Nuclear Materials and Energy (Dec 2024)
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 been frequently observed in NBI-heated H-mode discharges, resulting in the degradation of plasma confinement performance. Control of tungsten impurity is thus critical for the maintenance of high-confinement plasmas. In this work the impact of the n = 1 resonant magnetic perturbation (RMP) on the behavior of intrinsic low- and high-Z impurities in EAST H-mode discharges are experimentally studied, utilizing high-performance extreme ultraviolet spectroscopic diagnostics. In the dedicated discharge, ELM mitigation, ELM suppression, H-L back transition, RMP penetration occurs in succession with increasing RMP current (IRMP). When IRMP is below the threshold for H-L back transition, IRMP_H-L = 2.29 kA, increasing influx of C2+ and C3+ ions and decreasing influx of C4+ and C5+ ions are observed simutaneously with enhancement of the RMP field. This opposite time behavior in the influx of C4+ and C3+ ion is then observed to be magnified during the RMP penetration phase. It indicates a impurity 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 in C4+ influx after H-L back transition indicates C4+ ion mainly located at bottom of pedestal. A higher RMP coil currents 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+ ions the decontamination effect by this impurity screening layer is more efficient for these high-Z impurity ions in plasma 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 and suppression phase proved that this impurity decontamination effect by RMP field is dominant over the impact of ELM activity to core high-Z impurities transport since tungsten is frequently observed to accumulate during original ELM-free phase. Experimental results from this work would contribute to further understanding of the underlying mechanism how the RMP field impacts the impurity transport.
