Fusion Engineering and Design 58–59 (2001) 901–905

Toroidal field coil and central solenoid modernization forGlobus-M tokamak

V.A. Korotkov a,*, V.A. Belyakov a, S.E. Bender a, Yu.G. Krasikov a,E.N. Rumyantsev a, V.F. Soikin a, V.A. Yagnov b

a D.V. Efremo� Scientific Research Institute, STC ‘‘Sintez’’, Metallostroy, So�etsky pr., 1,PO Box 42 St. Petersburg 196641, Russia

b Troitsk Institute for Inno�ation’s and Thermonuclear Researches, Troitsk, Russia

Abstract

The Globus-M spherical tokamak placed in A.F. Ioffe Institute is in operation since March 1999. The Globus-Mtokamak peculiarity is its small aspect ratio plasma properties (R/a=1.5). Major parameters of the Globus-Mtokamak are: plasma major radius 0.36 m, plasma minor radius 0.24 m, toroidal magnetic field 0.5 T, plasma current0.3 MA and pulse length 0.3 s [1]. Manufacturing of toroidal field coils required high accuracy of processing and highquality of contact surfaces. Winding uniformity of the conductor, both between turns, and on layers in the radialdirection was required for the central solenoid. The ability to change a toroidal field coil together with providingaccess to the vacuum vessel manholes without disassembly of the machine is discussed. Calculations indicatingdependence of magnetic flux distribution from central solenoid winding quality are presented. © 2001 Elsevier ScienceB.V. All rights reserved.

Keywords: Globus-M tokamak; Toroidal field coil; Outer limbs; Central solenoid

www.elsevier.com/locate/fusengdes

1. Introduction

The purpose of toroidal field coil moderniza-tion was the maintenance of fast access to thein-vessel components within 1–2 h, that replace-ment or the repair of these elements was possibleduring one working shift. The access to manholesof a vacuum vessel accordingly should be realizedwithout disassembly and removal of diagnostic

stands and without disassembly of hard-to-reachelectrical joints, elements of the mount and watercooling system of the toroidal field coils. Thedetaching of cooling unions, draining of water,detaching of cross-overs and leads of poloidalfield coils, disassembly of clamps and intercoilstructures is allowable only in emergencies. Aunique possibility for fulfillment of the require-ments in conditions of restricted space was theintroduction of the additional joint in a toroidalfield coil.

The magnetic measurements made on the ma-chine have shown, there is an asymmetry inpoloidal field distribution near the central so-

* Corresponding author. Tel.: +7-812-4627-935; fax: +7-812-4644-623.

E-mail address: korotkov@sintez.niiefa.spb.su (V.A. Ko-rotkov).

0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0920 -3796 (01 )00501 -4

V.A. Korotko� et al. / Fusion Engineering and Design 58–59 (2001) 901–905902

lenoid. The calculations show, the given asymme-try is connected with winding heterogeneity of thecentral solenoid turn both on layers and in aradial direction. The possible ways of central so-lenoid modernization are determined.

2. Design principles

2.1. Toroidal field coil

The partial disassembly of machine wasplanned for the access inside the vacuum vessel.In this case, it is necessary to remove the bandagering, upper intercoil structures, upper and lowercoils PF3 and CC3 [2]. After that, the pins in theupper contact unit are taken out and the wedgedevices in the contact joint lock are unclamped.The upper outer limb of the toroidal field coil istaken out and the access is opened to the manholeof the vacuum vessel (Fig. 1).

Fig. 2. Design of the toroidal field coil.

Fig. 1. Old variant of the machine disassembly for the accessto the vacuum vessel manholes.

The toroidal field coil consists of the inner leg,upper and lower outer limbs and contact jointlock. The coil has three joints. The outer limb isbolted to the inner leg at the bottom and attachedwith a special bandage at the top. The upper andlower outer limbs are connected mid-plane seg-ment. This joint is for simplification of the ma-chine disassembly for replacement of the vacuumvessel (Fig. 2).

The problem was to modernize two toroidalfield coils closing the access to the manholes ofthe vacuum vessel. It was supposed to make theouter limbs from three parts with two joints inthese coils. Upper and lower contact zones areunchanged. In this case, disassembly of the ma-chine is reduced only to taking out of the averageouter limbs. The modernized coil consists of theinvariable inner leg and upper, lower and averageouter limbs and two pressure devices for themaintenance of the good contact (Fig. 3). Mod-ernized outer limb is installed in the same way as

V.A. Korotko� et al. / Fusion Engineering and Design 58–59 (2001) 901–905 903

usual. Also it fastens to the inner leg at the topand bottom and has the corbel on the lower outerlimb for the mount to the support. Upper andlower outer limbs together with average outerlimb form two contact units compressed withpressure devices. The pressure device consists ofthe steel clamp with the groove for the outer limb,pressure plate and inner wedge device for fixingseparate parts of the outer limb from each other.The disassembly of the modernized toroidal fieldcoil starts with taking out of the dowel-boltscompressing the pressure device. The pressureplate is taken out and the wedge device fixative onthe average outer limb is unclamped. The averageouter limb is turned from grooves in the steelclamps and is taken out. After that there is apossibility to remove average outer limb (Fig. 4).

Fig. 4. Design of the modernized toroidal field coil.

Fig. 3. New variant of the machine disassembly for the accessto the vacuum vessel manholes.

2.2. Central solenoid

Operation of the machine magnets includingcentral solenoid was simulated during design [3].Distribution of the magnetic flux from the ‘theo-retical’ central solenoid is exhibited (Fig. 5). Themagnetic measurements were carried out afterstarting of the machine. The measurements haveshown, that the measured distribution of the mag-netic flux in the central solenoid (Fig. 6) differsfrom distribution of the magnetic flux in ‘theoret-ical’ central solenoid. The distortion of magneticlines of force is visible in the area with coordi-nates R=0.1 m, Z= +0.2 m. It was decidedafter discussion and check of possible reasons ofthe given distortion that the main reason of theasymmetry in distribution of the magnetic flux(magnetic field) is a discontinuity of the centralsolenoid winding. Different variants of the devia-tions of the central solenoid from the ‘theoretical’form were considered. The segmented model of

V.A. Korotko� et al. / Fusion Engineering and Design 58–59 (2001) 901–905904

the ‘heterogeneous’ central solenoid was con-structed. It was divided into parts with differentlength and forms. The distribution of the mag-netic flux in such a model coincides with mea-sured one with good accuracy (Fig. 7). The strongheterogeneity is explained by an offset of theturns group of the central solenoid in axial andradial directions.

The distribution of poloidal fields under opera-tion with major currents (Fig. 8) was determinedfrom the model of the ‘heterogeneous’ solenoid.The asymmetry in distribution of poloidal fieldscan cause complexities in organization of thebreakdown region during magnetization reversal.In this case, correction of the given asymmetryand modernization of the central solenoid is prob-ably required. There are some ways for centralsolenoid modernization. At first, the upper het-

Fig. 6. Distribution of the magnetic flux from the real centralsolenoid (It is constructed by results of measurements).

Fig. 5. Distribution of the magnetic flux from the ‘theoretical’central solenoid.

erogeneous layer of the central solenoid can berewound. Secondly, the central solenoid can bemade using existing technology and a previoustechnological device. Large cross-section of theconductor and small radius of winding changesthe cross-section form without the use of thesemethods. The resulting sharp edges influence thedurability of insulation. Besides, it is necessary toincrease tensile force of the conductor for preven-tion of cross-section turning. It can result in insu-lation failure also. The central solenoid windingusing a conductor of decreased cross-section isoptimal. In this case the force of tension of theconductor is reduced at winding, the cross-sectionturning and distortion of its form is decreased.However, this variant results in decrease of thecoefficient of fullness and to increase of the turns.It will demand the coordination and moderniza-

V.A. Korotko� et al. / Fusion Engineering and Design 58–59 (2001) 901–905 905

Fig. 7. The rated distribution of the magnetic flux fromconstructed model of the ‘heterogeneous’ central solenoid.

Fig. 8. The rated distribution of the poloidal fields fromconstructed model of the ‘heterogeneous’ central solenoid inthe breakdown region with magnetization reversal.

tion of the power supply. A variant with simulta-neous winding two or four of the conductors inparallel is most optimal. In this case, the formdistortion of the conductor and cross-sectionturning is reduced without change of the turnsand the coefficient of fullness. The solution of themethod of modernization will be selected after thepresent operation period. Special attention tomanufacturing of the new central solenoid shouldbe given to the manufacturing technology andwinding quality control.

3. Conclusions

(1) The conducted modernization of thetoroidal field coil allows quick access to the in-vessel components and reduces time of the ma-chine disassembly.

(2) The conducted calculations and measure-ments of the magnetic field distribution of thecentral solenoid show the reasons for the poloidalfield heterogeneity of the machine.

(3) The possible ways of the central solenoidmodernization of the machine are scheduled.

References

[1] A.B. Alekseev, et al., Globus-M tokamak magnets, in:Proceedings of the 19th Symposium on Fusion Technol-ogy, Lisbon, Portugal, 1996, vol. 1, pp. 829–832.

[2] V.A. Belyakov, et al., Spherical tokamak Globus-M loadassembly, in: Proceedings of the 20th Symposium on Fu-sion Technology, Marseille, France, 1998, vol. 2, pp.1717–1720.

[3] V.K. Gusev, et al., Central solenoid for spherical tokamakGlobus-M, Fusion Technol. 34 (2) (1998) 137–146.