Large m=3/n=l locked mode in JT-60 and its stabilization

An m=3/n=l locked mode has been observed in JT-60 at safety factors slightly below three during the current ramp-up phase of high plasma current discharges. The locked mode grows suddenly and there is no Mirnov oscillation just before the onset of this mode. The maximum plasma current of Ohmic discharges is restricted to approximately 2.4 — 2.45 MA by disruptions, which are preceded by a large m=3/n=l locked mode. The locked mode is stabilized by high power neutral beam injection. The stabilizing effect depends on neutral beam heating power, plasma density, plasma current ramp-up rate and divertor/limiter configuration.