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RECENT PROGRESS IN HIGH FIELD SYSTEMS
M. Leupold
To cite this version:
M. Leupold. RECENT PROGRESS IN HIGH FIELD SYSTEMS. Journal de Physique Colloques,
1984, 45 (C1), pp.C1-9-C1-14. �10.1051/jphyscol:1984102�. �jpa-00223559�
JOURNAL
DEPHYSIQUE
Colloque C1, supplbment a u no 1, Tome 45, janvier 1984 page CI-9
R E C E N T P R O G R E S S IN H I G H FIELD S Y S T E M S
M . J . Leupold
Francis B i t t e r NatiomZ Magnet .kzboratory,
MIT *
Cambridge, Maasachuse t t s , U.S.A.sum& -
Les l a b o r a t o i r e s d ' a i m a n t s2
champs i n t e n s e s c o n t i n u e n t f a i r e dans l a t e c h n o l o g i e des aimants d e s p r o g r g s q u i p r o f i t e n t 5 l a communaute' d e s chercheurs t o u t e e n t i h r e . Dans c e r a p p o r t , nous p a r l o n s des a c t i v i t 6 s en cours concernant l e s aimants h y b r i d e s2
r e f r o i d i s s e m e n t2
eau e t supra- c o n d u c t e u r s , e t l e s a i m a n t s p u l s 6 s , e t nous commentons l e u r importance g b n 6 r a l e pour l a t e c h n o l o g i e d e s a i m a n t s .A b s t r a c t
-
High-field magnet l a b o r a t o r i e s c o n t i n u e t o make advances i n magnet technology which b e n e f i t t h e magnet community a s a whole. I n t h i s paper we r e p o r t on c u r r e n t water-cooled, s u p e r c o n d u c t i n g , h y b r i d , and p u l s e magnet a c t i v i t i e s and comment on t h e i r g e n e r a l s i g n i f i c a n c e t o magnet technology.INTRODUCTION
I n t h i s t a l k h i g h - f i e l d magnets a r e t a k e n t o b e t h o s e whose purpose is t o p r o v i d e f i e l d s f o r e x p e r i m e n t e r s , and t h e c l a s s e s i n c l u d e water-cooled, s u p e r c o n d u c t i n g hy- b r i d , and p u l s e magnets. S p e c i a l purpose o r d e v i c e magnets l i e beyond t h e s c o p e of t h i s t a l k . H i g h - f i e l d l a b o r a t o r i e s where r e s e a r c h measurements a r e b e i n g m a d e e x i s t i n v a r i o u s c o u n t r i e s throughout t h e world:
England 0
France 0
Germany 0
Japan 0
0 N e t h e r l a n d s 0 0
Poland @
USA 0
USSR 6
Clarendon L a b o r a t o r y , Oxford
The High F i e l d Laboratory of Grenoble I n s t i t u t e of T e c h n i c a l P h y s i c s , K a r l s r u h e High F i e l d Laboratory f o r Superconducting M a t e r i a l s , Tohoku U n i v e r s i t y
Osaka U n i v e r s i t y U n i v e r s i t y of Amsterdam U n i v e r s i t y of Nijmegen
I n t e r n a t i o n a l Laboratory of High Magnetic F i e l d s and Low Temperature, Wroclaw
F r a n c i s B i t t e r N a t i o n a l Magnet Laboratory,MIT Kurchatov I n s t i t u t e , Moscow
Aside from t h e r e s e a r c h programs which a r e s u p p o r t e d by t h e magnets i n u s e i n t h e s e f a c i l i t i e s , t h e magnets themselves a r e r e l e v a n t t o magnet technology a s v e h i c l e s f o r t r y i n g o u t new i d e a s . AS such t h e y a f f o r d o p p o r t u n i t i e s t o experiment w i t h new ma- t e r i a l s , t o r e f i n e computational t e c h n i q u e s , and t o improve r e l i a b i l i t y .
WATER-COOLED MAGNETS
I n s e r t s f o r h y b r i d s a r e i n c l u d e d i n t h i s c a t e g o r y . B i t t e r and p o l y - h e l i x a r e pre- s e n t l y t h e o n l y c o n s t r u c t i o n s t y l e s which a r e s u i t a b l e f o r r e a l l y h i g h performance.
A v a r i a n t of t h e B i t t e r c o n s t r u c t i o n i s t o g l u e t h e p l a t e s t o g e t h e r f o r t h e sake of a c h i e v i n g g r e a t e r s t r e n g t h and b e t t e r c o o l i n g h o l e r e g i s t r a t i o n . /1/
*supported by N a t i o n a l S c i e n c e Foundation
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984102
JOURNAL
DE
PHYSIQUEAt MIT and a t Nijmegen a new B i t t e r magnet o f f e r s improvements o v e r t h e ones i t i s d e s t i n e d e v e n t u a l l y t o r e p l a c e . I n t h e Nijmegen v e r s i o n i t g e n e r a t e s 20T i n a 32mm b o r e . The f i r s t MIT magnet w i l l b e a 50 mm i n s e r t f o r t h e h y b r i d . It c o n s i s t s of c o n c e n t r i c c o i l s i n s e r i e s , b u t where i t d i f f e r s from i t s p r e d e c e s s o r s i s t h a t t h e c o i l s a r e d i v i d e d a t t h e 150 mm d i a m e t e r i n s t e a d of a t 114.
L a r g e l y due t o a new i n s u l a t i o n m a t e r i a l s e v e r a l of our 54 mm magnets have been pushed t o an h o n e s t 20T. They o p e r a t e q u i t e h o t , i n e x c e s s of 130 C; t h e h e a t f l u x i s on t h e o r d e r of 800 w a t t s p e r s q u a r e cm.
P o l y - h e l i x magnets e x i s t i n Grenoble and a t Oxford i n t h e i r h y b r i d . The Oxford mag- n e t h a s g i v e n yoeman s e r v i c e l o n g e r t h a n any o t h e r . I n Grenoble t h e y have succeeded w i t h a 25T 50 mm magnet, and t h i s d e s i g n w i l l b e used i n t h e i r 30T h y b r i d .
The p o l y - h e l i x c o n s t r u c t i o n h a s i t s g r e a t e s t a d v a n t a g e i n t h e s t r u c t u r a l e f f i c i e n c y which d e r i v e s from s e v e r i n g t h e r a d i a l c o n t i n u i t y between r e g i o n s w i t h i n t h e winding.
By f r e e i n g t h e i n s i d e from r a d i a l t e n s i o n t h e r e i s a g r e a t r e d u c t i o n i n t h e hoop s t r e s s which i s by f a r t h e dominant component. / 2 /
SUPERCONDUCTING MAGNETS
New magnets i n t h i s c a t e g o r y i n c l u d e s e v e r a l 40 cm niobium-titanium magnets and some s m a l l e r niobium-tin magnets whose b o r e s measure a p p r o x i m a t e l y 5 cm. With t h e excep- t i o n of Turowski's magnet i n K a r l s r u h e /3/ t h e l a r g e ones a l l a r e p a r t of h y b r i d s y s - tems. The K a r l s r u h e magnet i s i n c l u d e d b e c a u s e i t s s t a t i s t i c s a r e s i m i l a r t o t h o s e o f t h e o t h e r s .
The K a r l s r u h e magnet g e n e r a t e s 8T when o p e r a t e d a t 4.2 K w i t h 1090 A. It i s n o t f u l l y s t a b i l i z e d under t h e s e c o n d i t i o n s ; t h e h e a t f l u x i s around 0.5 W/sq.cm. At 1 . 8 K i n a new c r y o s t a t i t i s d e s i g n e d t o make 10T w i t h 1350 A.
I n Grenoble t h e new h y b r i d system w i l l have a s u p e r c o n d u c t i n g magnet which c o n t r i - b u t e s 11T t o 30T c e n t e r l i n e f i e l d / 4 , 5 / I t i s a b i g magnet: 42 cm b o r e x 1099 cm O.D.
x 85 cm l o n g , and i t i s d e s i g n e d t o o p e r a t e a t 1 . 8 K. There a r e two c o n c e n t r i c c o i l s . The i n n e r i s a 22 l a y e r winding, and i t i s surrounded by a c o i l assembled from 60 double pancakes. T h i s magnet s t o r e s 22 M J i n i t s f i e l d . The o p e r a t i n g c u r - r e n t of o n l y 850 A i s a b o u t h a l f of t h a t of t h e o t h e r magnets i n t h i s group.
l o l o k u U n i v e r s i t y i n Japan / 6 / i s b u i l d i n g a new l a b o r a t o r y , and t h e i r program i s t o h a v e , among o t h e r t h i n g s , 3 h y b r i d magnets: 20, 25, and e v e n t u a l l y 30T. Two s u p e r - c o n d u c t i n g magnets have been b u i l t t h u s f a r . One, c a l l e d HM-A, h a s a 29 cm b o r e and g e n e r a t e s 8T. It i s n o t c r y o s t a b l e and c o n s e q u e n t l y i s c o m p a r a t i v e l y s m a l l . I t s b r o t h e r , HM-B, i s a 42 cm b o r e magnet which g i v e s 8T, and i t is c r y o s t a b l e . T h i s i s a 1470 A magnet which was b u i l t b y Toshiba o u t of double pancake c o i l s .
A somewhat s m a l l e r magnet i s t h e MIT h y b r i d s u p e r c o n d u c t o r which g e n e r a t e s 7.5T i n a 40 cm b o r e . / 7 / The O.D. i s o n l y 71 cm. The magnet i s by no means c r y o s t a b l e ; t h e h e a t f l u x i s .55 t o .6 W/sq.cm,but i t h a s n e v e r quenched d e s p i t e s m a l l s e c t i o n s hav- i n g been d r i v e n i n t o c u r r e n t s h a r i n g d u r i n g t e s t s . R e c e n t l y t h e q u e s t i o n of quench d e t e c t i o n i n t h i s magnet h a s been s t u d i e d a f r e s h . A new scheme which i s b a s e d on comparing a l l t h e p a i r s of a d j a c e n t pancakes h a s been t r i e d on an e x p e r i m e n t a l b a s i s and shows promise. / 8 / The problem i n quench d e t e c t i o n i s t o f i n d s m a l l r e s i s t i v e v o l t a g e s d e v e l o p i n g among much l a r g e r i n d u c t i v e v o l t a g e s .
Under development f o r s e v e r a l y e a r s h a s been niobium-tin i n m u l t i - f i l a m e n t a r y form, and now we a r e b e g i n n i n g t o s e e some magnets made from i t . The s i g n i f i c a n c e of t h e development i s i n i t s a p p l i c a b i l i t y t o l a r g e magnets f o r which t h e more f a m i l i a r t a p e form of niobium-tin i s i n a p p r o p r i a t e . These magnets do n o t f i n i s h t h e c h a p t e r on t h e s u b j e c t though, a s t h e y were wound from "green" c o n d u c t o r and r e a c t e d l a t e r . F o r l a r g e - s c a l e c o n s t r u c t i o n i t i s g e n e r a l l y h e l d t h a t t h e p r e f e r r e d way w i l l be t o wind w i t h p r e v i o u s l y r e a c t e d c o n d u c t o r .
MIT t o g e t h e r w i t h A i r c o i s b u i l d i n g a 15T magnet w i t h a 5 cm b o r e . There w i l l b e 4 c o n c e n t r i c c o i l s made from m u l t i - f i l a m e n t a r y niobium-tin c o n d u c t o r on t h e i n s i d e which w i l l b e surrounded by a niobium-titanium magnet c o n s i s t i n g of 3 c o i l s o n one form. The two innermost niobium-tin c o i l s have been t e s t e d t o f u l l f i e l d i n a water- c o o l e d magnet; t h e o t h e r two a r e p r e s e n t l y under c o n s t r u c t i o n . The s u b j e c t o f c o i l p r o t e c t i o n h a s r e c e i v e d a l o t of a t t e n t i o n . The c o n c e r n i s o v e r how one s e c t i o n i s a f f e c t e d by a quench i n t h e o t h e r , and i n p a r t i c u l a r , t h e build-up of c u r r e n t i n t h e i n n e r c o i l s i n t h e e v e n t of a quench i n t h e o u t e r o n e s . I n an i n g e n i o u s scheme de- v i s e d by Iwasa, t h e c u r r e n t b u i l t - u p i s opposed by a n o u t e r winding connected i n s e r i e s w i t h i t .
There i s a 13T m u l t i - f i l a m e n t a r y niobium-tin magnet b e i n g b u i l t f o r Tohoku Univer- s i t y f o r t h e purpose of e s t a b l i s h i n g a t e c h n i c a l f o u n d a t i o n f o r t h e i r 30T h y b r i d . It i s a two-coil magnet: t h e i n n e r one w i l l c o n t r i b u t e 4.5T, and t h e o u t e r of nio- bium-titanium w i l l g i v e 8.5T. The b o r e of t h e i n n e r c o i l i s 58 mm y i e l d i n g a c l e a r b o r e of 47 mm.
PULSE MAGNETS
P u l s e magnets c o n t i n u e t o b e b u i l t f i r s t , because t h e y o f f e r a r e l a t i v e l y inexpen- s i v e means t o h i g h f i e l d s and s e c o n d , b e c a u s e o n l y w i t h p u l s e magnets i s i t p o s s i - b l e t o a t t a i n f i e l d s much h i g h e r t h a n 30T. The p r i n c i p l e , conceived by K a p i t z a , i s t o o p e r a t e on a d u t y c y c l e w i t h r e s p e c t t o b o t h power and c o o l i n g . Energy i s de- l i v e r e d v e r y r a p i d l y from e i t h e r a s t o r a g e d e v i c e o r from a s o u r c e which i s s o l a r g e t h a t a s h o r t b u r s t won't b e missed. Among s t o r a g e d e v i c e s t h e most p o p u l a r a r e c a p a c i t o r s , b u t f o r l o n g e r p u l s e s e s p e c i a l l y , f l y w h e e l o r r o t a t i o n a l energy s t o r a g e i s more s u i t a b l e .
At b e s t , making measurements i n p u l s e d f i e l d s i s l e s s p o p u l a r t h a n i n DC f i e l d s s i m p l y b e c a u s e of t h e b a s i c p r e f e r e n c e which we a l l have f o r l e i s u r e o v e r h a s t e , b u t t h e n i n s t r u m e n t a t i o n i s more d i f f i c u l t , and s o a r e t h e t e c h n i q u e s . N e v e r t h e l e s s , p u l s e magnets a r e h e r e t o s t a y ; t h e h i g h f i e l d s o f f e r c h a l l e n g i n g o p p o r t u n i t i e s t o e x p e r i m e n t e r s , and b u i l d i n g t h e s e magnets o f f e r s immense c h a l l e n g e t o magnet tech- n o l o g i s t s .
The technology i s v a l u a b l e , f o r what i t t e a c h e s i s r e l e v a n t t o t h e s t r u c t u r a l d e s i g n of magnets g e n e r a l l y . I t i s a l s o v a l u a b l e t o t h e f u s i o n energy community which i s h e a v i l y committed t o v e r y l a r g e p u l s e d d e v i c e s .
P u l s e magnets f i t i n t o s e v e r a l c a t e g o r i e s . F i r s t , we have t o d i s t i n g u i s h between l o n g and s h o r t p u l s e s , and f o r s h o r t p u l s e magnets we have t o make a f u r t h e r d i s - t i n c t i o n between t h o s e which s u r v i v e and t h o s e which d e s t r o y themselves (and unfor- t u n a t e l y o f t e n t h e sample on which measurements a r e b e i n g made).
The d e s i g n of l o n g p u l s e magnets i s c o n s t r a i n e d by t h e r m a l c o n s i d e r a t i o n s . The windings become h e a t e d and must b e r e c o o l e d i n time f o r t h e n e x t s h o t . O f t e n t h e r e c o o l i n g t i m e d i c t a t e s t h e pace of t h e t e s t i n g ; i t i s seldom t h a t r e c h a r g i n g cap- a c i t o r s o r a c c e l e r a t i n g a f l y w h e e l l i m i t s t h e c y c l e time. T h e r m a l e f f e c t s a f f e c t d i f - f e r e n t magnet c o n s t r u c t i o n s i n o t h e r ways t o o . Non-uniform c u r r e n t d e n s i t y windings s u c h a s i n B i t t e r magnets w i l l respond t o h e a t i n g w i t h changing f i e l d c o n s t a n t s a s t e m p e r a t u r e n o n - u n i f o r m i t i e s f o r c e a r e d i s t r i b u t i o n of c u r r e n t d u r i n g a p u l s e . For s h o r t p u l s e s , t h o s e measured i n microseconds, magnet d e s i g n i s more h e a v i l y i n - f l u e n c e d by t h e n e c e s s i t y t o keep i n d u c t a n c e s s m a l l and by t h e need t o u s e t h e s t r o n g e s t c o n d u c t o r s . Whether t h e y a r e r e s i s t i v e o r n o t i s of r e l a t i v e l y l i t t l e importance.
P u l s e f i e l d f a c i l i t i e s c a t e r i n g t o o u t s i d e u s e r s a r e n o t t h e norm. I n s t e a d , we t e n d t o have p u l s e magnets which have been b u i l t by l a b o r a t o r i e s f o r t h e i r own programs.
Recent i n t e r e s t i n r e s e a r c h i n v e r y h i g h f i e l d s w a r r a n t s t h e i r i n c l u s i o n i n t h i s d i s c u s s i o n however.
C1-12 JOURNAL DE PHYSIQUE
Perhaps t h e m o s t v e n e r a b l e p u l s e magnet f a c i l i t y i s t h e one a t t h e Univery,itv of Amsterdam. / 9 / I t h a s been making f i e l d s of 40T s i n c e 1969. These a r e semi-con- t i n u o u s " ; t h e f i e l d s a r e on f o r about 1 s e c , and t h e y c a n b e r e g u l a t e d . The b o r e s i z e of 16 mm i s more o r l e s s t y p i c a l . The magnets a r e wound w i t h g r e a t c a r e o u t of h e a v i l y cold-worked copper. They a r e cooled w i t h l i q u i d neon t o a t e m p e r a t u r e of 25T u s i n g a l i q u e f i e r . Depending whether t h e y go t o 20 o r 40T, t h e r e c o o l time i s e i t h e r 20 minutes o r two h o u r s .
Kurchatov I n s t i t u t e i n Moscow r e p o r t e d a 50T magnet w i t h a 1 5 t o 20 mm b o r e a n d p u l s e d u r a t i o n s on t h e o r d e r o f 1 5 msec.
/ l o /
T h i s magnet i s i n t e r e s t i n g i n t h a t i t u s e s copper r e i n f o r c e d w i t h niobium-titanium a s t h e c o n d u c t o r .Kurchatov h a s a l s o succeeded w i t h lOOT f i e l d s i n b o r e s of 1 cm l a s t i n g on t h e o r d e r of s e v e r a l microseconds. The magnets l a s t f o r about 10 s h o r t s . They a r e e n e r g i z e d from 50 K J , 25 Kv c a p a c i t o r bank.
I n t h e l a b o r a t o r y a t Wroclow t h e r e i s a SOT p u l s e magnet w i t h a 1 3 mm b o r e and a 50 msec p u l s e d u r a t i o n .
At Osaka U n i v e r s i t y M. Date and h i s c o l l e a g u e s a r e working toward 100T.
I l l /
Pre- s e n t l y they a c h i e v e f i e l d s of JOT i n a 20 mm b o r e f o r t i m e s of 350 t o 400 micro- seconds. T h e i r magnet i s a r o b u s t l y c o n s t r u c t e d two-coil p o l y - h e l i x made of marag- i n g s t e e l . There i s no c o o l i n g ; none i s r e q u i r e d when t h e r e i s no copper. Using b e r y l l i u m copper t h e y have a c h i e v e d 44T.At t h e U n i v e r s i t y of Tokyo I n s t i t u t e o f S o l i d S t a t e P h y s i c s (ISSP) N. Miura and h i s coworkers have a t t a i n e d f i e l d s of 45T l a s t i n g f o r 27 msec i n 1 3 t o 20 mm b o r e s . /12/.
The c o i l s a r e l a y e r wound and r e i n f o r c e d w i t h a s t a i n l e s s s t e e l o u t e r c y l i n d e r . The winding i s immobilized w i t h epoxy r e s i n which i s p r e s s u r e impregnated w i t h 50 atm.
At MIT S. Foner h a s m a i n t a i n e d a p u l s e f i e l d c a p a b i l i t y o v e r t h e l a s t 20 y e a r s . To accommodate t h e r e c e n t i n t e r e s t i n h i g h f i e l d s h i g h speed d a t a a c q u i s i t i o n equip- ment i s b e i n g added, and t h e s e f a c i l i t i e s w i l l become a v a i l a b l e t o e x p e r i m e n t e r s on a r e g u l a r b a s i s . The magnets a r e layer-wound copper c o i l s w i t h b o r e s of 20 mm which a c h i e v e 45 t o 50T i n 10 msec p u l s e s .
B a r t of MIT's r e a s o n f o r making F o n e r ' s p u l s e d f i e l d s a v a i l a b l e on a f a c i l i t y b a s i s i s t o p r e p a r e f o r t h e time when we w i l l o f f e r h i g h e r f i e l d s a n d / o r l a r g e r b o r e s . There a r e two d e s i g n s t u d i e s under way: one i s concerned w i t h g e n e r a t i n g f i e l d s of 60 t o lOOT i n a b o r e of 20 mm and t h e o t h e r i s a p r o j e c t t o g e n e r a t e 1 second-long f i e l d s of 50T i n l a r g e b o r e s , on t h e o r d e r of 10 cm. I n almost every r e s p e c t t h e l a t t e r i s a l a r g e u n d e r t a k i n g ; i t i s made p o s s i b l e by t h e a v a i l a b i l i t y of t h e A l c a t o r C a l t e r n a t o r power s u p p l y (150 MW).
I n p u l s e magnets, r e g a r d l e s s of whether t h e y l a s t f o r s e v e r a l o r f o r hundreds of s h o t s , f a i l u r e i s almost always a s s o c i a t e d i n some way w i t h f a t i g u e . It i s probably s a f e t o s a y t h a t f a t i g u e p l a y s a p a r t i n t h e e v e n t u a l demise of a l l h i g h performance magnets. A common element i n t h e p u l s e magnets we have d i s c u s s e d i s some means of p r e v e n t i n g t h e development of s l a c k w i t h i n t h e windings. Most o f t e n i t i s t h e i n - s u l a t i o n system which i s degraded when t h e r e i s s l a c k and working can o c c u r . The p i c t u r e i s complicated by t h e f a c t of t e m p e r a t u r e changes and t h e r m a l d e f o r m a t i o n s . CONCLUSION
I n each c l a s s of magnet t h e p r o g r e s s which h a s been made should be m e a s u r e d a g a i n s t t h e problems s o l v e d . Obviously, t h e r e i s no p o i n t i n comparing 15T a c h i e v e d by a s u p e r c o n d u c t i n g magnet w i t h 150T f o r a few microseconds i n a p u l s e magnet, and y e t each marks a s i g n i f i c a n t accomplishment. Nor is t h e r e c o m p e t i t i o n among t h e s e v e r a l k i n d s of magnets a s t h e r e once was p e r c e i v e d t o be. There was a time when t h e r e l a - t i v e m e r i t s of s u p e r c o n d u c t o r s and water-cooled magnets were debated a s though a c h o i c e had t o b e made between them. Even a t MIT where we a r e h e a v i l y committed t o w a t e r magnets we acknowledge t h e e x i s t e n c e and p l a c e of o t h e r ways of making h i g h f i e l d s
.
Continued p r o g r e s s toward h i g h e r f i e l d s becomes more d i f f i c u l t a s t h e t e c h n o l o g y m a t u r e s . I n t h e b e g i n n i n g t h e r e were a l o t of new i d e a s t o t r y o u t , and p r o g r e s s sometimes came s w i f t l y a s t h e b e s t o n e s proved t h e m s e l v e s . With s u p e r c o n d u c t o r s , s o o n a f t e r we l e a r n e d t h a t t h e r e had t o b e f i l a m e n t s , we l e a r n e d t h a t t h e y had t o b e t w i s t e d . B e f o r e t h a t we l e a r n e d t h a t s u p e r c o n d u c t o r s had t o be s t a b i l i z e d . I n w a t e r magnets t h e B i t t e r d e s i g n was a b r e a k t h o u g h and was r e g a r d e d a s t h e u l t i m a t e u n t i l t h e t h r u s t toward h i g h e r f i e l d s prompted t h e in-depth s t r u c t u r a l a n a l y s i s which c u l m i n a t e d i n t h e p o l y - h e l i x . / 2 / By a d d r e s s i n g a fundamental s t r u c t u r a l pro- blem t h e p o l y - h e l i x c o n c e p t h a s found i t s way i n t o t h e d e s i g n of u l t r a - h i g h f i e l d p u l s e magnets.
What we n e e d n e x t a r e b e t t e r m a t e r i a l s , and a s t h o s e a r e developed o u r c h a l l e n g e w i l l be i n l e a r n i n g t o u s e them. The c a l l f o r b e t t e r m a t e r i a l s i s a c r o s s t h e b o a r d i n c o n d u c t o r s , i n i n s u l a t o r s , and i n r e i n f o r c i n g m a t e r i a l s . The g r e a t e s t promise seems t o b e i n t h e a r e a of c o m p o s i t e s where s e v e r a l c o n s t i t u e n t s c a n b e b l e n d e d t o produce p r o p e r t i e s t a i l o r e d t o s p e c i f i c r e q u i r e m e n t s .
F a t i g u e e f f e c t s h a v e g e n e r a l l y been i g n o r e d i n m a g n e t s , l a r g e l y o u t of i g n o r a n c e on t h e p a r t o f magnet d e s i g n e r s working i n t h e c o n t e x t o f DC magnets. However, t h e q u e s t f o r h i g h f i e l d s w i l l demand t h e u t m o s t from m a t e r i a l s which w i l l n e v e r b e s t r o n g enough f o r c o m f o r t a b l e m a r g i n s . I n d e e d , i f t h e y were, d e s i g n e r s would s o o n push f o r e v e n h i g h e r f i e l d s . But i n o r d e r t o make utmost u s e of a m a t e r i a l , we must know i t s s t r e n g t h , n o t i n t h e a b s t r a c t , b u t i n r e l a t i o n t o t h e s e r v i c e c o n d i t i o n s which f o r magnets means f a t i g u e . O b v i o u s l y , economics e n t e r s t h e d e s i g n p r o c e s s a s s e r v i c e l i f e i s t r a d e d a g a i n s t performance. But t h e r e a r e two problems: t h e f i r s t i s t h a t f a t i g u e p r o p e r t i e s o f most, m a t e r i a l s and n o t j u s t t h e advanced ones under c o n d i t i o n s o f t e n e n c o u n t e r e d i n magnets, e g . c r y o g e n i c e n v i r o n m e n t s , a r e n o t w e l l known, and t h e second i s t h a t f a t i g u e e f f e c t s a r e p r e s e n t on b o t h macro and micro- s c o p i c s c a l e s . I n p u l s e magnets, l o n g b e f o r e t h e r e i s f a i l u r e from m e t a l f a t i g u e , t h e s t r u c t u r e a s a whole s u f f e r s from t h e l a r g e r - s c a l e e f f e c t s of i n s u l a t i o n crumbl- i n g and c r e a t i n g v o i d s w i t h i n which m o t i o n becomes p o s s i b l e . When we u n d e r s t a n d f a t i g u e , we w i l l b e a b l e t o make t h e f u l l e s t u s e of t h e m a t e r i a l s which a r e a v a i l - a b l e t o u s .
Saying we need b e t t e r m a t e r i a l s i s a vague s t a t e m e n t which encompasses a complex in- t e r a c t i o n between i d e a s , t e c h n i q u e s , d e s i g n , and a v a i l a b i l i t y . D e c i d i n g where t o s t a r t i s much l i k e t r y i n g t o u n r a v e l a n u n t i d y b a l l of s t r i n g . P r o m i s i n g i d e a s s u c h a s niobium s t r e n g t h e n e d copper /13/ have been d e m o n s t r a t e d i n t h e l a b o r a t o r y . C l o s e r t o r e a l i t y a s m a t e r i a l s from which magnets c a n b e made a r e m e t a l l u r g i c a l l y bonded c o m p o s i t e s o f copper and s t a i n l e s s s t e e l which a r e h e a v i l y c o l d worked t o p r o d u c e h i g h s t r e n g t h s . However, once someone s u c c e e d s i n making a m a t e r i a l w i t h t h e r e - q u i s i t e p r o p e r t i e s , ' t h e n e x t q u e s t i o n i s how t o u s e i t t o make a magnet. A s l a b of copper and s t a i n l e s s s t e e l sandwiched t o g e t h e r might b e bandsawed i n t o a s p i r a l . On t h e o t h e r h a n d , t h e same c o n s t i t u e n t s i n t h e form of a m u l t i - p l y c y l i n d e r would sug- g e s t machining i n t o a h e l i x . There i s no s u g g e s t i o n t h a t machining e i t h e r would b e e a s y . The f o u r t h k n o t i n o u r b a l l of s t r i n g is a v a i l a b i l i t y . What a r e t h e i n c e n - t i v e s f o r t h e i n d u s t r i e s which p o s s e s s t h e equipment f o r c a r r y i n g o u t t h e s e p r o c e s s - e s t o d i v e r t i t from p r o d u c t i o n f o r t h e s a k e of t h e r e l a t i v e l y s m a l l q u a n t i t i e s which we might s e e k ? I s u g g e s t t h a t t h e f i e l d s p e r s e t o g e t h e r w i t h t h e i d e a t h a t t h e s e magnets a r e i n f a c t v e h i c l e s f o r t e s t i n g m a t e r i a l s under extreme c o n d i t i o n s might b e s u f f i c i e n t j u s t i f i c a t i o n f o r i n d u s t r i a l i n t e r e s t .
Even w i t h wonder m a t e r i a l s we c a n s t i l l e x p e c t s u r p r i s e s when we b u i l d magnets w i t h them. I am reminded of t h e motion p i c t u r e "The Man i n t h e White S u i t " w i t h Alec Guinness made o v e r 30 y e a r s ago. As a c h e m i s t h e d i s c o v e r e d a m a t e r i a l of i n c r e d i - b l e s t r e n g t h and t o u g h n e s s and d e c i d e d t o make some i n t o a s u i t of c l o t h e s a s a de- m o n s t r a t i o n . H i s s u i t m a t e r i a l a f t e r a l l t h e problems of working w i t h something s o tough were s o l v e d and a m a r v e l l o u s s u i t was f i n a l l y made, t u r n e d o u t t o be chemical- l y u n s t a b l e and d i s i n t e g r a t e d s p o n t a n e o u s l y i n s h o r t o r d e r . Watch f o r t h e unex- p e c t e d ! Lex Murphii!
Cl-14 JOURNAL DE PHYSIQUE
The table summarizes the activities of the various laboratories with respect to the different classes of magnets.
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