Penetration of n = 2 resonant magnetic field perturbations in EAST

摘要

This paper presents the penetration of n = 2 magnetic field perturbations, where n is the toroidal mode number. The n = 2 intrinsic error field (IEF) is measured in an ohmic heating plasma using the compass scan method, i.e. the toroidal asymmetry in the threshold current for the penetration of n = 2 resonant magnetic perturbations (RMPs). Its amplitude is 55.5 A in equivalent coil current or B_(r,3/2) = 0.1 G and the toroidal phase of the IEF is around 170.6° (129°). Phasing scans (scans of the phase difference between the upper and lower coil currents) of the n = 2 RMPs are carried out to obtain the effects of the n = 2 spectrum on field penetration. The observed dependence of the field penetration on the spectrum is consistent with those of simulations using the MARS-F code. One of the interesting phenomena is that then = 2 mode often stimulates an n = 1 mode. The dominant poloidal harmonic of the n = 1 mode is m = 2, and the dominant poloidal harmonic of the n = 2 mode is m = 3. The evolution of the n = 1 mode has two stages, i.e., an initial small island growth stage, and a later saturation stage. In the initial stage, the amplitude of the n = 1 magnetic island grows, while the phase remains fixed. When the amplitude of the magnetic island exceeds a certain threshold, it enters the second stage, in which the magnetic island is locked into another phase and its amplitude starts to saturate. The phase in the initial small island stage depends linearly on the phase of the applied n = 2 RMP, which suggests that the n = 2 mode is directly driven by the coupling between the n = 1 and n = 2 modes. The phase in the second stage is either locked to the phase close to the previously measured n = 1 IEF, or is locked to the phase close to the n = 2 response field. This suggests that the final phase of the n = 1 mode depends on competition between the locking effect induced by the n = 1 IEF and the nonlinear coupling. effect between the two modes. This might be an issue in the MHD control application using high-n RMPs in the future ITER device.