Impact of ELM filaments on divertor heat flux dynamics in NSTX

Abstract The ELM induced change in wetted area (A_wet) and peak heat flux (q_peak) of divertor heat flux is investigated as a function of the number of striations, which represent ELM filaments, observed in the heat flux profile in NSTX. More striations are found to lead to larger A_wet and lower q_peak. The typical number of striations observed in NSTX is 0-9, while 10-15 striations are normally observed in other machines such as JET, and the ELM contracts heat flux profile when the number of striations is less than 3-4 but broadens it with more of them. The smaller number of striations in NSTX is attributed to the fact that NSTX ELMs are against kink/peeling boundary with lower toroidal mode number (n = 1-5), while typical peeling-ballooning ELMs have higher mode number of n = 10-20. For ELMs with smaller number of striations, relative A_wet change is rather constant and q_peak change rapidly increases with increasing ELM size, while A_wet change slightly increases leading to a weaker increase of q_peak change for ELMs with larger number of striations, both of which are unfavourable trend for the material integrity of divertor tiles.