Influence of n = 4 RMPs on pedestal structure and stability in EAST

Changes of pedestal structure and stability caused by n = 4 resonant magnetic perturbations (RMPs) for control of Edge Localized Modes (ELMs) in the EAST tokamak in a target plasma close to the ITER high-Q scenario are studied in this paper. Here, n is the toroidal mode number of RMPs. It is shown that plasma pedestal density profiles are significantly changed at different stages, i.e. ELM mitigation and suppression, during the application of n = 4 RMPs, compared to the phase without RMPs. The pedestal top density decreases while the separatrix density slightly increases, leading to a reduction in the pedestal density gradient at the stage of ELM mitigation. There is a further sudden increase of separatrix density and hence drops of pedestal density gradient after accessing to ELM suppression, which is consistent with the picture that a nonlinear bifurcation in edge magnetic topology at the transition from ELM mitigation to suppression. The decrease in pedestal top density and pedestal density gradient causes the reduction of edge pressure gradient and bootstrap current. Stability analysis using the ELITE code shows that the dominant modes in the phase without RMP are low-n peeling-ballooning modes (PBMs). The growth rate of the low-n PBMs decrease with decreasing edge pressure gradient and bootstrap current, which is consistent with the observation of ELM mitigation and suppression during the application of RMPs. Different effects from edge plasma toroidal rotation, temperature, separatrix density etc on the stability of PBMs are also investigated via numerical modelling based on the reference case in EAST, which could be helpful for the optimization of operational window for accessing to ELM suppression in the future studies.