THE FTU MAGNETIC STRUCTURE

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THE FTU MAGNETIC STRUCTURE

R. Andreani, L. Bettinali, A. Cecchini, M. Gasparotto, L. Lovisetto, A.

Pizzuto, G. Righetti

To cite this version:

R. Andreani, L. Bettinali, A. Cecchini, M. Gasparotto, L. Lovisetto, et al.. THE FTU MAGNETIC STRUCTURE. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-153-C1-156.

�10.1051/jphyscol:1984132�. �jpa-00223687�

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JOURNAL DE PHYSIQUE

Colloque C l , supplCment a u n o 1, Tome 45, janvier 1984 page Cl-153

T H E F T U MAGNETIC S T R U C T U R E

R. Andreani, L. Bettinali, A. Cecchini, M. Gasparotto, L. Lovisetto, A. Pizzuto and G.B. Righetti

Associazione EURATOM-ENEA.suZZa Fusione, Centro Ricerche Energia Frascati, C.P. 65, 00044 Frascati, Roma, I t a l y

Resume

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Le Tokamak doit travailler d des champs magnetiques toroidaux et po- loidaux tres Qleves pour plus de

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chocs, afin d'atteindre les performances ddsirdes. Pour verifier la possibilitd de construire des bobines de champ to- ro'idal capables de soutenir les contraintes mGcaniques, des analyses aux Ble- ments finis & trois dimensions et d'importants programmes d'essais mecaniques .S temperature cryogenique ont Btd effectues. Les principaux rdsultats obtenus sont present&. Les performances, les charges et les contraintes des bobines de champ sont aussi resum8es.

Abstract

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The FTU Tokamak, i n order t o produce t h e r e q u i r e d performance must be operated a t very h i g h l e v e l s o f t o r o i d a l and p o l o i d a l magnetic f i e l d and f o r a l a r g e number o f shots (> 104). I n order t o assess t h e f e a s i b i l i t y o f a magnet capable o f t h i s performance, a t h r e e dimensional f i n i t e element s t r e s s analysis and an extensive m a t e r i a l cryogenic t e s t program has been developed. The main r e s u l t s obtained are i l l u s t r a t e d . The objectives, t h e loads and t h e stresses on the p o l o i d a l f i e l d windings are a l s o reported.

1. INTRODUCTION

The FTU machine i s designed t o combine t h e good confinement p r o p e r t i e s o f a compact tokamak w i t h a medfum h i g h t o r o i d a l f i e l d (8 T) w i t h a strong plasma a d d i t i o n a l heating (8 MW) a t t h e lower h y b r i d resonance frequency). The e n t i r e load assembly i s kept a t l i q u i d n i t r o g e n temperature (- 196 OC) t o keep t h e power consumption i n t h e t o r o i d a l magnet and i n t h e p o l o i d a l f i e l d windings w i t h i n reasonable l i m i t s . The basic mechanical s t r u c t u r e o f t h e machine i s provided by t h e m o n o l i t h i c t o r o i d a l magnet supporting the vacuum chamber and t h e p o l o i d a l f i e l d windings.

2. FTU TOROIOAL MAGNET

2.1. General Description. The FTU t o r o i d a l magnet w i l l have t o work e x t e n s i v e l y (104 shots) a t the nominal value o f t h e t o r o i d a l mangetic f i e l d , S.e. 8 T w i t h 1.5 S

c u r r e n t f l a t t o p time. The magnet c o n s i s t s o f 12 modules. Each module includes two copper c o i l s enclosed i n a s t a i n l e s s s t e e l casing. I n order t o l i m i t the s t r a y f i e l d ( r e f e r r e d t o t h e t o r o i d a l f i e l d a t t h e plasma boundary) w i t h i n 6% two i n s u l a t i n g spacers a r e located between t h e two c o i l s . Each c o i l has 42 t u r n s obtained from wedge-shaped ETP hardened copper sheets. The e l e c t r i c a l connections between f o l l o w -

i n g t u r n s are made i n t h e o u t e r r e g i o n by brazing. Sheets o f glass r e i n f o r c e d epoxy,

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^0.22^mmt h i c k , i n s u l a t e t h e t u r n s from each other. I n s u l a t i o n t o ground i s made by s h e l l s o f t h e same m a t e r i a l , 4 mm t h i c k . Each c o i l s h a l l be vacuum impregnated w i t h a s u i t a b l e epoxy resin. The main aim f o r the assembly procedure i s t o achieve a w e l l defined force t r a n s f e r from t h e c o i l t o t h e s t a i n l e s s s t e e l casing d u r i n g the opera- t i o n a t l i q u i d i n i t r o g e n temperature ^(% 77 "K). To t h i s purpose the s t a i n l e s s s t e e l casing w i l l be preloaded, a t room temperature, up t o 70 MPa by means o f p r e c i s e l y adjustable wedges located between t h e casing and t h e c o i l .

2.2. Stress Analysis. To evaluate t h e s t r e s s d i s t r i b u t i o n i n t h e o v e r a l l s t r u c t u r e ,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984132

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Cl-154 JOURNAL DE PHYSIQUE

an extensive 3-dimensional f i n i t e element computation has been c a r r i e d o u t by means o f t h e "BERSAFE" code /l/. The main r e s u l t s are:

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maximum e q u i v a l e n t stresses (Von Mises): 545 MPa f o r the s t a i n l e s s s t e e l and 230 MPa f o r t h e copper;

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maximum compressive r a d i a l s t r e s s on t h e i n s u l a t i n g m a t e r i a l t o ground: 50 MPa and maximum t e n s i l e s t r a i n along t h e l a y e r s o f t h e i n s u l a t o r : 2

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maximum shear s t r e s s i n t h e f i b e r g l a s s between turns: 5 MPa.

2.3. Mechanical Tests. The c y c l i c behaviour o f s t e e l a t 77OK, o f copper a t 173OK and 213OK and o f t h e i n s u l a t i n g m a t e r i a l have been i n v e s t i g a t e d using specimens reproducing as w e l l as p o s s i b l e t h e r e a l operating c o n d i t i o n s i n c l u d i n g thermal c y c l i n g /2,3/. The main r e s u l t s o f these t e s t s are:

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no sensible damage i n t h e s t e e l a t working s t r e s s l e v e l ;

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no p l a s t i c deformation o f t h e copper such as t o d e t e r i o r a t e t h e i n s u l a t i n g m a t e r i a l ;

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undetectable mechanical damage and an accettable e l e c t r i c a l degradation a f t e r 2 104 cycles on t h e specimens reproducing t h e c r i t i c a l p o r t i o n o f t h e magnet c o i I s /4/.

Tests o f non l i n e a r f r a c t u r e mechanics on t h e s t e e l have been c a r r i e d o u t a t cryogenic temperature. The m a t e r i a l toughness has been determined a t -180°C using t h e J - i n t e g r a l method /5/. I n Figure 1 t h e J - i n t e g r a l versus t h e crack extension i n c r e - ment Aa i s reported. These curves have been obtained f o r 3 specimens 1TCT Type using the "unloading compliance" method. The JIC values r e l a t e d t o a o o f 956 MPa and t h e corresponding has been assumed v a l i d I n t h e e l a s t i c as w e l l as i n t h e p l a s t i c condition: JKU: values are reported i n Table I. The f o f I 8 S i n g r e l a t i o n CI

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= [ ( l - v Z ) Kfc/E] where v i s t h e Poissonls r a t i o and E i s t h e Young's modulus.

Fig. l

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J - i n t e g r a l versus crack extension TABLE I

*

^Aa^(mm) ³

a f l o w J IC( kN/m) K ~ ~ ( M P ~ J ~ )

Specimen (MPa) mean value mean value

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The crack propagation r a t e da/dN has been t e s t e d u s i n g one specimed lTCT type. A sine shaped load a t 20 Hz w i t h a s t r e s s r a t i o between the minimum and t h e maximum values o f 0 . 1 has been applied. I n Figure 2 t h e crack propagation r a t e versus the s t r e s s i n t e n s i t y f a c t o r range AK i s reported. From t h e experimental r e s u l t s , the f o l l o w i n g P a r i s ' l a w has been obtained: da/dN = 0.217 10-9 (Ak)4 (mm/cycle).

SPEC 1 T = -180

'C

Fig. 2

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Crack growth r a t e versus s t r e s s i n t e n s i t y f a c t o r range.

2.4. Concluding Remarks. Comparing s t r e s s analysis r e s u l t s w i t h experimental data, i t can be argued t h a t t h e t h i s t o r o i d a l f i e l d magnet appears capable of t h e re- quested performance (104 shots a t 8 T). A l a r g e s a f e t y margin appears t o be present even a t a t o r o i d a l f i e l d o f 9 T. As f a r as f r a c t u r e mechanics t e s t s are concerned, t h e s t e e l has a very good toughness b u t a r e l a t i v e l y h i g h crack propagation r a t e . 3. FTU POLOIDAL FIELD SYSTEM

3.1. Poloidal F i e l d Windings. The FTU machines includes f o u r sets o f p o l o i d a l f i e l d windings /3,6/.

a) Ohmic h e a t i n g c o i l s (transformer TR), used t o s t a r t t h e discharge (plasma break- down voltage up t o 40 V), provide most o f t h e f l u x swing (5.1 Vs) r e q u i r e d t o r a i s e and keep f l o w i n g t h e plasma c u r r e n t according t o a preprogrammed waveform. The TR windings have been arranged i n such a way t o g i v e a very low value o f t h e s t r a y

f i e l d (- 50 gauss) i n t h e plasma r e g i o n w i t h a n u l l on the plasma c e n t e r l i n e . b) V e r t i c a l f i e l d windings (VF) whfch provide t h e preprogrammed e q u i l i b r i u m f i e l d (up t o 0,65 T) and c o n t r i b u t e t o t h e f l u x v a r i a t i o n d u r i n g t h e r i s e o f plasma cur- r e n t w i t h 1.3 Vs. With o n l y a couple o f c o i l s i t i s p o s s i b l e t o produce a magnetic f i e l d c o n f i g u r a t i o n , i n the plasma region, compatible w i t h a plasma cross s e c t i o n very close t o c i r c u l a r , d u r i n g pp v a r i a t i o n s up t o pp = 1,8.

c) Plasma h o r i z o n t a l p o s i t i o n feedback windings (F) which, together w i t h a s e t o f simple passive c i r c u i t /3/, are capable o f c o r r e c t i n g h o r i z o n t a l plasma column d i s - placements due t o e r r o r i n preprogramming o r t o plasnta s o f t d i s r u p t i o n s , producing a v e r t i c a l f i e l d up t o

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750 gauss. The F c o i l s are p o s i t i o n e d i n such a way t o have a zero mutual inductance w i t h the plasma. Furthermore, i n order t o n o t p e r t u r b t h e plasma c u r r e n t by magnetic coupling between t h e feedback c o i l s and t h e VF and TR c o i l s , an e x t e r n a l decoupling transformer has been designed t o compensate f o r t h e mutual inductances.

d) Plasma v e r t i c a l p o s i t i o n feedback windings (H) which, together w i t h a s e t of pas- s i v e c i r c u i t s , are capable o f c o r r e c t i n g plasma column v e r t i c a l displacements, producing an h o r i z o n t a l f i e l d up t o f 200 gauss. The p o s i t i o n o f t h e H c o i l s has been chosen t o produce an h o r i z o n t a l f i e l d i n the plasma r e g i o n i n t h e e q u a t o r i a l plane uniform w i t h i n

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2%.

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Cl-156 JOURNAL DE PHYSIQUE

3.3 Loads and Stresses on t h e Windings. Each c o i l has been designed t o stand, by i t s e l f , t h e electromagnetic r a d i a l forces. The maximum t a n g e n t i a l stresses i n each winding have been compuded w i t h o u t t a k i n g i n t o account the cooperation between turns. Table I 1 shows the t a n g e n t i a l stresses f o r each winding and f o r t h e i n n e r , o u t e r and c e n t r a l t u r n . P o s i t i v e values correspond t o t e n s i l e stresses. The a x i a l loads are t r a n s f e r r e d t o t h e t o r o i d a l magnet through t h e mechanical s t r u c t u r e . As a r e s u l t o f a p r e l i m i n a r y i n v e s t i g a t i o n , t h e maximum a x i a l forces on t h e c o i l s under normal and f a u l t operating c o n d i t i o n s are reported i n Table 111. P o s i t i v e values i n - d i c a t e outward bound forces.

TABLE I 1

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Maximum t a n g e n t i a l stresses (kg/mm2) I n n e r r a d i u s Middle radius Outer radius

TABLE I11

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Maximum electromagnetic a x i a l forces on c o i l s (t)

P- P P p - p p p - -

NORMAL OPERATING CONDITIONS FAULT CONDITIONS

REFERENCES

/l/ HELLEN T.K., FLACK V., Berkely Nuclear Laboratories

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Report RD/B/N4057 (1977) /2/ PIZZUTO A., Prcc. o f 1 2 t h Symposium o f Fusion Technology, J u l i c h 13

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^{17 Sep-}

tember 1982

/3/ FRASCATI TOKAMAK UPGRADE

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Associazione Euratom-ENEA, Centro Ricerche Energia F r a s c a t i

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Report 82.49 (1982)

/4/ BEATRICI D., BETTINALI L., RUM1 B., Proc. o f 7 t h I n t . Conf. on Magnet Techno- logy, Karlsrue, 30 March

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3 A p r i l 1981, IEEE Trans May MAG-17 5, (1981) 2305 /5/ ROLFE

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BARSOM, Fracture and Fatigue Control i n Structure-Practice HALL 1977 /6/ GASPAROTTO M., LOVISETTO L., RIGHETTI G.B., Proc. o f 1 2 t h Symposium o f Fusion

Technology, J i i l i c h 13

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17 September 1982.