Suppression and mitigation of inter-ELM high-frequency Alfvén-like mode by resonant magnetic perturbation in EAST

Suppression and mitigation of a high-frequency Alfvén-like mode (HFAM) between type-I edge localized modes (ELMs) during ELM mitigation by resonant magnetic perturbation (RMP) is observed for the first time in the EAST tokamak. This mode is located near the edge pedestal region. The modeling result of the Alfvén continuum shows that the HFAM is located near the elipical Alfvén eigenmode (EAE) gap. During the application of n = 1 RMP for ELM mitigation, the HFAM can be fully suppressed when the RMP amplitude exceeds a threshold, below which the HFAM is mitigated. The suppression is caused by a reduction of pedestal height induced by RMP. In the case using n = 3 RMP, the mode is localized toroidally at specific phase depending on the phase of applied RMP, i.e. locked in the three-dimensional equilibrium formed by RMP. The dominant toroidal mode number of HFAM is around n = −6 and it reduces to −3 during the application of n = 3 RMP, which indicates the existence of possible nonlinear coupling between the HFAM and RMP. Here the negative mode number denotes that the mode rotates in electron diamagnetic drift direction. The observation reported here improves the understanding of pedestal dynamics and its stability in RMP ELM control.