Observation of the m = 1 mode during the ramp phase of the sawtooth oscillations in LHCD plasmas on the HT-7 tokamak

A new oscillation with m = 1 and n = 1, where m and n are the poloidal and toroidal mode numbers, respectively, has been observed with lower hybrid current drive (LHCD) on the HT-7 tokamak. This mode appears during the sawtooth ramp, saturates, damps and finally disappears before the subsequent sawtooth collapse and before the appearance of the precursor oscillations. The necessary condition for its observation is that the sawtooth period exceeds 6–7 ms and that the LHCD power exceeds the threshold value of about 90 kW. We propose that the mode may be destabilized by the current density gradient, the evolution of which is determined by the suprathermal electrons generated by the LHCD.