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 (I_RMP). When I_RMP is below the threshold for H-L back transition, I_{RMP_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.