Distinctive dependence of RMP-ELM coupling on plasma shape, and its optimization for robust ELM control in KSTAR

In recent KSTAR experiments, a critical dependence of RMP-ELM coupling on plasma shape was found to be as important as q$_{\mathrm{95}}$. In application of low-n RMPs, small variations of lower triangularity ($\delta _{\mathrm{Lower}})$ made dramatic changes on RMP coupling. Specifically, as $\delta_{\mathrm{Lower}}$ increases, the coupling to plasma core appears weakened, while the edge coupling gets strengthened effectively for ELM suppression. So far, such $\delta_{\mathrm{Lower}}$ dependence window for RMP-ELM suppression is surprisingly narrow ($\Delta \delta _{\mathrm{Lower}}=$0.08), while the other shape dependence is unclear. In 2017, a further investigation reveals that such a strict condition can be relaxed by allowing a small up-down asymmetry on plasma shape. Applying this new optimized plasma shape made substantial improvements on the reliability and robustness of RMP-ELM control. As results, the ELM suppressions were successfully demonstrated in a wide range of q$_{\mathrm{95}}$ (4.0-6.4 for n$=$1 and 3.3-3.8 for n$=$2 RMPs) even with a fixed RMP phasing, achieving a record-long sustainment of ELM-suppression (\textgreater 30sec).