A Strain span from -0.15% t o +0.50% is therefore required. The 0.5% s t r a i n must be equal t o t h e sum of (1) 0.13% from the tensile loading of the conductor, ( 2 ) 0.055% f o r d i f f e r e n t i a l thermal contraction from 650 C t o 4 .2 C , and ( 3 ) a s t r a i n o f 0.315% associated with unwinding the conductor from the reaction spool (1.08 cm divided by the radius of the react ion spool) . The required radius of the react ion spool is thus 3 .43 meters . I f cmax i s d i f f e r e n t f o r 2 . 5 ~ d i a m e t e r f i l a m e n t s from t h a t assumed, then the spool radius will be changed. The design point for this ETF conductor i s thus Jc = 0.76 J, = 0.84 Jc (E=O) = 4.0 x lo4 Amps/cmZ; a s shown i n Figure 4, a c r i t i c a l c u r r e n t margin of 40%, or a temperature margin of 1.6 K .

The conductor and coi ls are expected to experience several hundred excitation cycles, each involving a 0 .13% decrease in t ens i le s t ra in , and a number of ther- mal cyc les to room temperature each involving an addi t ional 0.055% decrease i n t e n s i l e s t r a i n . No de- c rease i n c r i t i ca l cu r ren t r e su l t s i n 0 .87~ d i ame te r filament conductor from c y c l i c s t r a i n u p t o peak s t r a i n values which l i e below t h e r e v e r s i b l e s t r a i n l i m i t , ~i~~ = 0.85%, even for l o 5 or more cycles15. Assuming t h e same cyclic behavior for 2.51~ D f i l amen t s , fo r t he design maximum s t r a i n of 0.50% the conductor will be ope ra t ing s a fe ly a t l e s s t han 60% of i t s i r r e v e r s i b l e s t r a i n l i m i t .

loca l deformat ion of t r ip le t s d u r i n g operation i s a concern because the NbaSn may be damaged l o c a l l y . Al- though the conductor would continue to operate because i t i s cryostable , s ignif icant heat loads and voltage drops could develop. The minimum Lorentz force, on the 12 Tesla grade, i s approximately 180 k g per axial cm of conductor length, d r i v i n g the cable of the con- duc tor aga ins t the web of i t s channel housing. Although t h i s i s n o t a l a rge l oad , i t i s appl ied to s t rands which a r e mainly fully-annealed copper (anneal occurs during reaction of the Nb3Sn). Where t r i p l e t s o f t h e c a b l e

Experiment - T r i p l e t Bending under Load. Large

' 612

cross the web, they are well supported. As they cross over the open side of the channel, they are pressed aga ins t the oppos i te ly-cross ing t r ip le t s which l i e aga ins t t he web. Even though the cab le wi l l have been annealed and pre-compacted before reaction to a s su re t h a t t h e t r i p l e t s a r e deformed across each other a t crossing points t o provide greater bearing area for adequate support subsequent to reaction, because of the f luted nature of the t r iplets support on e v e r y t r i p l e t c ross ing po in t i s n o t assured. Support on a t l e a s t every-o ther t r ip le t c ross ing po in t i s expec ted , how- ever . To t e s t f o r p o s s i b l e t r i p l e t bending under load i n t h i s s i t u a t i o n , a t r i p l e t of annealed 0.288 cm D copper strands was loaded between supports with the equivalent of the 11 kgm per cm load expected from t h e Lorentz force. Plastic bending was observed a t 3 t o 4 cm support separation, a l a rger d i s tance t h a n the 2.5 cm dis tance for suppor t a t every-other t r iplet- upon-tr iplet crossing. Thus, support a t every-other t r i p l e t c ros s ing po in t should be adequate t o prevent t r i p l e t bending, and the pre-compacted cable should n o t deform s i g n i f i c a n t l y under Lorentz load.

Credible Events a n d S t a b i l i t y . An assessment was made of possible credible events which might d r ive t he superconductor normal. Frictional heating resulting from major forces occurs only on the ex te r ior sur faces of the s ta inless s teel channel , a poor thermal con- ductor . T h u s , the heat will be deposited mainly in the helium. The only s ignif icant heat ing t h a t the superconductor cable sees i s from movement or defor- mation within the channel. The most s i g n i f i c a n t possible event, sl ight deformation under Lorentz load, provides a thermal input of 57 mJ/cc of conductor s t r and , f a r l e s s t han t he minimum postulated thermal input of 100 mJ/cc f o r t h i s program. The 100 mJ/cc minimum per turba t ion tha t has been specif ied was u t i - l ized, nevertheless , for the analysis of recovery for this conductor.

each temperature for measurements that have been made on various surfaces and conductors i s shown in Figure 5. No d i s t i n c t i o n has been made between horizontal and ve r t i ca l o r i en ta t ions of the conductor, since b o t h o r i en ta t ions a r e assumed to achieve the conservative hea t t r ans fe r cha rac t e r i s t i c s shown. I t i s apparent from Figure 5 t h a t t h e 2 T grade will recover uncon- d i t i o n a l l y from normalizat ion, s ince the ent i re heat generat ion curve l ies below the heat transfer curve. The time for recovery i s 0.06 seconds, d u r i n g which 7.4% of the helium within the conductor channel i s boiled away. With t h i s small helium b o i l o f f , a maxi- mum o f 50% of the helium space i s f i l l e d w i t h g a s , leaving an adequate reservoir of l iqu id helium so t h a t no helium replenishment i s necessary. Bubble c lear ing channels are short and a1 mm in d iameter in ' th i s g rade , allowing the removal of bubbles generated by nuclear heating or AC l o s s e s .

Computer analysis shows the 12 T grade of the con- ductor, with a cr i t ical temperature of 9.4 K and a current-sharing temperature of 5.2 K , t o be s t a b l e o n a cold-end recovery basis. The superconducting regions propagate in from the ends of the normal region with a velocity of 122 cm/sec, a half- turn normal recovery time of 6 .3 seconds. During th i s t ime, 450% of the helium volume w i t h i n the conductor channel a t the cen- t e r of normalized length i s boiled away. Replenishment of t h i s helium volume during the 6.3 seconds should be easily achieved, since replenishment of helium on a s teady-s ta te bas i s was found t o occur in channels with fewer holes for a comparable 1 eve1 of heat t ransfer ' .

A composite curve of the lowes t hea t t ransfers a t

REFERENCES 1. W . A . Fietz, Proceedings of the Cryogenic

Engineering Conference, Madison, August 1979, t o be published.

2. Part 1, Final Report, 12 Tesla Toroidal Field Coils Engineering Test Facility (ETF), U n i o n Carbide Corp. Subcontract No. 22Y-07881C.

3. J . P . Heinr ich e t . a l . , "Scoping Study of a 12 T Toroidal Field Coil for the Fusion Engineering Test Faci l i ty ," This Proceedings.

Verdier, Advances in Cryogenic Engineering, 2 3 , p. 358, 1978. G . Claudet, C . Meuris, J . Parain, B . Turck, IEEE Trans. E, P . 340, 1979.

5. J . R . Mil ler , L . Dresner, and J . W . Lue, Proc. Eng. Problems of Fusion Research, Knoxville, Tennessee, P . 1282 (1977), L . Dresner, "Heating-Induced Flows in Cable-in-Conduit Conductors," t o be published in the proceedings of the 1979 Cryogenic Engineering Conference, Madison, Wisconsin, August 1979. Lue, Mil ler a n d Dresner, "Stability of Cable-in-Conduit Conductors," t o be published in the proceedings of the 1979 Cryogenic Engineering Conference, Madison, Wisconsin, August 1979.

6. L . Dresner, "Stabil i ty of Cable-in-Conduit Force- Cooled Conductors: Elementary Theory," Oak Ridge National Laboratory Publication ORNL/TM-6657.

7. L . Dresner, "Heating-Induced Flows in Cable-in- Conduit Conductors," t o be published in the pro- ceedings of the 1979 Cryogenic Engineering Con- ference, Madison, Wisconsin, August 1979. Lue, Miller and Dresner , "S tab i l i ty o f Cable-in-Conduit Conductors," t o be published in the proceedings of t h e 1979 Cryogenic Engineering Conference, Madison, Wisconsin, Augus t 1979.

8. R . E . Schwall, S . S . Shen, 3. W . Lue, J . R. Miller , and H . T . Yeh, Advances in Cryogenics, 4, p . 427, 1979.

9. R . E . Schwall, F. J . Reles , and J . P . Heinrich, IGC Report #779-1, Boiling Helium Heat Transfer in Large Cabled Conductor Channels, t o be published in Advances in Cryogenics, Volume 25.

Problems of Fusion Research, Knoxville, Tennessee, p . 920 (1977).

11. R . Quay, e t . a l . , P roc . of the Seventh Symposium on Engineering Problems of Fusion Research, Knoxville, Tennessee, October 1977, p . 926.

4. G . Eon Mardion, G . Claudet, P . Seyfer t , and J .

1 0 . D. S . Hackley and J . P . Waszczak, Proc. Eng .

12. E . S e i b t , IEEE Trans., E, p . 804 (1979). 13. P . A . Sanger, E . Adam, E . Gregory, W . Marancik,

E . Mayer, G . Rothschild, and M . Young, IEEE Trans. MAG15, p. 789 (1979).

a n d M . S . Wal'ker, M . J . Cutro, B . A . Z e i t l i n , G . M. Ozeryansky, R . E . Schwall, C . E . Oberly, J . C . Ho, J . A . Woollam, IEEE Trans. , w, p . 80 (1979).

-- 15. J . W. Ekin, IEEE Trans. m, . p . 197 (1979)

TABLE 1 - REFERENCE DESCRIPTION OF THE SELECTED G E / I G C ETF CONDUCTOR

Grade

D e s c r i p t i o n o f Channel O p e r a t i n g C u r r e n t

Wid th H e i g h t S ide Th ickness P l a t e T h i c k n e s s

D e s c r i p t i o n of Cab le H o l e D and A x i a l S p a c i n g

No. o f T r i p l e t s No o f S t r a n d s S t rand D iamete r % He l ium i n Channel

NbgSn Type (For Reference On ly ) 7 I " \

M:ximum. M i n . S t r a i n I \ D l

Cu S t a b i l i z e r , A n n e a l e d RRR

Area/Cab le p ( B ) , I r r a d i a t e d

1, (4.2 K, E = 0) I (4.2 K, C = .I:- Grade F i Y l y Normal Heat Transfer

Per Unoccluded Surface Per S t rand Sur face

H i q h F i e l d ( 1 2 T)

15,000 Amperes

6.38 cm 1.66 cm 0.58 cm 0.59 cm 1.06 cm, 3.18 cm

48 1 6

44% 0.288 cm

9 . 4 K I n t e r n a l B r o n z e

4 . 8 x IO4 Amps 0.50%, -0.15

4.0 x l o 4 Amps2 0.525 cm2

150 8 x 10-8 Rcm 2.60 cm2 25,200 Amperes 21,000 Amperes 1,335 Amps/cmE

0.24 Watts/cm2 0.16 Watts/cm2

Low F i e l d (2 T )

15,000 Amperes

6.38 cm 0.83 cm 1.04 cm 0.21 cm 1.06 cm, 3.18 cm

23 69

44% 0.165 cm

15.3 K I n t e r n a l B r o n z e

+0.37%, -0.00%

61.5 x Ig5 Amp/cm2 0.034 cm

150 2.2 x 10-8 ncm 1.44 cm2 25.200 Amoeres 21;OOG Amperes 2,233 Amps/cmZ

0.18 Watts/cm2 0.12 Watts/cm2

673

a ) va r i a t ion o f GD L C P Concept(")

[b) Variat ion of GE LCP Concept''')

D. Orientat ion poor f o r NbgSn s t r a i n D. I n t r o d u c t i o n o f s t e e l d i f f i c u l t

D. Needed hea t t ransfer > 0.4 W/c& D. Fab. & handl ing a f te r reac t ion

r eac t ion

:) var i a t ion o f G E LCP & KFK Concepts(l2)

0. Hole r e g i s t r a t i o n

(d) Selected Concept for ETF (Pool Boiling) NbgSn

Grade 2 T

Extruded Steel Channels

A . No hole regis t ra t ion requirgd A . Large heat transfer surface A . Large spaces for bubble clearin5 A . Inexpensive fabr icat ion A . Low AC l o s ses A . No-solder , low Nb3Sn s t r a i n 0. Diffusion barr ier needed f o r Sn

i g . 1 Conductor concepts examined f o r pool 3il ing or subcooled superfluid cooling modes.

L

a) Variation of Westin house LCP Concept(13') A . Large heat transfler surface A . Low AC lo s ses 0 . Bend s t r a i n on Nb3Sn may be h i g h D. Support plates expensive

T u b

[A A . D i f fus ion -ba r r i e r fo r Sn not needed D . Soldering & handing a f t e r r e a c t i o n

0. Not t e s t ed and may not gain from HIF for fab .

(c) Variation of Karlsruhe (KFK) LCTConcepdli A . Wide f l a t conduc to r fo r low Nb3Sn

A . Low AC l o s ses D. Limi ted sur face for hea t t ransfer

(d) Best Al ternate For ETF [Heat Induced flow

s t r a i n

,Stainless Steel r C u o r S tee l

12 T Grade

A . No so lde r , low NbgSn s t r a i n A . Large surface heat t ransfer A. Low A C l o s ses , Fab. cos t reasonable D. Diffusion barr ier needed f o r Sn _ _ _ - _ - - - - - -

F i g . 2 Conductor concepts examined f o r supercr i t ical forced f low or heat induced flow cooling modes.

Notes fo r F igs . 1 & 2 :

A , Advantage D, Disadvantage

O e 2 L Dat; a t , 1 2 :, 4.: K for 1;C 1;t. , 1 Bronze with 0 . 8 7 ~ D F i l . ( J ack Ekin)(I5)

0

-1.0 - .8 -.6 - .4 - .2 0 .2 .4 .6 .8 E , $

Figure 4 . Dependence o f the Nb3Sn Cri t ical Current Density on S t r a i n a t 1 2 T, 4 .2 K.

4

(a) Monolithic Strands (Internal Bronze)t - Thin Inorganic

Insulation Per LCP

DFHC Copper

Ta Diffusion

Bronze and Nb3Sn

Bar r i e r

0.288"cm +/

(b) Sintered Composite of Substrands

T h i n Inorganic Insu la t ion Per LCP

OFHC Copper

External Bronzet$nd Nb3Sn Substrandti ' or Internal Bronze and NbgSn Substrand

3 g . 3 Generic candidate multif i lamentary ib3Sn t r i p l e t s . *For 12 T grade. ?Or w i t h Cu :enter , bronze outs ide for each strand or w i t h :wo C u s t r ands and one superconductor s t rand. -tSn on substrand surface before s t rand fabr i - : a t ion d i f fuses in to Cu and Nb dur ing reac t ion ) f cab le .

1 TABLE 2 - STRESSES IN THE CHANNEL AND INSULATION

MAXIMUM [ SIDE ,"$;,":SSIVE EQUIVALENT STRESS O N ELASTIC BUCKLING STRESS IN INSULATION [ STRESS FOR

94.0 50.0 115

MAXIMUM VOR MISES COMPRESSIVE _ _ CONDUCTOR

GRADE THE CHANNEL ( k s i ) 316LN STAINLESS STEEL

Design Limit

Established BY 213 Of 140,000 psi % of 100,000 Assumed Buckling Limit y i e l d f o r 316LN p s i i t S t a in l e s s S t ee l*

91 .8 50.0 1/5.1

H i g h Field A t Bore

A t Grade Division 87.6

66 .O 0 1/37

49.3 1/37

'Assumes tha t s t ee l w i l l r e t a in p rope r t i e s du r ing t he r eac t ion of the Nb3Sn or t ha t t he cab le w i l l be co-wound w i t h the channel a f te r reac t ion .

ttAssumes (1 ) 100,000 Psi tolerance achieved by p iece se le t ion on G-10 and

t i m e scale of ETF. ( 2 ) development of t o l e rance of o rgan ic i n su la t ion t o lo5 rods i n t h e

I

0 ' 4 7 - 1 Selected Conservative Composite Heat

0 .3

0 .2 cu E V --. 3 0.1 .e, a

0

Transfer Curve - Excess Heating

.// -

2 4 6 8 10 12 AT, K

Figure 5 . Heat Transfer Curve a n d Heat Generation Curves for t h e 2 T and 12 T Grades