Nuclear Fusion (Jan 2024)
Origin of the up/down poloidal asymmetric dependence of ELM control in ITER-like RMP configuration in KSTAR
Abstract
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 q _95 $ \gtrsim $ 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 q _95 ∼ 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 q _95 ∼ 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 q _95 , 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 q _95 ∼ 4 and (ii) symmetric behavior at q _95 ∼ 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 ( ξ _X ), 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 ξ _X variation follows the same quantitative trend as observed in experiments. However, no or little plasma pressure dependence has been found, though ξ _X increases with plasma pressure. At the ITER-relevant low q _95 ∼ 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.
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