BOUT++ electromagnetic turbulence simulations of edge plasma dynamics during thermal quench

Damage of plasma facing components during tokamak disruption is a major concern for ITER and other future devices; and yet the underlying physics of disruption hasn’t been fully understood. In this study, BOUT++ six-field turbulence model with flux-driven capability is applied to investigate plasma turbulence and transport dynamics at the tokamak edge region, as well as the divertor power loads during the thermal quench phase of disruption. We find that with excessive particle and power are applied at the pedestal region for a short period of time (~10-20% of stored thermal energy within 0.1-1ms) to mimic the intensive particle and energy outflow from the core during the onset of thermal quench, two transport mechanisms - ExB edge turbulence and stochastic parallel diffusion, play important roles due to giant ELM-like instabilities as a result of rapid pedestal build-up. Enhanced edge turbulence is responsible for the surging divertor heat load and broadened heat flux at the early stage of thermal quench. At the late stage, turbulence induced magnetic fluctuation becomes large enough (>10⁻³) to completely break flux surfaces such that stochastic field-lines can directly connect pedestal top plasma to the divertor target plates or first wall, further impacting divertor heat load.