Magnetic island formation and rotation braking induced by low-Z impurity penetration in an EAST plasma

Abstract

Recent observations of the successive formations of the $4/1,3/1$, and $2/1$ magnetic islands as well as the subsequent braking of the $2/1$ mode during a low-Z impurity penetration process in EAST experiments are well reproduced in our $3D$ resistive MHD simulations. The enhanced parallel current perturbation induced by impurity radiation predominately contributes to the tearing mode growth, and the $2/1$ island rotation is mainly damped by the impurity accumulation as results of the influence from high $n$ modes.

Influence of low-Z impurity on the stabilization of m/n = 2/1 tearing/locked modes in EAST

“…A series of TMs with different frequencies ( f 4/1 , f 3/1 , f 2/1 and f 3/2 ) and multiple helicities (m/n = 4/1, m/n = 3/1, m/n = 2/1 and m/n = 3/2) are excited successively, as shown in figure 3 (selected and amplified from the shaded region of figure 2). Similar results for those magnetic islands at different rational surfaces are excited successively during the massive gas injection (MGI), as in [33,34], and the formation of the current sheet is the reason for the excitation of TMs, as reproduced by the NIMROD code. The relative positions of those modes are located at ρ(r/a) ≈ 0.8 (q = 4, R ≈ 2.23 m), ρ(r/a) ≈ 0.65 (q = 3, R ≈ 2.17 m), ρ(r/a) ≈ 0.5 (q = 2, R ≈ 2.1 m or R ≈ 1.67 m in high field side (HFS)), and ρ(r/a) ≈ 0.39 (q = 1.5, R ≈ 2.06 m), respectively, and those modes are propagated in the electron diamagnetic drift direction that is marked by the blue bold arrow shown in figure 1.…”

“…Three important results regarding the relationship between the low-Z impurity concentration and the tearing/locked modes are summarized as follows. (1) A series of TMs with multiple helicities (m/n = 4/1, m/n = 3/1, m/n = 2/1, and m/n = 3/2) are excited successively by the influx of low-Z (carbon) impurity concentration, which has been reproduced using the NIMROD code [34]. (2) The m/n = 2/1 LMs can be formed by the redistribution of impurity concentration, and the 'O'-point of the magnetic islands is locked by the tungsten protector limiter with toroidal separation of Δφ ≈ 0, where the electromagnetic interaction decreases dramatically when the separation of Δφ 0.2π is satisfied.…”

Species dependence of the impurity injection induced poloidal flow and magnetic island rotation in a tokamak

“…3D simulations including Fourier components with toroidal mode numbers n = 0 − 5 are considered here, and all other setups remain same as the 2D simulations. The n = 1 mode dominates the entire impurity penetration process, and the poloidal mode number m = 2 TM is induced by the impurity radiation and becomes the dominant MHD instability rapidly after the impurity injection [7,26]. The initial destabilization of the TM mainly comes from the current profile steepening due to the impurity radiation edge cooling, as indicated by the 2/1 magnetic island width w = 4…”