Observation of resonant tearing mode induced by energetic-ion redistribution due to sawtooth collapse in HL-2A NBI plasmas

Abstract

Recent observations in HL-2A tokamak give new experimental evidence of resonant tearing modes (resonant-TMs) caused by the re-distribution of energetic ions (EIs) due to sawtooth collapses in NBI plasmas. The m/m=2/1 (m and n are the poloidal and toroidal mode numbers) resonant-TMs with frequencies chirping down rapidly are found after the strong busts of fishbones and closely followed sawtooth collapses. The frequencies of fishbones and resonant-TMs chirp down from 10 to 7 kHz and 5 to 2 kHz rapidly, which are higher and lower than the 4-6 kHz existing weak TMs near q=2 (q is the safety factor) rational surface. The fishbones and existing TMs propagate in ion and electron diamagnetic directions, respectively. In contrast, the resonant-TMs propagate in electron diamagnetic directions. This suggests that the counter-direction EIs, which generated by the redistribution of EIs due to sawtooh collapses, excite the resonant-TMs. The simulation results from M3D-K code show that the counter-passing EIs play an important role on the excitation of resonant-TMs. The results can help broadening and understanding the interaction mechanism between energetic particles and TMs.