Broadening the SOL width with small ELMs

The pulsed heat load due to large ELMs is an existential problem for future devices of ITER size and compact FPPs because that load would produce unacceptable damage to the divertor plates. On the other hand, the divertor heat flux width for inter-ELMs is too narrow as predicted by the heuristic drift-based (HD) model and an experimental (Eich) scaling due to the suppressed turbulence transport in typical H-mode plasmas. The research for small ELMs becomes important because of following desired features: (1) Good energy confinement; (2) Comparable with inter-ELM level of divertor heat flux; (3) Broadening divertor heat flux due to larger turbulent fluxes ejected from the pedestal into the SOL; (4) Compatible with divertor plasma detachment operation; and (5) Quasi-continuous particle and power exhaust. In this presentation, we will briefly overview recent small ELM experimental results from multiple ITPA tokamaks and theory and simulations on current physics understanding of small ELMs. In order to identify the key drivers of small ELM instabilities, such as ideal, or resistive, or drift, or kinetic peeling-ballooning modes, we will compare simulations results from BOUT++ two-fluid and gyro-Landau-fluid (GLF) models for ITER 15 MA baseline scenario and 10 MA steady-state operation scenario. From nonlinear simulations, we will also dicuss the connection between small ELMs and marginal peeling-ballooning instabilities.