MHD activity driven by NBI in the W7-AS stellarator

The m = 2, n = 1; m = 3, n = 1 resonant modes have been previously observed in the low-frequency band of 1.5 kHz by the soft-X diodes in W7-AS during iota scans about the resonant values i = 0.5 and i = 0.33. A different kind of resonance observed in the high-frequency range 20-40 kHz by the Mirnov coils and the soft-X diodes in low-iota operation (i(a) = 0.34) of W7-AS during NBI heating is shown. The most remarkable feature of this new effect (apart from the high frequency) is its correlation with the NBI operation. It was not always observed during NBI, but whenever it was observed, it always disapeared on a time scale of 0.5 msec imediately after the NBI was switched off, although the plasma was sustained for a further 10-20 msec. (orig.)