Fusion Engineering and Design 51–52 (2000) 1059–1064 Study of plasma current start-up of the middle-sized tokamak with force-balanced coils Tomoyuki Murakami a, *, Yasuhiro Komatsu b,1 , Hiroaki Tsutsui b,2 , Shunji Tsuji-iio b,3 , Ryuichi Shimada b,4 , Satoru Murase a,5 , Susumu Shimamoto a,6 a Department of Electrical Engineering, Graduate School of Engineering Uniersity, Aoba -yama 05, Sendai 980 -8579, Japan b Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2 -12 -1, O-okayama, Meguro -ku, Tokyo 152 -8550, Japan Abstract For a high field tokamak device, we have developed force-balanced coils (FBCs) which have nonuniform-pitch multi-pole helical-configuration to reduce centering electromagnetic forces without using cancellation coils. As a superconductivity magnet, the FBC configuration is favorable to the achievement of high critical current densities because of a J c B effect. In the tokamak operation with the FBCs, the rise of the toroidal field synchronizes with the plasma current ramp up because the FBCs function both as toroidal field coils (TFCs) and as primary coils for ohmic heating. The torsional force exerted on the FBCs is comparable and the net centering force is reduced to 20% compared with those on conventional TFCs of the same dimensions. © 2000 Elsevier Science B.V. All rights reserved. Keywords: High magnetic field; Tokamak device; Force balance technique www.elsevier.com/locate/fusengdes 1. Introduction Since high magnetic field is attractive for raising the performance of magnetic confinement fusion devices and superconducting magnet energy stor- age systems, concepts for force reduction configu- rations against a significant Lorentz force exerted on the magnets themselves have been investigated [1–3]. The electromagnetic forces on the coil ca- bles are caused by an interaction between the currents in the windings and the magnetic fields induced by them. To vanish the Lorentz force in the minor radius direction and raise critical cur- rents of the superconducing coils, a multi-layer winding where the current flows parallel to the magnetic field vector have been proposed by Furth et al. [1]. A theoretical analysis of force balanced magnetic energy storage coils have been conducted by Eyssa et al., where it has been * Corresponding author. Tel.: +81-22-2177116; fax: +81- 22-2639161. E-mail address: murakami@es.titech.ac.jp (T. Murakami). 1 Tel.: +81-3-57343328; fax: +81-3-57342959. 2 Tel.: +81-3-57343328; fax: +81-3-57342959. 3 Tel.: +81-3-57343328; fax: +81-3-57342959. 4 Tel.: +81-3-57343328; fax: +81-3-57342959. 5 Tel.: +81-22-2177116; fax: +81-22-2639161. 6 Tel.: +81-22-2177116; fax: +81-22-2639161. 0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S0920-3796(00)00180-0