Study of slowing down mechanism of locked-mode-like instability in helical plasmas

The mode slowing down mechanism of the locked-mode-like instability without a large magnetic island is investigated, based on the LHD experimental analysis. The mode frequency coincides with E  ×  B rotation frequency at the resonant surface and the slowing down is caused by two processes. One is the resonant surface moving to the small E  ×  B rotation frequency region and the other is the slowing down of the E  ×  B rotation frequency near the resonant surface. Both processes are almost the same as those of the locked-mode-like instability with a large magnetic island. The results suggest that the slowing down occurs even though the precursor does not have a large magnetic island. However, when the external RMP is imposed, the mode frequency in the slowing down phase sometimes does not coincide with the E  ×  B rotation frequency. Moreover, the mode amplitude during the slowing down phase increases with the decrease of the mode frequency both with and without the imposed external RMP, which suggests that the instability growth in the slowing down phase is more strongly related to the mode frequency than the E  ×  B rotation frequency because the mode sometimes does not rotate with the E  ×  B rotation.