Study of plasma current start-up of the middle-sized tokamak with force-balanced coils

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 Jc‖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.