Divertor peak heat flux reduction induced by ELM filaments in NSTX

The occurrence of small ELMs with high (> 4 MW) neutral beam injection is observed to decrease the divertor peak heat flux in NSTX. Small ELMs are routinely observed in many discharges, resulting in < 50% transient increase in deposited power on the divertor. ELM filaments are observed with these small ELMs. The ELM filaments observed from divertor imaging are consistent with magnetic perturbations and filament rotation from the Mirnov diagnostic. The ELM peak heat flux decreases with the number of ELM filaments, and the divertor heat flux width increases with the number of ELM filaments. The divertor peak heat flux decreases during the ELM rise time with two or more ELM filaments. With one or no ELM filament, the divertor heat flux increases during the ELM rise time. Compared with the ELM-free phases in discharges, the integral power decay width (λ_int) at the ELM peak time can reach ~ 300% larger than in the ELM-free phase. Broadening of the heat flux footprint makes the divertor peak heat flux at the ELMs peak <40% of the divertor peak heat flux in ELM-free periods. The divertor heat flux width during these small ELMs and inter-ELM periods does not decrease with the plasma current.