MHD activities and related impurity behaviour in JT-60 discharges

An m=3 non-rotating mode has been observed at safety factors slightly below three during the current ramp-up phase. The maximum plasma current of the OH plasma is restricted to approximately 2.4-2.45 MA by disruptions, which are preceded by this locked mode. The locked mode is stabilized by high power NB heating; a maximum plasma current of 3.2 MA has been attained routinely. The characteristics of this locked mode suggest that it is kink-like. A periodic m=1 burst mode similar to a fishbone has been observed. The mode frequency depends on the plasma rotation frequency; it is shown that this result is consistent with the theoretical prediction. Magnetic oscillations with narrow bandwidth at a frequency higher than 10 kHz are excited by NB injection into divertor discharges. A candidate for an instability due to this excitation is discussed. Frequent edge localized modes (ELMs) have been observed in lower X-point discharges, and it is found that frequent ELMs occur in the regime of βp/qeff > or approx. 0.25. (author). 8 refs, 7 figs