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.
摘要：本文研究了在EAST托卡马克中，针对接近ITER高Q场景的目标等离子体，n=4共振磁扰动（RMPs）用于控制边缘局域模（ELMs）时，台基结构及稳定性的变化。这里，n是RMPs的环向模数。结果表明，与无RMP阶段相比，在施加n=4 RMPs期间，等离子体台基密度在不同阶段（即ELM缓解和抑制阶段）发生显著变化。在ELM缓解阶段，台基顶部密度降低，而分界面密度略有增加，导致台基密度梯度减小。在进入ELM抑制后，分界面密度进一步突然增加，台基密度梯度随之下降，这与边缘磁拓扑在从缓解到抑制转变时发生非线性分叉的图像一致。台基顶部密度和台基密度梯度的降低引起了边缘压力梯度和自举电流的减小。使用ELITE代码的稳定性分析表明，在无RMP阶段，主导模为低n的剥离-气球模（PBMs）。随着边缘压力梯度和自举电流的减小，低n PBMs的增长率降低，这与RMPs施加期间ELM缓解和抑制的观测结果一致。此外，基于EAST参考案例的数值模拟还研究了边缘等离子体环向旋转、温度、分界面密度等因素对PBMs稳定性的不同影响，这些结果可能有助于未来研究中优化实现ELM抑制的运行窗口。