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PROGRESS TOWARD 10 TESLA ACCELERATOR DIPOLES
W. Hassenzahl, G. Gilbert, C. Taylor, R. Meuser
To cite this version:
W. Hassenzahl, G. Gilbert, C. Taylor, R. Meuser. PROGRESS TOWARD 10 TESLA AC- CELERATOR DIPOLES. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-271-C1-278.
�10.1051/jphyscol:1984155�. �jpa-00223711�
JOURNAL DE PHYSIQUE
Colloque C l , supplement au n o 1, Tome 45, janvier 1984 page Cl-271
W. Hassenzahl, G. Gilbert, C. Taylor and R. Meuser
Accelerator and Fusion Research Division, Lawrence Berkeley Laboratory, University of CaZifornia, Berkeley, California 94720, U.S.A.
RQsumQ
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Un champ central de 9T a 6td obtenu dans un dipole refroidi 1 1,8K w e 1'hQlium I1 1 pression atmosphQrique. Trois dipoles diffgrents en Nb Ti sans fer ont atteint respectivement des champs centraux de 8, 8,6 et 9,1 T correspondant aux caract6ristiques Qchantillons courts de leurs con- ducteurs 1 1,8K. Dans l'hdlium I 1 4,3K, les champs centraux sont reduits de 1,5 1 2T.Des aimants de 10 teslas ont Qt6 conQus pour utiliser soit du Nb Ti 1 1,8K soit du Nb3 Sn 1 4,2K. 11s ont de trPs petites ouvertures (40 h 45 rum) de
trSs grandes densites de courant dans le supraconducteur (plus de 1000 ~ / m m ~ ) et un tr$s faible rapport Cu/SC ( N 1). Des aimants 1 couches et 1 blocs ont QtQ dQveloppQs. 11s utilisent un cable du type Rutherford.
Des cycles de champ de 0 2 6T ont dt.6 realis& ldes vitesses allant jusqu'1 1T/s. Les pertes 1 1T/s sont de 36W/m. A une vitesse plus reprzsentative de 0,2T/s elles tombent P 2W/m.
On discutera Qgalement de notre programme d'utilisation du Nbg Sn et du Nb Ti pour les aimants d'acc6lQrateurs 1 10T.
Abstract
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A 9.1 T central f i e l d has been achieved in a Nb Ti dipole oper- ating in pressurized helium I1 a t 1.8 K. Three d i f f e r e n t Nb Ti dipoles, without iron yokes, have achieved central f i e l d s of 8.0, 8.6, and 9.1 T--
a l l short sample performance f o r the conductors a t 1.8 K. In helium I , a t 4.3 K, t h e maximum central f i e l d s a r e from 1.5 t o 2.0 T lower.
Ten-tesla magnets have been designed f o r both Nb Ti operating a t 1.8 K and Nb3Sn operating a t 4.2 K. They a r e based on a very small beam aperture, (40 t o 45 mm), very high current density in the superconductors (over 1000 ~ l m m z ) , and a very low r a t i o of s t a b i l i z i n g copper t o superconductor (about 1 ) . Both layer and block designs have been developed t h a t u t i l i z e Rutherford Cable.
Magnet cycling from 0 t o 6 T has been carried out f o r f i e l d change r a t e up t o 1 Tls; the c y c l i c heating a t 1 Tls i s 36 W per meter. A t a more repre- s e n t a t i v e r a t e of 0.2 T/s t h e heating r a t e i s only 2 Wlm.
Progress in our program t o use Nb3Sn and NbTi superconductor, i n 10 T accelerator magnets will also be discussed.
INTRODUCTION
Existing high-energy proton accelerators1 have reached a s i z e t h a t appears t o be a l i m i t of machines using conventional magnets, and the f i r s t accelerator using superconducting magnets, t h e Fermilab Energy ~ o u b l e r 2 , i s j u s t beginning t o operate. Because superconducting magnets allow both s i z e and operating costs t o
su his
work was supported by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, High Energy Physics Division, U.S. Department of Energy, under Contract No. DE-AC03-76SF00098Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984155
C1-272 JOURNAL DE PHYSIQUE
be reduced t h e y appear t o be t h e c l e a r choice f o r f u t u r e high-energy synchro- t r o n s 3 ~ 4 , unless some o t h e r a c c e l e r a t i o n technology i s developed.
The optimum f i e l d and t h e bore o f t h e superconducting magnets destined f o r f u t u r e machines a r e n o t c e r t a i n . The choice o f these parameters w i l l be based on trade- o f f s among many f a c t o r s . Lower f i e l d s mean 1 arger accelerators, which increases t h e c o s t o f t u n n e l i n g and conventional services. The volume o f superconductor increases f a s t e r than t h e design f i e l d , however, and a t v e r y h i g h f i e l d , greater than about 11 T, t h e c u r r e n t c a r r y i n g c a p a c i t i e s o f conductors a v a i l a b l e a t present become t o o small f o r consideration. Synchrotron r a d i a t i o n increases as t h e l o c a l f i e l d t o t h e f o u r t h power and f o r a 20 TeV accelerator, may l i m i t t h e f i e l d t o about 10 T because o f t h e increased r e f r i g e r a t i o n load. Though the economic o p t i - mum may be somewhat lower, a l i k e l y upper l i m i t imposed b y these c o n s i d e r a t i o n i s
about 10 T which was s e t as an u l t i m a t e design goal f o r t h e magnets we a r e develop- i n g a t t h e Lawrence Berkeley Laboratory.
The design bore o f an a c c e l e r a t o r i s determined by cost, f i e l d q u a l i t y , alignment accuracy, and t h e ease w i t h which t h e beam can be steered. Recent experience a t Fermilab i n d i c a t e s t h a t a 75 mm i n n e r winding diameter i s s a t i s f a c t o r y f o r synchro- t r o n operation. We have used 58.5 mm f o r our r e c e n t magnet development program and see no fundamental l i m i t a t i o n down t o about 40 mm. Special c o n s i d e r a t i o n must be given t o t h e ends o f small magnets however, due t o d i f f i c u l t y i n bending conductors around t h e small r a d i u s a t t h e pole.
CONDUCTOR
Two commercial superconductors: an a l l o y o f niobium and t i t a n i u m (Nb-Ti), and a compound o f niobium and t i n (Nb3Sn), can be used i n t h e f i e l d range near 10 T f o r a c c e l e r a t o r dipoles. The approximate f i e l d l i m i t s f o r these m a t e r i a l s are shown i n Table I , and some c r i t i c a l c u r r e n t d e n s i t i e s a r e given i n Fig. 1.
Table I
Maximum Design F i e l d f o r Superconducting Accelerator Dipoles
M a t e r i a1 Temperature Maximum F i e l d
N b T i Nb-Ti NbjSn
The conductor used i n the Nb-Ti magnets constructed a t LBL has had a r e l a t i v e l y low copper t o supercondcutor r a t i o , between 0.86 and 1.5. By reducing t h i s r a t i o as the f i e l d increases, t h e c u r r e n t d e n s i t y i n t h e windings can be maintained a t a high value, which keeps t h e t o t a l superconductor requirement down. The c u r r e n t d e n s i t y i n t h e s t a b i l i z i n g copper, which a f f e c t s s t a b i l i t y and p r o t e c t i o n , changes very 1 i t t l e . A copper-to-superconductor r a t i o o f about 1.0 has been selected f o r our f u t u r e developmental magnets.
Recently a m u l t i f i l a r n e n t a r NbjSn conductor having a h i g h c r i t i c a l c u r r e n t den- s i t y has been d e v e ~ o p e d ~ ~ ~ t h a t uses a t i n r i c h region, and does n o t have a bronze m a t r i x i n i t i a l l y . This avoids the c o s t l y annealing t h a t i s c h a r a c t e r i s t i c o f bronze process Nb3Sn. The c h a r a c t e r i s t i c s o f m a t e r i a l manufactured b y IGC and r e c e n t l y t e s t e d by LBL a r e equal t o t h e b e s t c r i t i c a l c u r r e n t d e n s i t i e s shown i n Fig. 1, o r about 1500 ~lrnm2 a t 9 T.
I f Nb-Ti i s t o be used i n magnets w i t h f i e l d s above about 8 T, t h e o p e r a t i n g tem- p e r a t u r e must be reduced below 4.3 K, t h e temperature o f l i q u i d helium i n equlib- r i m w i t h helium gas a t atmospheric pressure. A r e c e n t and e f f e c t i v e development i n cryogenics has been t h e o p e r a t i o n o f magnets a t 1.8 K i n l i q u i d helium a t atmo- spheric pressure, which has been s t r a i g h t f o r w a r d , c o n s i d e r a b l y simpler, and more
r e l i a b l e than many expected. The experience o f l a b o r a t o r i e s t h a t work w i t h magnets i n t h i s temperature range has been t h a t the a n t i c i p a t e d problem o f superleaks has n o t m a t e r i a l i z e d because good seals ( i n p a r t i c u l a r , h e l i a r c welds) f o r l i q u i d helium are a l s o s u p e r f l u i d t i g h t .
NbTl (4.4 K) Best \, \
Nbg Sn LCP - LBL Optimized
NbTi (4.4 K) FNAL Specification
Field (Tesla)
XBL 8210-3095
Fig. 1.
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C r i t i c a l c u r r e n t d e n s i t i e s i n commercially a v a i l a b l e supercondcutors.The t o p curve f o r NbjSn corresponds t o Nb3Sn tape and some r e c e n t l y developed h i g h - i n t e r n a l Sn m a t e r i a l .
MAGNETS
During t h e p a s t year we have b u i l t and t e s t e d t h r e e Nb-Ti dipoles. Two are o f t h e l a y e r design, as shown i n Fig. 2, and one i s o f t h e block design, as shown i n Fig. 3. D e t a i l s o f magnet design and sane t e s t r e s u l t s have been described e l s e ~ h e r e ~ ~ ~ * ~ * ~ . The c h a r a c t e r i s t i c s o f these magnets a r e summarized i n Table 11. The Nb-Ti d i p o l e s t e s t e d g e n e r a l l y reach about 30% higher f i e l d s a t 1.8 K than a t 4.3 K.
Table I 1
Smmary o f Small-Bore, Nb-Ti Accelerator Dipoles Designed and Tested a t LBL
Magnet Design F i e l d (T) Cu: Sc T r a i n i n g
Name 4.3K 1.8K Ratio
D-9A 4 l a y e r 5.9 8.0 1.5 No
D-9B 4 l a y e r 7.1 8.6 1.0-1.3 Yes
D-1OB 8 pancakes 7.0 9.1 0.9 Yes
JOURNAL DE PHYSIQUE
Laminated Pole Island
10 TESLA DIPOLE MAGNET LAYER WINDING
XBL 825-9775 F i g . 2
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Cross s e c t i o n o f D-9A, a t y p i c a l 4 l a y e r or 4 s h e l l d i p o l e .,
Laminated S. S. ClampsS. S. Axial ' Tie Bars
Laminated S. S. Center Support
10 TESLA DIPOLE MAGNET BLOCK WINDING
XBL 825-9774
Fig. 3
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Cross s e c t i o n o f a block design c o i l having 8 f l a t pancakes.I n t h e l a y e r design t h e c o i l winding can support an inward r a d i a l l o a d i n t h e s t r a i g h t s e c t i o n and, when clanped under the maximum stress, remains separated from t h e s t a i n l e s s s t e e l bore-tube by about 0.5 mm. A l a y e r o f l-mm-thick n y l o n mono- f i l a m e n t i s wound over each l a y e r t o ( 1 ) provide a pre-canpression b e f o r e the sub- sequent l a y e r s are applied, (2) a i d t h e e x t e r n a l r i n g s i n supplying t h e f i n a l preload, and ( 3 ) provide a path f o r helium t o f l o w c i r c u m f e r e n t i a l l y around the c o i l . The ends o f t h e c o i l , which are n o t s e l f supporting, c o n t a c t t h e bore-tube and are under b o t h a r a d i a l load and sane c i r c u m f e r e n t i a l l o a d due t o t h e nylon banding and e x t e r n a l r i n g s .
The ends o f t h e c o i l l a y e r s are staggered, w i t h t h e o u t e r c o i l s shorter, t o reduce the maximum f i e l d a t t h e innermost t u r n o f the f i r s t layer. The h i g h - f i e l d r e g i o n i s i n t h e s t r a i g h t s e c t i o n o f l a y e r one. Neglecting end e f f e c t s , t h e maximum f i e l d r i s e i n t h e s t r a i g h t s e c t i o n o f l a y e r 1 i s about 3 % on the f i r s t t u r n .
The D-9A d i p o l e performed b e t t e r than any o t h e r f i r s t c o i l i n a s e r i e s t h a t we have constructed. A t 4.3 K t h e f i r s t quench was a t 80% o f s h o r t sample currents, and t h e second was a t 100%. The performance i n He I 1 a t 1.8 K was s i m i l a r , w i t h t h e f i r s t two quenches a t 90% and 1 0 0 % o f s h o r t s m p l e , 7.2 and 8.0 T, r e s p e c t i v e l y . During t h e second t e s t o f t h i s c o i l , which i s now underway, we hope t o l e a r n i f r e l a x a t i o n o f p r e s t r e s s i n t h e non-metal l i c p o r t i o n s windings w i l l a f f e c t p e r f ormance.
I n t h e block o r pancake c o i l , t h e c o i l s e c t i o n s a r e wound from heavy r e c t a n g u l a r c a b l e i n f l a t pancake o r race-track p a i r s . The p a r t i c u l a r geometry was chosen t o enable heavy Nb3Sn cable t o be wound i n t o a small-aperture d i p o l e .
To t e s t t h e new geometry f o r f a b r i c a t i o n p r a c t i c a l i t y and a l s o t o t e s t the magnet f o r t r a i n i n g and o t h e r p e r t i n e n t behavior, we b u i l t D-1OB u s i n g Nb-Ti cable.
The f i r s t quench i n D-1OB a t 4.3 K was about 80% o f i t s short-sanple, o r sane 5.5 T c e n t r a l f i e l d . The t r a i n i n g was moderately slow, some 25 quenches t o s h o r t sample.
It i s n o t c l e a r whether t h i s slow t r a i n i n g i s due t o a superconductor s t a b i l i t y problem, which might r e s u l t from a low copper-to-superconductor r a t i o , t o struc- t u r a l and pre-stress problems, o r t o conductor o r manufacturing d e f e c t s i n one pan- cake p a i r . Most quenches occured i n t h e pancake p a i r 3 and 4 o f t h e bottom h a l f . The maximum c e n t r a l f i e l d reached a t 4.3 K i s 7.0 T, and we b e l i e v e i t t o be short- sample performance because t h e same c u r r e n t i s reached b e f o r e and a f t e r o p e r a t i o n a t h i g h e r c u r r e n t s i n He 11. Also, the character o f t h e quenches, which i s q u i t e reproducible, i s t h a t o f conductor o p e r a t i n g a t s h o r t sample. Some t r a i n i n g a l s o occurred i n He 11, i n which a peak f i e l d o f 9.1 T was reached.
C y c l i c Losses
I n He I 1 one can measure t h e heat generated by magnet c y c l i n g by m o n i t o r i n g t h e bath temperature movements. And by observing simultaneously t h e v o l t a g e and cur- r e n t i n t h e c o i l . The losses as a f u n c t i o n o f f i e l d change r a t e f o r magnets D-9A, D-9B and D-1OB are shown on Fig. 4.
I n s p e c t i o n o f t h e t h r e e curves shows t h a t t h e D-106 behavior i s l i n e a r , which means the loss i s h y s t e r e t i c i n nature, o c c u r r i n g i n t h e superconductor i t s e l f . The D-9A behavior i s m o s t l y q u a d r a t i c on t o p o f a small l i n e a r term, showing t h a t most o f the l o s s i s eddy o r coup1 i n g type i n t h e cable w i t h a small component due t o the l i n e a r o r h y s t e r e t i c p o r t i o n . The D-9B l o s s l i e s between t h e o t h e r two.
We can estimate t h e r a t i o s o f t h e h y s t e r e t i c t y p e losses t o be found i n t h e t h r e e magnets based on t h e m a t e r i a l constants f o r t h e superconducting cables i n magnets, as l i s t e d i n Table 111. The r a t i o s o f t h e l i n e a r slopes are i n good agreement w i t h those expected from t h e m a t e r i a l constants, i.e., 1.00; 2.83; 7.44, as shown i n t h e equations below Table 111.
J O U R N A L D E PHYSIQUE
Fig. 4
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Losses i n c o i l s D-9A, D-96 and D-1OB. The d i f f e r e n c e s can be ascribed t o d i f f e r e n t f i l a m e n t s i z e s and d i f f e r e n t q u a n t i t i e s o f conductor i n the c o i l .Table I11
Cable Length (m) NO. Strands D i a. Strands (mm) Cu: S.C.
No. F i 1 aments Filament Dia.Jp) Vol. Cable (c ) Vol. S.C. (cm3)
0-9 Ba
D-9A Layers 1+2 Layers 3+4
a. For 0-9B use VS.C. = 1.59x103cm3 and d = 20.7 ~ m .
The dominant f a c t o r i n intra-cable c o u p l i n g l o s s i s t h e c o n t a c t r e s i s t a n c e a t the s t r a n d cross-over points. As pressure on t h e cable increases t h i s c o n t a c t r e s i s t - ance decreases and t h e q u a d r a t i c t y p e losses increase. The observed c o u p l i n g los'ses i n Fig. 4 are q u a l i t a t i v e l y i n agreanent w i t h t h e precompression a p p l i e d d u r i n g manufacture. Magnet D-9A had the most canpressive pre-stress, D-96 less, and D-1OB t h e l e a s t .
FUTURE PROGRAM
We a r e i n t h e process o f c o n s t r u c t i n g two a d d i t i o n a l d i p o l e s h a v i n g nominal 50 mm c o l d bores. One i s a four-layer design c a l l e d D-9C and i s s i m i l a r t o t h e two mag- nets, D-9A and D-9B. The d i f f e r e n c e s are m a i n l y i n t h e conductor, which i s graded.
The conductor strands i n t h e two cables used i n D-9C are from t h e same b i l l e t and have a 1.0 copper-to-superconductor r a t i o . The cable f o r l a y e r s 1 and 2 has 2 1 strands o f 0.79 mm (0.0318") and t h a t f o r l a y e r s 3 and 4 has 27 strands o f 0.67 mm (0.023") conductor. The c u r r e n t d e n s i t y i n t h e o u t e r two l a y e r s i s 60 h i g h e r than i n t h e i n n e r two layers. Special f a b r i c a t i o n f i x t u r e s f o r winding t h i s c o i l are complete, and h a l f o f one l a y e r has been wound.
The D-1OA c o i l i s a wind-and-react NbgSn c o i l made o f 8 pancake layers. It i s s i m i l a r i n cross s e c t i o n t o D-lOB, which was described above and shown i n cross s e c t i o n i n Fig. 2. The conductor i s wrapped w i t h a g l a s s i n s u l a t i o n t h a t can with- stand the abrasion o f winding and t h e - 7 0 0 ~ ~ r e a c t i o n temperature.
The conductor f o r D-10A i s a 11 s t r a n d Nb3Sn, Rutherford cable supplied b y t h e Intermagnetic General Canpany. Each s t r a n d i s 1.7 mm (0.068") diameter and has 2 t o 3 vm d i a n e t e r f i l a m e n t s .
The f i r s t h a l f o f D-1OA and t h e o u t e r two l a y e r s o f D-9C have been f a b r i c a t e d . Both D-9C and D-1OA should be canpleted and tested, i n c l u d i n g magnetic f i e l d measurements, i n 1983. I n a d d i t i o n , because i t now appears t h a t t h e n e x t h i g h energy p r o t o n a c c e l e r a t o r i n the U.S. may operate a t a lower f i e l d we are t u r n i n g o u r a t t e n t i o n t o graded 2-layer, 40 mm b o r e dipoles.
ACKNOWLEDGEMENTS
The authors wish t o acknowledge t h e h e l p o f t h e f o l l o w i n g members o f t h e super- conducting group's s t a f f i n designing, f a b r i c a t i n g and t e s t i n g these c o i l s : C. Peters, J. Rechen, R. Wolgast, S. Caspi, A. Borden, R. Schafer, J. O I N e i l l , R. Hannaford, L. Parsons, T. Young, D. Kemp, and R. Althaus.
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" P r o p e r t i e s and Performance o f High Current Density Sn-Core Process MF NbjSn," IEEE Trans. on Magnets, Vol. MAG-19, No. 3, May 1983, pp. 1135-1138.
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"A Layer-Wound, 8.7-Tesla Superconducting Dipole Magnet," IEEE Trans. Nucl.
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Schafer, C. Taylor and R. Wolgast, "A 9.1 T Iron-Free Nb-Ti Dipole Magnet w i t h Pancake Windings," i b i d , pp. 2578-2580.
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t o be published i n t h e Proceedings o f t h e X I 1 I n t e r n a t i o n a l Conference on P a r t i c l e Accelerators, Fermilab, August 1983.
