Turbulence Spreading Effects on the ELM Size and SOL Width

BOUT++ turbulence simulations were performed to investigate the impact of turbulence spreading on the edge localized mode (ELM) size and divertor heat flux width (lambda(q)) broadening in small ELM regimes. This study is motivated by EAST experiments. BOUT++ linear simulations of a pedestal radial electric field (E-r) scan show that the dominant toroidal number mode (n) shifts from high-n to low-n, with a narrow mode spectrum, and the maximum linear growth rate increases as the pedestal E-r well deepens. The nonlinear simulations show that as the net ExB pedestal flow increases, the pressure fluctuation level and its inward penetration beyond the top of the pedestal both increase. This leads to a transition from small ELMs to large ELMs. Both inward and outward turbulence spreading are sensitive to the scrape-off-layer (SOL) plasma profiles. The inward turbulence spreading increases for the steep SOL profiles, leading to increasing pedestal energy loss in the small ELM regime. The SOL width (lambda(q)) is significantly broadened progressing from the ELM-free to small ELM regime, due to the onset of strong radial turbulent transport. The extent of the SOL width (lambda(q)) broadening depends strongly on outward turbulence spreading. The fluctuation energy intensity flux Gamma(epsilon) at the separatrix can be enhanced by increasing either pedestal Er flow shear or local SOL pressure gradient. The lambda(q) is broadened as the fluctuation energy intensity flux Ge at the last close flux surface (LCFS) increases. Local SOL ExB flow shear will restrain outward turbulence spreading and the associated heat flux width broadening. Operating in H-mode with small ELMs has the potential to solve two critical problems: reducing the ELM size and broadening the SOL width.