Role of multi-scale MHD/turbulence in pedestal stability and transport of wide-pedestal QH-mode*

Wide-pedestal QH-mode discovered on DIII-D is an attractive scenario for future fusion reactors as it exists with low edge rotation and good H-mode confinement without ELMs. Two experimental observed scale-separated MHD/turbulence modes are identified in BOUT++ reduced MHD model linear and nonlinear simulations: a) low frequency, low k peeling mode rotates in the ion diamagnetic drift direction; b) higher frequency, intermediate-high k electron drift wave propagates in the electron diamagnetic drift direction. In the experiments it was observed that ELMs could occur in wide-pedestal QH-mode with an increase of the amplitude of the low k mode and a decrease of the intermediate-high k mode. This indicates that the intermediate-high k electron drift wave could have an impact on the low k peeling mode and thus an impact on the ELM dynamics. BOUT++ simulation with only low k initial perturbation leads to ELM crash as linear peeling-ballooning theory predicted; meanwhile the all-scale simulation forms a "turbulence mixed" state with no ELM crash, which indicates a novel nonlinear criterion for the onset of ELMs. This work presents improved physics understanding on the connection between multi-scale MHD/turbulence and the existence of wide-pedestal QH-mode.