Fast Turbulence Phase Transition in a Flux-Driven Global Edge-SOL Simulation of a Tokamak Plasma.

A power ramp from L- to H-mode-like conditions is carried out in a 50 ms long global full-f turbulence simulation of ASDEX Upgrade with the grillix code. After 32 ms of slow profile evolution, a sudden change in transport triggers a pedestal buildup within ∼100  μs. This transition results from an abrupt shift from drift-wave (DW) to kinetic-ballooning-mode (KBM) turbulence. The self-consistently evolved E×B flow shear first increases slowly and then sharply. Geodesic acoustic mode and Alfvénic flow oscillations intensify prior to the transition and subsequently die out. This is followed by slow oscillations of the transport and profiles, reminiscent of a dithering I-phase or edge-localized-mode (ELM) cycles observed experimentally. This study paves the way for a better understanding of confinement regime transitions.