Origin of the up/down poloidal asymmetric dependence of ELM control in ITER-like RMP configuration in KSTAR

Recent edge-localized modes (ELM) control experiments via resonant magnetic perturbation (RMP) in KSTAR have shown a strong up/down poloidal asymmetric coupling dependence. Specifically, in lower single null (LSN) plasmas at q95 5, the lower two-row (middle/bottom) RMPs among ITER-like three-row (top/middle/bottom) in-vessel control coils (IVCC) in KSTAR were more effective in suppressing ELM-crashes than the upper two-row (top/middle) RMPs. In contrast, at q95 ∼ 4, the upper two-row RMPs turned out to be more effective than the lower counterpart. Since the ITER baseline scenario is planned to operate at q95 ∼ 3, the understanding of the origin of such up/down poloidal asymmetric coupling dependence, as well as the prediction about ITER-relevant conditions at a lower q95, would be quite important and potentially impactful to the RMP ELM control in ITER. A linear, resistive, single-fluid MHD code MARS-F has been utilized to address and model the up/down poloidal asymmetric RMP coupling dependence. Specifically, based on two contrasting exemplary discharges with up/down poloidal (i) asymmetric at q95 ∼ 4 and (ii) symmetric behavior at q95 ∼ 5, among tens of otherwise similar discharges, a systematic MARS-F modeling has been thoroughly conducted. As a result, the plasma response investigation suggests that the X-point displacement (ξX), rather than any other figures of merit, would be a directly relevant metric for the up/down poloidal asymmetric coupling in RMP-driven, ELM-crash suppression in KSTAR. Based on a sensitivity study of the edge safety factor in MARS-F modeling, the ξX variation follows the same quantitative trend as observed in experiments. However, no or little plasma pressure dependence has been found, though ξX increases with plasma pressure. At the ITER-relevant low q95 ∼ 3 in a scaled KSTAR equilibrium, such modeling predicts the upper two-row RMPs would be more favorable in suppressing the ELM-crashes than the lower counterpart.