Two-stage crash process in resistive drift ballooning mode driven ELM crash

Abstract

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 with a scale-separated four-field reduced MHD (RMHD) model using the BOUT++ framework. In the early nonlinear phase, the small first crash is triggered by linearly unstable RDBMs, and m/n=2/1 magnetic islands are nonlinearly excited by nonlinear coupling of RDBMs as well as their higher harmonics. Here, m is the poloidal mode number, n is the toroidal mode number, the q = 2 rational surface exists near the pressure gradient peak, and q is the safety factor. Simultaneously, middle-n RDBM turbulence develops but is poloidally localized around X-points of the magnetic islands, leading to the small energy loss. The second large crash occurs in the late nonlinear phase. Higher harmonics of m/n=2/1 magnetic islands well develop around the q = 2 surface via nonlinear coupling and make the magnetic field stochastic by magnetic island overlapping. Turbulence heat transport develops at X-points of higher harmonics of m/n=2/1 magnetic islands, resulting in the turbulence spreading in the poloidal direction. The large second crash is triggered when the turbulence covers the whole poloidal region so that the magnetic island generation and magnetic field stochastization before the large crash can be interpreted as ELM precursors. It is concluded that the ELM trigger is attributed to the turbulent spreading in the poloidal direction in synchronization with the magnetic field stochastization and the crash is driven by E × B convection rather than the conventional Rechester–Rosenbluth anomalous electron heat transport.

Nonlinear simulations of the peeling-ballooning instability of super H-modes in the HL-3 tokamak

“…Moreover, only n = 0 mode of pressure perturbation are included in the simulation. As illustrated in [33], at the second crash in the ELM crash process, the zonal pressure and zonal flow play dominant roles in determining the ELM size. This part is not well considered in our research.…”

“…Indeed, the ELM size is closely correlated with the saturation level. The nonlinear interaction including n = 0 generation will be essential to determine the ELM size [33,44]. Further study may be executed according to the optimized model in [44], which can simulate the full annular tokamak with the consideration of n = 0 mode for ϕ and A ∥ better.…”