Investigating the Role of ELMs in Triggering RMP ELM Suppression*

H-mode pedestal profiles early in the inter-ELM phase are used to investigate if ELMs trigger RMP ELM suppression in ITER-like DIII-D discharges. Plasma response simulations are usually made at times just before the ELM crash, but recent theory [Callen, UW-CPTC 16-4] suggests that the pedestal modifications due to an ELM crash trigger ELM suppression. Testing this model requires profile and equilibrium analysis just after an ELM crash as input for plasma response simulations. Measurements suggest that this correlation may be fortuitous. Applying the RMP in an ELMing H-mode starts a slow pedestal evolution taking 10s-100s of ms to ELM suppression through a phase with higher frequency, smaller size “mitigated” ELMs. Fast measurements during these mitigated ELMs show that this slow evolution is robust against fast transient pedestal changes (≤5ms) due to the remaining ELMs. ELM suppression has also been obtained by applying the RMP in L-mode, with the discharge transitioning directly to an ELM suppressed H-mode without ever ELMing, suggesting that an ELM crash isn’t a necessary condition for ELM suppression.