ALCATOR DCT MAGNETIC SYSTEMS DESIGN

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Submitted on 1 Jan 1984

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ALCATOR DCT MAGNETIC SYSTEMS DESIGN

D. Montgomery, O. Schultz, R. Thome

To cite this version:

D. Montgomery, O. Schultz, R. Thome. ALCATOR DCT MAGNETIC SYSTEMS DESIGN. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-149-C1-152. �10.1051/jphyscol:1984131�. �jpa-00223686�

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J O U R N A L D E PHYSIQUE

Colloque C l , suppl6ment a u no 1, Tome 45, janvier 1984 page Cl-149

ALCATOR DCT MAGNETIC SYSTEMS DESIGN

D.B. Montgomery, O.H. S c h u l t z and R.J. Thome

PZasma Fusion Center, MIT Cambridge, M 02139, U.S.A.

&sum& - Le r a p p o r t d 6 c r i t un tokamak de grand rayon Q g a l 2 2 m g t r e s , avec 24 b o b i n e s s u p r a c o n d u c t r i c e s d e 1 5 0 X 200 cm, 2 champ maximum d e 10 t e s l a e t un s y s t s m e 2 champ p o l o i d a l t o t a l e m e n t s u p r a c o n d u c t e u r . Tous l e s systemes d e b o b i n a g e u t i l i s e n t l e c o n c e p t d e c o n d u c t e u r s 2 r e f r o i d i s s e m e n t i n t e r n e .

A b s t r a c t - A 2 m e t e r m a j o r r a d i u s tokamak w i t h 24(150 X 200 cm)lO t e s l a peak f i e l d s u p e r c o n d u c t i n g c o i l s and a n a l l s u p e r c o n d u c t o r PE s y s t e m i s d e s c r i b e d . A l l c o i l s y s t e m s u t i l i z e i n t e r n a l l y - c o o l e d c o n d u c t o r c o n c e p t s .

MACHINE DESCRIPTION

The A l c a t o r DCT i s a 2 m e t e r m a j o r r a d i u s , 7 t e s l a o n - a x i s f i e l d machine which f o l - lows i n t h e A l c a t o r t r a d i t i o n o f a compact h i g h power d e n s i t y , h i g h performance approach, b u t u n l i k e i t s p r e d e c e s s o r , i s d e d i c a t e d t o l o n g p u l s e i s s u e s . The l o n g p u l s e r e q u i r e m e n t i n a compact machine h a s d i c t a t e d t h e u s e o f s u p e r c o n d u c t i n g mag- n e t s f o r b o t h t h e t o r o i d a l and p o l o i d a l c o i l s y s t e m s .

The machine h a s a r e l a t i v e l y l a r g e plasma a s p e c t r a t i o of 5 , more t y p i c a l of r e a c t o r s t h a n present-day tokamaks. The l a r g e r a s p e c t r a t i o a l l o w s t h i s machine t o have a v e r y l a r g e i n d u c t i v e d r i v e (35 v o l t - s e c o n d s ) a l l o w i n g p u l s e t i m e s of s e v e r a l m i n u t e s a t d e n s i t i e s t o o h i g h f o r RE s t e a d y - s t a t e c u r r e n t d r i v e , which w i l l however, b e pos- s i b l e a t reduced d e n s i t i e s . S t e a d y - s t a t e RF power w i l l i n c l u d e 5 MW of ICRF h e a t i n g and 4 MW of LH c u r r e n t d r i v e . The machine h a s 24 t o r o i d a l f i e l d c o i l s , "Dee" shaped t o accommodate e l o n g a t e d plasmas (K = 1 . 6 ) and t o i n c o r p o r a t e a s i n g l e n u l l p o l o i d a l d i v e r t o r . The s t o r e d e n e r g y i n t h e t o r o i d a l f i e l d i s 550 M J .

As shown i n ( F i g . l ) , t h e s u p e r c o n d u c t i n g c o i l s y s t e m s a r e c o n t a i n e d i n a common vacuum s y s t e m a s i s t y p i c a l of tokamak r e a c t o r d e s i g n s . Each t o r o i d a l c o i l i s i n a sep- a r a t e heavy-walled c r y o s t a t which s e r v e s a s b o t h a s t r u c t u r a l s u p p o r t and c r y o g e n i c containment v e s s e l . The c e n t r a l OH c o i l and i n d i v i d u a l p o l o i d a l f i e l d c o i l packages a r e c o n t a i n e d i n i n d i v i d u a l n o n s t r u c t u r a l v e s s e l s , o f s p e c i a l c o n s t r u c t i o n t o i n - c r e a s e t h e i r t o r o i d a l r e s i s t a n c e .

The plasma chamber and a c c e s s p o r t s u t i l i z e a second i n d e p e n d e n t vacuum system, a l - lowing vacuum-break a c c e s s t o t h e v e s s e l w i t h o u t warm-up of t h e s u p e r c o n d u c t i n g mag- n e t s . V e r t i c a l a c c e s s i s p r o v i d e d between e v e r y c o i l , b u t h o r i z o n t a l a c c e s s i s o n l y p r o v i d e d a t e v e r y o t h e r c o i l i n o r d e r t o a l l o w f o r s t r u c t u r e t o r e s i s t t h e o v e r t u r n - i n g moments. The c o l d mass i s s u p p o r t e d from t h e c e n t r a l s e c t i o n of t h e c o m o n vacuum chamber, a l l o w i n g removal o f b o t h c o v e r s . The c e n t r a l s e c t i o n i s i n t u r n s u p p o r t - ed from t h e f l o o r b y out-board l e g s , a l l o w i n g removal of a l l PE c o i l s i f r e q u i r e d .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984131

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JOURNAL DE _PHYSIQUE

TOROIDAL FIELD MAGNETS

The DCT d e s i g n r e q u i r e s a 1 0 t e s l a p e a k f i e l d a t t h e c o n d u c t o r . The two TF magnet a p p r o a c h e s g e n e r a l l y c o n s i d e r e d f o r f i e l d s above 8 T a r e Nb3Sn a t 4.2 K, o r NbTi a t 1 . 8 K. Niobium-tin h a s t h e d i s a d v a n t a g e o f r e q u i r i n g s t r i n g e n t q u a l i t y c o n t r o l t o a s s u r e undamaged m a t e r i a l p r o p e r t i e s , and 1 . 8 K NbTi r e q u i r e s a more c o m p l e x c r y o g e n i c system and a "double c a s e " magnet s u p p o r t c o n c e p t . W e h a v e c h o s e n Nb3Sn b a s e d on i t s a d v a n t a g e s of a s i m p l e r r e q u i r e m e n t o n c r y o g e n i c s y s t e m s and a g r e a t e r e n e r g y m a r g i n f o r s t a b i l i t y a g a i n s t d i s t u r b a n c e s , p a r t i c u l a r l y a t h i g h e r f i e l d s .

W e h a v e s e l e c t e d t h e n i o b i u m - t i n c o n d u c t o r of t h e t y p e u s e d i n t h e Westinghouse LCP c o i l and i n t h e MIT 1 2 t e s l a t e s t c o i l . (Approximately twenty-two t o n n e s of t h i s c o n - d u c t o r h a v e been f a b r i c a t e d by A i r c o , a p p r o x i m a t e l y 40% t h e t o t a l needed f o r a l l t h e DCT c o i l s . ) While A l c a t o r DCT w i l l u s e t h e b r o n z e - p r o c e s s NbgSn c o n d u c t o r used i n LCP, we h a v e proposed f o u r improvements: ( 1 ) h e a t t r e a t m e n t a f t e r w i n d i n g , which w i l l r e l i e v e t h e n e e d f o r s e v e r a l d i f f i c u l t q u a l i t y c o n t r o l s t e p s d u r i n g w i n d i n g ; (2) e l i - m i n a t i o n o f t h e s u b s t r u c t u r e s u p p o r t p l a t e s which w i l l l e a d t o a more economical con- s t r u c t i o n ; (3) u s e o f p o o l - b o i l i n g h e l i u m o u t s i d e t h e ground wrap t o p r o v i d e a s i m p l e t e c h n i q u e f o r removing a v e r a g e h e a t l o a d s w h i l e s t a g n a n t h e l i u m i n t e r n a l t o t h e con- d u c t o r p r o v i d e s a h i g h d e g r e e of s t a b i l i t y ; ( 4 ) u s e of a d u a l c y c l e h e a t t r e a t m e n t and a n I n c o l o y 9 0 3 s h e a t h , which t o g e t h e r d r a m a t i c a l l y r e d u c e t h e s t r a i n d e g r a d a t i o n of t h e m a t e r i a l . /l/

The e n e r g y margin a g a i n s t t h e e f f e c t of plasma d i s r u p t i o n s and o t h e r d i s t u r b a n c e s , is p r e d i c t e d t o b e a b o u t 500 &/cm3 o f w i r e f o r t h e c o n d u c t o r . T h i s m a r g i n .has been con- f i r m e d o n s u b s c a l e c o n d u c t o r s u t i l i z i n g f e w e r f u l l s c a l e s t r a n d s , and is c o n s i s t e n t w i t h l e v e l s p r e d i c t e d by t h e a n a l y t i c a l models. D i s r u p t i o n s a r e e x p e c t e d t o d e p o s i t a p p r o x i m a t e l y 50 &/cm3 and h e n c e t h e c o n d u c t o r s h o u l d h a v e a l a r g e margin of s a f e t y i n t h i s r e g a r d .

The w i n d i n g w i l l b e c o n s t r u c t e d from s i x d o u b l e p a n c a k e s which a r e 11 l a y e r s d e e p . At 1 0 t e s l a peak f i e l d , t h e c o n d u c t o r o p e r a t i n g c u r r e n t w i l l b e 23 kA, 58% of t h e c r i t i - c a l c u r r e n t a t 1 0 T, 4 . 2 K, and 32% v o i d f r a c t i o n . A s i n t h e MIT 1 2 t e s l a c o i l 121,

we p r o p o s e t o h e a t - t r e a t t h e c o n d u c t o r a f t e r w i n d i n g t o e l i m i n a t e t h e danger o f a c c i - d e n t a l damage d u r i n g w i n d i n g . A f t e r h e a t t r e a t i n g , t h e t u r n s c a n be e a s i l y s e p a r a t e d , r e i n s u l a t e d , and t h e p a n c a k e s t h e n epoxy i m p r e g n a t e d . Each d o u b l e p a n c a k e w i l l b e epoxy impregnated s e p a r a t e l y and w i l l c o n s i s t of a s i n g l e 150 m e t e r l e n g t h of con- d u c t o r . We b e l i e v e t h a t t h i s a b i l i t y t o epoxy i m p r e g n a t e a n d f u l l y i n s u l a t e t h e w i n d - i n g g i v e s r i s e t o a w i n d i n g pack o f more p r e d i c t a b l e modulus t h a n a non-impregnated winding, and one w i t h b e t t e r i n s u l a t i o n c h a r a c t e r i s t i c s .

It i s n o t n e c e s s a r y t o c i r c u l a t e h e l i u m f o r s t a b i l i t y ( l o c a l p r e s s u r e - d r i v e n mass f l o w s p r o v i d e a d e q u a t e t r a n s i e n t h e a t t r a n s f e r , / 3 / ) b u t i t i s n e c e s s a r y t o remove t h e a v e r a g e h e a t l o a d o v e r a c y c l e due t o d i s t u r b a n c e s a n d h e a t l e a k a g e . While t h i s could b e done by low v e l o c i t y c i r c u l a t i o n o f t h e s u p e r c r i t i c a l h e l i u m , i t may b e e a s i e r t o s i m p l y p r o v i d e a c c e s s f o r a t m o s p h e r i c p r e s s u r e 4 . 2 K h e l i u m t o t h e f a c e s o f t h e w i n d i n g p a n c a k e s . The c o n d u c t a n c e o f t h e t h e r m a l p a t h , even t h r o u g h t h e epoxy- g l a s s ground i n s u l a t i o n of t h e p a n c a k e s , would b e s u f f i c i e n t t o remove t h e a v e r a g e h e a t . Each d o u b l e p a n c a k e w i l l t h e r e f o r e b e a s s e m b l e d w i t h s p a c e r s t o p r o v i d e 1 . 5 mm gaps t o p r o v i d e s u p p l e m e n t a l e x t e r n a l c o o l i n g .

The w i n d i n g pack w i l l b e e n t i r e l y s u p p o r t e d by t h e h e a v y w a l l c a s e which s u r r o u n d s it.

The c o n d u c t o r s t e e l c o n d u i t i s n o t e x p e c t e d t o c a r r y a p p r e c i a b l e i n - p l a n e t e n s i o n l o a d , ( a l t h o u g h i t i s c a p a b l e of c a r r y i n g s i g n i f i c a n t l o a d s ) , b u t must c a r r y t h e r a - d i a l compression l o a d s w i t h i n t h e w i n d i n g p a c k . We h a v e performed m e c h a n i c a l t e s t s on a r r a y s o f c o n d u c t o r s which h a v e been p o t t e d i n t o a s u p p o r t c a s e , and i n f e r f r o m t h e l i n e a r b e h a v i o r t h a t t h e a v e r a g e r a d i a l p r e s s u r e s o f 50 MPa o n t h e l o w e s t f i e l d c o n d u c t o r s c a n b e r e a d i l y accommodated. The p o t t i n g a p p e a r s t o p l a y a n i m p o r t a n t r o l e . A l c a t o r DCT i n a s i n g l e - n u l l d i v e r t o r c o n f i g u r a t i o n e x p e r i e n c e s a s t r o n g non- symmetric o u t - o f - p l a n e l o a d . The method f o r r e s i s t i n g t h e o v e r t u r n i n g moments i s t o u t i l i z e s t i f f top-to-bottom e l e m e n t s between e v e r y o t h e r c o i l , and t o u s e f a s t e n e r s azd k e y s o n a l l s l i p s u r f a c e s .

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Fig. 1 - Elevation view of Alcator DCT.

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JOURNAL DE PHYSIQUE

POLOIDAL FIELD MAGNETS

We have chosen t o u s e s u p e r c o n d u c t i n g p o l o i d a l f i e l d c o i l s on t h e DCT machine t o pro- v i d e t h e most economic s o l u t i o n f o r t h e c a p a b i l i t y t o r u n l o n g p u l s e s . Were t h e PF c o i l s t o b e c o p p e r r a t h e r t h a n s u p e r c o n d u c t i n g f o r example, t h e y would consume more t h a n 100 MW a t f u l l e x c i t a t i o n . The p r o p o s e d c o i l set shown i n ( F i g . l ) , i s a h y b r i d system i n t h a t any g i v e n c o i l p e r f o r m s m u l t i p l e f u n c t i o n s , f o r example, b o t h an i n - d u c t i v e d r i v e and a n e q u i l i b r i u m f u n c t i o n , o r b o t h a d i v e r t o r and a s h a p i n g f u n c t i o n . This approach i s t y p i c a l of n e x t - s t e p d e s i g n s , b u t i s a d e p a r t u r e from tokamaks i n which t h e i n d u c t i v e d r i v e (OH c o i l s ) and e q u i l i b r i u m (EF c o i l s ) a r e i n d e p e n d e n t de-

coupled s y s t e m s .

The PF c o i l s w i l l b e exposed t o f i e l d changes of a b o u t 3 T / s d u r i n g t h e c u r r e n t ramp, and 0 . 3 T / s d u r i n g t h e l o n g f l a t t o p . The c e n t r a l s o l e n o i d w i l l a d d i t i o n a l l y b e exposed t o a 30 T / s rate d u r i n g t h e i n i t i a l plasma break-down p h a s e . D i s r u p t i o n s pro- duce a b o u t 0 . 3 T c h a n g e s i n 1 0 m i l l i s e c o n d s a s c o n t r o l l e d by t h e vacuum v e s s e l t i m e c o n s t a n t . These f i e l d c h a n g e s , t o g e t h e r w i t h c o n t r o l f i e l d v a r i a t i o n s r e q u i r e u s e of a r e a s o n a b l y f i n e l y d i v i d e d PF c o n d u c t o r t o a s s u r e s t a b i l i t y and a r e a s o n a b l e l e v e l of a c l o s s .

I n t e r n a l l y c o o l e d c o n d u c t o r s , a g a i n i n t h e form of t h e Westinghouse/Airco c o n d u c t o r a r e under s t u d y a s t h e l e a d i n g PF c o n d u c t o r c h o i c e . T h a t c o n d u c t o r form i s s u f f i c i - e n t l y s u b d i v i d e d and h a s a l a r g e c o i l m a n u f a c t u r i n g d a t a b a s e through LCP. While such PF c o n d u c t o r s might w e l l u t i l i z e Nb3Sn s t r a n d s , t h e lower f i e l d s t y p i c a l of t h e PF c o i l s a l l o w NbTi s t r a n d s (7.0 t e s l a c e n t r a l s o l e n o i d , 2.0 t o 5.0 t e s l a , EF c o i l s ) . The i n t e r n a l l y c o o l e d c o n d u c t o r s p e r m i t f u l l epoxy i m p r e g n a t i o n and t h e r e f o r e s u c h s u p e r c o n d u c t i n g PF c o i l s can s h a r e i n t h e l a r g e d a t a b a s e f o r impregnated copper con- v e n t i o n a l c o i l s . Non-impregnated pool-cooled c a b l e c o n d u c t o r s have a v e r y l i m i t e d m e c h a n i c a l p e r f o r m a n c e d a t a b a s e .

A l c a t o r DCT can u s e t h e Westinghouse/Airco s h e a t h i n e x a c t form ( a l t h o u g h made from 316 LN s t a i n l e s s - s t e e l ) i n o r d e r t o u t i l i z e t h e e x i s t i n g t u b e m i l l w i t h o u t f u r t h e r development. A PF c o n d u c t o r would t y p i c a l l y need t o h a v e a copper t o s u p e r c o n d u c t o r r a t i o of 5 t o 1 t o g i v e a d e q u a t e s t a b i l i t y u n d e r p u l s e f i e l d c o n d i t i o n s and h e n c e would n e c e s s a r i l y h a v e a lower o v e r a l l c u r r e n t d e n s i t y t h a n t h e DCT TF c o n d u c t o r s . A s h e a t h of t h e Westinghouse/Airco dimension c o u l d c a r r y a c a b l e c a p a b l e of c a r r y i n g 12.5 kA, h a l f t h a t of t h e TF c o n d u c t o r . We would p r o p o s e t o u s e two o r more such con- d u c t o r s wound-in-hand t o b u i l d up a s u i t a b l e t o t a l c u r r e n t l e v e l of 25 kA t o 50 kA.

REFERENCES

1. ( a ) M.M. S t e e v e s , M.O. Hoenig, C . J . C y d e r s , " E f f e c t s of I n c o l o y 903 and Tantalum C o n d u i t s on C r i t i c a l C u r r e n t i n Nb3Sn Cable-In-Conduit Conductors" p r e s e n t e d a s paper IP-1-6 a t t h e 1 9 8 3 CEC-ICMC 1 9 8 3 C o n f e r e n c e , Colorado S p r i n g s , CO., Aug. 15-19, 1983. (b) M.O. Hoenig, M.M. S t e e v e s and C . J . Cyders,"NbjSn ICCS Technology I and 11. P r e s e n t e d a t t h e MT-8 Magnet Technology Conference, Grenoble, F r a n c e , S e p t . 1983.

2. M.O. Hoenig, M.M. S t e e v e s , A.G. Montgomery and M.M. Olmstead, " P r o g r e s s i n t h e ICCS-HFTF 12 T e s l a C o i l Program", IEEE T r a n s . Mag MAG-17, No. 1 (1981).

3. L. D r e s s n e r , " P a r a m e t r i c Study of t h e S t a b i l i t y Margin of Cable-In-Conduit Super- c o n d u c t o r s Theory". IEEE T r a n s . Mag MAG-17, No. 1 (1981).