COMPARISON OF PUMPING LIMITER, DOUBLE-NULL AND SINGLE-NULL DIVERTOR CONDITIONS FOR THE ASDEX UPGRADE TOROIDAL FIELD MAGNET

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COMPARISON OF PUMPING LIMITER,

DOUBLE-NULL AND SINGLE-NULL DIVERTOR CONDITIONS FOR THE ASDEX UPGRADE

TOROIDAL FIELD MAGNET

O. Jandl, E. Springmann, B. Streibl

To cite this version:

O. Jandl, E. Springmann, B. Streibl. COMPARISON OF PUMPING LIMITER, DOUBLE- NULL AND SINGLE-NULL DIVERTOR CONDITIONS FOR THE ASDEX UPGRADE TOROIDAL FIELD MAGNET. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-163-C1-169.

�10.1051/jphyscol:1984134�. �jpa-00223689�

JOURNAL DE PHYSIQUE

Colloque Cl, suppl6ment au n o 1, Tome 45, janvier 1984 page Cl-163

COMPARISON OF PUMPING LIMITER, DOUBLE-NULL AND SINGLE-NULL DIVERTOR CONDITIONS FOR THE ASDEX UPGRADE TOROIDAL FIELD MAGNET

0. Jandl, E. Springmann and B. Streibl

Maz-PZanck-Institut fiir PZasmaphysik, EURATOM Association, 0-8046 Garching, F.R. G.

Resume

-

pouvant fonctionner avec une configuration ti un point de champ nu1 (SN) ou deux points (DN). De plus un limiteur pomp6 est envisage.

L'etude des bobines de champ toroidal est decrite et les contraintes sur les supports des bobines TF c o m p a r d e s p o u r l e s t r o i s t y p e s de con- figuration. Une attention speciale a 6t6 donnge 2 la configuration

(SN). C'est une option possible pour AU et pour INTOR et NET.

Abstract - ASDEX Upgrade (AU) is a tokamak experiment with external po- loidal field coils. It will be provided with divertors of single-null

(SN) and double-null (DN) configurations. Additionally, a pumping limi- ter (L) is envisaged / l / . The design principle of the toroidal field

(TF) magnet for AU is described and the stressing of the supported TF coils compared for the three operation modes. Special consideration is given to the SN divertor as favourit option for AU as well as for INTOR and NET.

The local poloidal field (PF) required for the production of the SN stagnation point is significantly larger in comparison with limiter to- kamaks such as JET, though the total sum of PF currents differs only little. The support of the lateral forces produced in the toroidal field (TF) coils by the PF thus requires a very rigid structure in or- der to protect the TF coils from being overstressed by shearing and bending. The mechanical effects of the SN loading have been little discussed up to now. They are difficult to understand because of the lack of symmetry of the PF currents with respect to the torus plane.

SMALL CASING

Fig. 1 - Plan view of the toroidal field magnet for AU Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984134

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1. BASIC DESIGN FEATURES

The t o r o i d a l maanet c o m ~ r i s e s 16

steel bmci-p2 prelim~mw f~mtm water-cooled coils and t h e t u r n -

o v e r s t r u c t u r e (TOS) , which h o u s e s t h e o u t e r p a r a l l e l f l a n k e d p a r t of t h e c o i l s ( F i g . 1 ) . The TOS c o n t a i n s 8 l a r g e c a s i n g s , which a r e s p l i t i n t h e t o r u s p l a n e by f l a n g e s . To pro- v i d e f o r c e l o c k i n g i n t o r o i d a l d i - r e c t i o n , two s m a l l c a s i n g s , located s y m m e t r i c a l l y above and below t h e t o r u s p l a n e , l i n k a d j a c e n t l a r g e c a s i n g s . Due t o t h e m i s s i n g v e r t i - c a l c o n n e c t i o n of t h e s m a l l c a s i n g s e a c h l a r g e c a s i n g h a s t o t a k e u p i n t h e t o r u s p l a n e t h e s h e a r f o r c e s r e s u l t i n g from t h e l a t e r a l l o a d i n g of two c o i l s . The s m a l l c a s i n g s p r o v i d e ample space f o r 8 l a r g e h e r i z o n t a l a c c e s s p o r t s . The dimen- s i o n s a v a i l a b l e of t h e s e o p e n i n g s a r e l a t e r a l l y t h e f u l l gap w i d t h o f

bottom screw a d j a c e n t c o i l s

( a

t i c a l l y 960 mm. Each l a r g e c a s i n g c o n t a i n s two a c c e s s p o r t s d i r e c t l y above and below t h e t o r u s t lane F i g . 2 - C o i l s u p p o r t a r e a s f l a n g e w i t h a 330 by 330 k 2 c l e a r

opening. The remaining openings f o r a c c e s s t o t h e plasma and t h e r e - c e s s e s f o r t h e c o i l w a t e r m a n i f o l d s a r e shown i n F i g . 1 . Due t o l a c k of s p a c e t h e p o s i t i o n of a d j a c e n t w a t e r m a n i f o l d s a l t e r n a t e s w i t h r e s p e c t t o t h e t o r u s p l a n e . The d o u b l e pancake copper c o i l s a r e c o n s t a n t t e n - s i o n D-shaped ( F i g . 2 ) . T h e i r b o r e opening i s 2.96 m v e r t i c a l l y and 1.9 m h o r i z o n t a l l y . The magnet i s d e s i g n e d t o produce a maximum TF of Bo = 4 T on t h e plasma a x i s w i t h a r a d i u s of Ro = 1.65 m.

The c e n t e r i n g f o r c e (CF) e x e r t e d by t h e TF i s s u p p o r t e d v i a a v a u l t , which i s formed by t h e 16 t a p e r e d i n n e r c o i l l e g s . Mismatch due t o m a n u f a c t u r i n g t o l e r a n c e s w i l l be compensated by means of e p o x y - f i l l e d s t e e l b l a d d e r s . The r e s u l t i n g v a u l t compression s t r e s s a c t i n g o v e r t h e l a t e r a l c o i l s u r f a c e s CFSA of F i g . 2 p r o v i d e s f o r c e l o c k i n g a l s o w i t h r e s p e c t t o s h e a r i n g . A t a f r i c t i o n c o e f f i c i e n t of 0 . 3 9 , t y p i c a l f o r i n - s u l a t i o n on s t e e l , a s h e a r f o r c e a s l a r g e a s t h e CF p e r c o i l ( 1 5 . 7 MN a t Bo = 4 T ) c o u l d be t r a n s f e r r e d between a d j a c e n t c o i l s . Hence t h e v a u l t e d i n n e r c o i l l e g s a c t m e c h a n i c a l l y l i k e a c l o s e d c y l i n d e r w i t h c r o s s - s e c t i o n CFSA. The l a t e r a l f o r c e s e x e r t e d on a TF c o i l o u t s i d e t h e TOS c a n t h u s immediately b e c o u n t e r a c t e d i n t h e v a u l t by h o r i z o n t a l and v e r t i c a l s h e a r f o r c e s . T h i s i s of prime i n t e r e s t f o r t h e DN and SN l o a d i n g d i s t r i b u t i o n s . The r e m a i n i n g l a t e r a l PF f o r c e s a r e t r a n s f e r r e d v i a t h e a r e a s TOSA of F i g . 2 i n t o t h e TOS and a l s o c o u n t e r a c t e d t h e r e by s h e a r i n g f o r c e s . Shear s l e e v e s and dowels a r e t h u s r e q u i r e d on t h e c a s i n g j o i n t s . F i g u r e 2 a l s o shows t h e a u x i l i a r y f i x t u r e s f o r assem- b l i n g and a d j u s t i n g t h e c o i l s . The s t e e l b r a c i n g t a p e s w i l l be removed a f t e r assembly of t h e complete t o r o i d a l magnet.

2 . ELECTROMAGNETIC PF FORCES

The e l e c t r o m a g n e t i c f o r c e s e x e r t e d on a TF c o i l due t o t h e i n t e r a c t i o n of c o i l c u r r e n t and e x t e r n a l PF a r e p r o p o r t i o n a l t o t h e p r o d u c t of t h e TF on t h e plasma a x i s (Bo) and t h e plasma c u r r e n t ( I ) . The t h r e e c o n f i g u r a t i o n s D N , L and SN a r e t h u s always comparedPat t h e same pro- d u c t Ip X Bo.

Starting from the L con£ iguration, the quadrupole PF component of the diver-

=

significantly in order to shift the stagnation point(s) towards the plas-

A z ma boundary. Additionally a hexapole

P F component is required to readjust the right height-to-width ratio ofthe plasma column (triangularity). Both PF components are quickly decaying from the PF coils, arranged outside the TF coils, towards the plasma.

Consequently these divertor-specific field components increase considerab- ly the lateral magnetic load in the winding of the TF coils, as demonstra- ted in Figs. 3 and 4.

The L force distribution, familiar from JET, is compared in Fig. 3 with that of the DN case. Both configura- tions show odd symmetry with respect to the torus plane ( z = 0)

.

In Fig. 4 the SN force distribution is shown. Thereby the stagnationpoint Fig.3 - DN and L loading distribution was assumed above the torus plane.

The SN loading resembles near the stagnation point the DN distribution, however,with a reduced hexapole PF component. Below the torus plane it resembles the L case. Hence the SN force distribution does not show simple z = 0 symmetry. The main pro- blem of the divertor force distribu- tions are the increased forces outside the TOS, i.e. in the region between the apex of the coils and the vault.

There the maxima of DN and L differ by more than a factor of three and those of SN and L by a factor of two.

The dominant actions of the PF forces on a coil can immediately be derived from Figs. 3 and 4. For DN and L the net forces of upper and lower coil half rotate due to the odd z = 0 sym- metry the complete coil around the X axis. The quadrupole forces twist the coil around a vertical axis. The opposite twists of upper and lowerhalf of a coil comPensate each other.

Fig.4 - SN loading distribution Hence the ioii could balanace this mode elastically without the assistance of the TOS or the vault. In the SN case this compensation exists no longer, hence the total coil is rota- ted around a vertical axis. This displacement mode shows even symmetry to z = 0 . Of course, the modes of odd z = 0 symmetry are also present.

The even and the odd SN modes can be separated by decomposing the cur- rent distribution as shown in Fig. 5. The SNA current distribution produces a magnetic flux which in the torus plane is directed horizon- tally. This flux distribution causes the displacement modes of even

Cl-I66 JOURNAL DE PHYSIQUE

SN-stagnation point above torus plane

--

m 0.825MA m 1.675MA

hor~zontal flux lz=OI B flux iz.01

Fig.5 - Decomposition of the PF currents for the SN case

z = 0 symmetry. The magnetic f l u x of t h e SNB c u r r e n t d i s t r i b u t i o n i s i n t h e t o r u s p l a n e d i r e c t e d v e r t i - c a l l y and c a u s e s t h e odd d i s p l a c e - ment modes. The decomposition of t h e SN l o a d i n g i n t o two symmetric l o a d i n g s , SNA and SNB, p e r m i t s t o a p p l y a f i n i t e element (FE) model which r e p r e s e n t s c o i l and TOS o n l y above t h e t o r u s p l a n e ( F i g . 6 ) . Hence t h e same reduced FE model can be used f o r D N , L and t h e SN c a s e s . Only t h e boundary c o n d i t i o n s i n t h e t o r u s p l a n e have t o be changed f o r t h e SNA l o a d i n g . Once t h e s t r e s s e s from t h e two l o a d i n g s of t h e S N c a s e a r e known f o r t h e upper h a l f of t h e s t r u c t u r e , t h o s e f o r t h e lower h a l f f o l l o w immediately from t h e symmetry c o n d i t i o n s . The maxi- mum of a s t r e s s component i s a l - ways given by t h e sum of t h e abso- l u t e v a l u e s o f t h e SNA and SNB s t r e s s e s .

3. THE FINITE-ELEMENT (FE) MODEL The s t r e s s a n a l y s i s was performed w i t h t h e FE model shown i n F i g s . 6 and 7 . T h i s model r e p r e s e n t s t h e main f o r c e t r a n s f e r p r o p e r t i e s b e t - ween c o i l s and TOS a s w e l l a s v i a t h e f l a n g e c o n n e c t i o n j o i n t s of t h e TOS c a s i n g s . However, s i m p l i f i - c a t i o n s were met w i t h r e s p e c t t o t h e TOS r e c e s s e s . Only t h e l a r g e and s m a l l p o r t n e a r t h e t o r u s plane, t h e l a r g e s t r e c e s s e s of t h e TOS, were a c c u r a t e l y r e p r e s e n t e d . The a l t e r n a t i o n of t h e w a t e r m a n i f o l d r e c e s s e s was n e g l e c t e d f o r t h e s a k e of h i g h e r o r d e r s t r u c t u r a l symmetry i n l a t e r a l d i r e c t i o n .

Hence 1 6 w a t e r m a n i f o l d r e c e s s e s i n - s t e a d of t h e a c t u a l 8 , were assumed above t h e t o r u s p l a n e . To compen- s a t e f o r t h e r e s u l t i n g r e d u c t i o n i n s t i f f n e s s , t h e a d j a c e n t c i r c u l a r p o r t openings o f F i g . 1 were n e g l e c - t e d . Also t h e v e r t i c a l o p e n i n g s a r e n o t c o n s i d e r e d a t p r e s e n t . I n t h i s c a s e t h e FE model needs o n l y tocom- p r i s e t h e s e c t o r shown i n F i g . l , i . e . one c o i l and t h e a d i a c e n t h a l - v e s of a l a r g e and a s m a i l c a s i n g . Fig.6- F i n i t e element (FE) model

The c o i l i s r e p r e s e n t e d up t o t h e v a u l t by 3 0 beam s e c t i o n s . A l t e r n a t i v e l y t h r e e dimensional ( 3 D ) e l e - ments c o u l d have been t a k e n . However, t h e i n t e r n a l bending ( M b ) and

t w i s t i n g ( M t ) moments a s w e l l a s t h e s h e a r f o r c e s ( Q ) p r o v i d e d by beam e l e m e n t s a r e d i r e c t l y r e l a t e d t o t h e e x t e r n a l ( l a t e r a l ) f o r c e s . Conse- q u e n t l y beam e l e m e n t s a r e much b e t t e r s u i t e d f o r t h e i n t e r p r e t a t i o n of r e s u l t s t h a n t h e i n v o l v e d s t r e s s d i s t r i b u t i o n s p r o v i d e d by 3 D e l e m e n t s .

However, a t a l a t e r s t a g e 3D elements w i l l be a p p l i e d f o r more d e t a i l e d i n v e s t i g a t i o n s of l o c a l e f f e c t s . I n t h i s c a s e t h e i n s u l a t i o n com- p o n e n t s w i l l a l s o be meshed a s de- t a i l e d a s p o s s i b l e (computer capa- c i t y ) . I n t h e v a u l t r e g i o n t h e c o i l i s r e p r e s e n t e d by p l a t e e l e - ments l i k e t h e TOS. T r u s s e l e m e n t s

( h i n g e d b a r s ) c o n n e c t c o i l and TOS l a t e r a l l y a t c r o s s - s e c t i o n s which a r e s t i f f e n e d by b u l k heads (Fig.7).

With a few i t e r a t i o n s a l l t r u s s e s which showed t e n s i o n f o r c e s c o u l d be e l i m i n a t e d , t h u s o n l y compres- s i o n a l f o r c e s were t r a n s f e r r e d i n - Fig.7 - Cross-sectional subdivision of t o t h e TOS a f t e r t h e l a s t i t e r a -

the FE model t i o n . The number of c a s i n g j o i n t s of t h e FE model a g r e e s w i t h r e a l i t y . However, t h e c o r r e c t c i r c u m f e r e n t i a l p o s i t i o n c o u l d n o t be r e a l i z e d f o r e a c h of t h e 12 j o i n t s . The FE model r e p r e s e n t s t h e j o i n t s by beam e l e - ments i n o r d e r t o t r a n s f e r s h e a r and t e n s i o n f o r c e s between t h e two c a s i n g s .

Each c o i l c o n s i s t s of 2 4 i n s u l a t e d t u r n s which a r e bonded by e p o x y r e s i n (vacuum i m p r e g n a t i o n ) . Consequently t h e beam t w i s t i n g and bending

s t i f f n e s s of t h e t o t a l c o i l c r o s s - s e c t i o n were i n s e r t e d and t h e con- s t a n t s of e l a s t i c i t y averaged o v e r t u r n copper and i n s u l a t i o n . For t h e p l a t e s of t h e TOS t h e e l a s t i c i t y modulus of s t e e l (E = 2 0 0 GPa) was i n s e r t e d .

Boundary c o n d i t i o n s a r e r e q u i r e d where t h e s t r u c t u r e o f F i g . 1 i s c u t by t h e symmetry p l a n e s of t h e FE model, i . e . w i t h i n t h e v a u l t and t h e TOS. These boundary c o n d i t i o n s a r e d e s c r i b e d by means o f t r a n s l a t i o - n a l and r o t a t i o n a l d i s p l a c e m e n t c o n s t r a i n t s , which f o l l o w from t h e symmetry c o n d i t i o n s of t h e c o r r e s p o n d i n g l o a d i n g . For a p l a n e of odd symmetry t h e i n p l a n e d i s p l a c e m e n t s and c o n s e q u e n t l y t h e o u t - o f - p l a n e r o t a t i o n a r e b l o c k e d . For a p l a n e of even symmetry t h e o u t - o f - p l a n e d i s p l a c e m e n t and t h e i n p l a n e r o t a t i o n s a r e b l o c k e d . The symmetry r e - l a t i o n s of t h e t o r u s p l a n e a r e g i v e n i n F i g . 5 . The l a t e r a l p l a n e s t h r o u g h o u t show odd f o r c e symmetry. The n a t u r a l boundary c o n d i t i o n s s u p p r e s s a l l p o s s i b l e r i g i d body motions of t h e FE model. Only i n t h e SNA c a s e a r i g i d body r o t a t i o n around t h e t o r u s a x i s remains uncon- s t r a i n e d . I n t h i s c a s e t h e s t i f f n e s s m a t r i x of t h e FE model c a n n o t be i n v e r t e d . However, t h i s r o t a t i o n i s n o t e x c i t e d by t h e e x t e r n a l f o r c e s . Consequently it can be e l i m i n a t e d by b l o c k i n g on t h e TOS E - t r a n s l a - t i o n a l d e g r e e of freedom i n and normal t o a l a t e r a l p l a n e of symmetry.

Thereby t h e s t r e s s d i s t r i b u t i o n w i l l n o t a t a l l be a f f e c t e d . 4. STRESS ANALYSIS OF THE TF COILS

For t h e SN c a s e t h e most i m p o r t a n t r e s u l t s a r e shown i n F i g s . 8 t o 11 a s a f u n c t i o n of t h e c r o s s - s e c t i o n numbers (CSN) of F i g . 7 . The p l o t s c o v e r t h e upper p a r t of a c o i l up t o t h e v a u l t , i . e . t h e r e g i o n which was r e p r e s e n t e d by b a r s . The SNA and SNB components a r e p l o t t e d sepa- r a t e l y . Consequently t h e r e s u l t s f o r t h e lower p a r t of t h e c o i l can a l s o be e x t r a c t e d from t h e p l o t s . The c o r r e s p o n d i n g r e s u l t s f o r t h e DN and L l o a d i n g show q u a l i t a t i v e l y t h e same d i s t r i b u t i o n s l i k e t h e SNB l o a d i n g , t h e q u a n t i k a t i v e d i f f e r e n c e s f o l l o w from Table 1 .

The l a t e r a l d i s p l a c e m e n t s a r e p l o t t e d - i n F i g . 8 . I n t h e r e g i o n suppor- t e d by t h e TOS t h e SNB and n e a r t h e v a u l t t h e SNA l o a d i n g dominate t h e d i s p l a c e m e n t p a t t e r n . The s h e a r f o r c e s ( Q ) of F i g . 9 a r e s t r o n g l y

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Ipx Bo = 3.9 MAT

l

0.L - _n

Cross section number & s s section numbw

Fig.8

-

Upper hatf of coil

0.m- - IpxBo=39MAT - Upper half of coil I P ~ B o = ~ . ~ M A T

-0.w-

Cmss sechan number Crms section number

Fig.10-Distribution of coil bending Fig.11 -Distribution of coil twisting

moments (Mb) lmments (Mt)

influenced by the concentrated force transfer into the TOS via trusses.

The supporting influence of the TOS tip (CSN 18, 19) on the coil is obvious. For the upper half of the structure altogether 20 % of the to- tal forces transferred into the TOS and 30 % of the coil forces of the unsupported part of the coil are taken up by the TOS tip. The distri- bution of the bending moments ( M b ) , shown in Fig. 10, is dominated by the SNB loading. The maximum of Mb outside the TOS coincides in lo- cation with that of the force distribution.

Bending stress [MP~]

Twisting shear [MP~] 4.8

Lateral displacement

[ m 4

Table 1

-

Comparison of stresses and displacements of a TF coil for the three PF configurations of

ASDEX Upgrade

Due to the increasing reduction of the coil cross-section towards the vault the bending stress shows its maximum (18 MPa) on CSN 31 of the lower half of the coil. The bending stresses on the watermanifold are comparatively small ( 7 MPa). Near the TOS tip, at CSN 1 7 , the zero points of SN loading, displacement and bending moment coincide rather well.

There a hinge can thus be assumed on the coil. The TOS tip istoonear to this location in order to balance a marked part of the moment formed by the external loading around this hinge. Most of this moment has thus to be taken up by the vault. The odd SNB loading is counteracted there

(CSN 31) mainly by the shear force of Fig. 9, which entails a vertical

f r i c t i o n a l s h e a r s t r e s s w i t h i n t h e v a u l t . The even SNA l o a d i n g i s main- l y c o u n t e r a c t e d on CSN 31 by t h e t w i s t i n g moment ( M t ) , shown i n F i g . 11.

The moment M t e n t a i l s a h o r i z o n t a l f r i c t i o n a l s h e a r s t r e s s w i t h i n t h e v a u l t . O u t s i d e t h e v a u l t i t c o u l d most e f f e c t i v e l y be t a k e n up by a c l o s e d c a s i n g around t h e c o i l , e . g . a welded s t e e l c a s i n g ( t h e r m a l prob- lems) o r an epoxy c a s i n g w i t h c r o s s i n g KEFLAR f i b r e s wound around t h e c o i l under an a n g l e of 45 d e g r e e s . However, t h e v a u l t c u r r e n t d e n s i t y of t h e c o i l winding i s f i x e d by t h e r e q u i r e d a d i a b a t i c p u l s e d u r a t i o n of t h e magnet and t h e a d d i t i o n of a c a s i n g would t h u s i n c r e a s e c o n s i d e r a b l y t h e

t o t a l tokamak system. C o n s e q u e n t l y , t h e TF c o i l s of AU d i s p e n s e w i t h a c a s i n g i n and n e a r t h e v a u l t r e g i o n .

I n t h e b a r e c o i l c r o s s - s e c t i o n t w i s t i n g moments show a v e r y inhomogeneous d i s t r i b u t i o n of s h e a r s t r e s s e s . A d d i t i o n a l l y t h e winding i n s u l a t i o n , a l s o s t r e s s e d by t h e t w i s t i n g s h e a r , c a n o n l y t a k e up s m a l l s h e a r s t r e s s e s of a b o u t 35 MPa a t t h e h i g h e s t o p e r a t i o n a l t e m p e r a t u r e of 70' C . The t w i s t i n g moments t h u s l i m i t a t t h e d e s i g n s h e a r of 20 MPa t h e SN o p e r a t i o n of AU

t o I p X Bo 5 4.5 MAT. I n t h i s c a s e , a t t h e maximum plasma c u r r e n t of 1.6 MA ( q = 2 . 2 ) , a f r i c t i o n c o e f f i c i e n t of 0.15 i s s u f f i c i e n t f o r t h e e q u i l i b r i u m of t h e v a u l t .

I n T a b l e 1 t h e t h r e e c o n f i g u r a t i o n s D N , SN and L a r e compared a t

I p X Bo = 3.9 MAT w i t h r e s p e c t t o t h e maximum bending s t r e s s and t w i s t i n g s h e a r on t h e v a u l t (CSN 31) a s w e l l a s t h e maximum d i s p l a c e m e n t . The DN c a s e shows t h e l a r g e s t bending s t r e s s e s , t h e SN c a s e t h e l a r g e s t t w i s t i n g s h e a r . With r e s p e c t t o t h e s e v a l u e s t h e c o r r e s p o n d i n g s t r e s s e s of t h e L c a s e a r e more t h a n a f a c t o r of t h r e e s m a l l e r .

5. CONCLUSIONS

The mechanical problems f o r t h e TF magnet of a tokamak w i t h a r e a c t o r - r e l e v a n t axisymmetric d i v e r t o r a r e d e c i d e d l y a g g r a v a t e d . However, t h e combination of f r i c t i o n a l v a u l t s u p p o r t and TOS, a s chosen f o r A U ,

e f f e c t i v e l y s u p p o r t s t h e l a r g e l a t e r a l SN o r DN d i v e r t o r f o r c e s produced by e x t e r n a l PF c o i l s .

For INTOR o r NET c o n d i t i o n s t h e l a r g e r a s p e c t r a t i o w i l l r e d u c e t h e me- c h a n i c a l problems. I n c a s e o f a welded-in s u p e r c o n d u c t i n g winding, t h e c o i l c a s i n g t a k e s up most e f f e c t i v e l y t h e SN t w i s t i n g s h e a r . A d d i t i o n a l l y , due t o t h e low o p e r a t i o n a l t e m p e r a t u r e , much l a r g e r s h e a r s t r e s s e s a r e a l l o w a b l e f o r t h e c o i l winding. Consequently t h e f o r c e problems of t h e TF magnet s h o u l d b e s o l v a b l e f o r a t o r o i d a l d i v e r t o r of a f u s i o n r e a c t o r . REFERENCES

/ l / ASDEX Upgrade P r o j e c t Team, IPP 1/217, May 1983.