Two-stage Crash Process in Resistive Drift Ballooning Mode Driven ELM Crash
We report a two-stage crash process in edge localized mode (ELM) driven by resistive drift-ballooning modes (RDBMs) numerically simulated in a full annular torus domain. In the early nonlinear phase, the first crash is triggered by linearly unstable RDBMs and m/n = 2/1 magnetic islands are nonlinearly excited via nonlinear couplings of RDBMs. Simultaneously, middle-n RDBM turbulence develops but is poloidally localized around X-points of the magnetic islands, leading to the small energy loss. Here m is the poloidal mode number, n is the toroidal mode number, the q = 2 rational surface exists at the pressure gradient peak, and q is the safety factor, respectively. The second crash occurs in the late nonlinear phase. Low-n magnetic islands are also excited around the q = 2 surface via nonlinear couplings among the middle-n turbulence. Since the turbulence develops from the X-points of higher harmonics of m/n = 2/1 magnetic islands, it expands out poloidally. The second crash is triggered when the turbulence covers the whole poloidal region. A scan of toroidal wedge number N, where full torus is divided into N segments in the toroidal direction, also reveals that the first crash process becomes more prominent with the higher toroidal wedge number where the RDBMs play a dominant role. These results indicate that nonlinear interactions of all channels in the full torus domain can significantly affect the trigger dynamics of ELMs driven by the RDBMs.