Impact of ELM filaments on divertor heat flux dynamics in NSTX

Abstract The ELM induced change in wetted area (Awet) and peak heat flux (qpeak) 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 Awet and lower qpeak. 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 Awet change is rather constant and qpeak change rapidly increases with increasing ELM size, while Awet change slightly increases leading to a weaker increase of qpeak change for ELMs with larger number of striations, both of which are unfavourable trend for the material integrity of divertor tiles.