Effective control of intrinsic impurities using n = 1 resonant magnetic perturbation (RMP) in EAST H-mode plasma

Abstract

Since 2021, EAST tokamak has been operated with full tungsten divertors. Tungsten accumulation has beenfrequently observed in NBI-heated H-mode discharges, resulting in the degradation of plasma confinementperformance. Control of tungsten impurity is thus critical for the maintenance of high-confinement plasmas. Inthis work the impact of the n = 1 resonant magnetic perturbation (RMP) on the behavior of intrinsic low- andhigh-Z impurities in EAST H-mode discharges are experimentally studied, utilizing high-performance extremeultraviolet spectroscopic diagnostics. In the dedicated discharge, ELM mitigation, ELM suppression, H-L backtransition, RMP penetration occurs in succession with increasing RMP current (IRMP). When IRMP is below thethreshold for H-L back transition, IRMP_H-L = 2.29 kA, increasing influx of C2+ and C3+ ions and decreasing influxof C4+ and C5+ ions are observed simutaneously with enhancement of the RMP field. This opposite time behaviorin the influx of C4+ and C 3+ ion is then observed to be magnified during the RMP penetration phase. It indicates aimpurity screening layer formed between the locations where C4+ and C3+ ions distribute during RMP appli­cation based on our previous analysis (W.M. Zhang et al 2024 Nucl. Fusion 64 086004). A large step of increase inC4+ influx after H-L back transition indicates C4+ ion mainly located at bottom of pedestal. A higher RMP coilcurrents threshold capable of impurity screening is found for high-Z impurity ions of Cu25+, Mo30+, W42+, i. e.0.53–0.75 kA, than that for C4+ and C5+, i. e. 0.33 kA. Meanwhile, it is found that comparing to C4+ and C5+ ionsthe decontamination effect by this impurity screening layer is more efficient for these high-Z impurity ions inplasma core region, e.g. up to 70 % reduction in the impurity density, leading to a significant reduction of ra­diation power. Furthermore, the continuous reduction of core high-Z impurities level both in ELM mitigation andsuppression phase proved that this impurity decontamination effect by RMP field is dominant over the impact ofELM activity to core high-Z impurities transport since tungsten is frequently observed to accumulate duringoriginal ELM-free phase. Experimental results from this work would contribute to further understanding of theunderlying mechanism how the RMP field impacts the impurity transport.