ELM triggering by high-n field perturbations in tokamak plasmas

The effects of resonant magnetic perturbation (RMP) on edge localized mode (ELM) dominated by ballooning mode instability are studied with the nonlinear magnetohydrodynamic (MHD) code CLT for HL-3-like plasma parameters. Nonlinear MHD simulations reveal that the behavior of ballooning modes at nonlinear saturation is sensitive to the initial perturbation. The dependence arises from the fact that the linear growth rates of the most unstable modes do not differ significantly, with high toroidal mode number (n) modes having similar growth rates. Previous studies have shown that nonlinear coupling between low-n RMP with a single mode and ballooning modes reduces the amplitude of the natural ballooning mode, which is considered an underlying mechanism for mitigation. However, our simulations indicate that High-n sidebands, as generated by realistic RMP coils, can suppress ballooning modes and trigger ballooning modes with the same toroidal mode number as the applied perturbation field. The impact of high-n perturbations on ballooning mode instability is attributed to their role as initial perturbation seeds: compared with the dominant low-n RMP, the high-n sidebands produce weaker modifications to the overall equilibrium topology. The simulation results indicate that, in addition to the nonlinear coupling of the dominant RMP modes with ballooning mode instability, the sidebands of RMP may also contribute to the reduction in the amplitude of ELM bursts.