Characteristics of EAST grassy ELMs and its impact on the divertor heat flux width*

For a 60s steady-state long pulse high β_p EAST grassy ELM discharge, BOUT++ linear simulations show that the unstable modes cover a range from low-n (n=10~15) with characteristics of peeling-ballooning modes (P-B) to high-n (n=40~80) modes driven by drift-Alfvén instabilities (DAI). Even though the DAI dominate the linear growth phase with a wide n-spectrum and the fluctuation peaks on high-field side, nonlinear simulations show that the ELM crash is trigged by P-B modes on low-field side while the DAI delays the onset of the ELM and enhances the energy transport. The fluctuation amplitude drops by an order of magnitude and the ELM crashes disappear if the peeling drive is removed. The temporal evolution of the power loading shows no obvious decay from the maximum of the ELM power pulse after the onset of the ELM power and the elm size is small (< 2%). The turbulence thermal diffusivity is larger than the critical value indicating grassy ELM falls into the turbulence dominated regime. The divertor heat flux width is 2 times larger than the estimates based on the HD model and the Eich’s ITPA multi-tokamak scaling. The heat flux width is inversely proportional to Er-shear due to the enhancement of the SOL parallel transport and suppression of radial transport.