Destabilization of low-frequency modes (LFMs) driven by a thermal pressure gradient in EAST plasmas with qmin ≤ 2

Mode structures and excitation conditions for the low-frequency modes (LFMs) have been investigated in experimental advanced superconducting tokamak (EAST) plasmas with q(min) <= 2. Two different stages/categories of the LFM instabilities are observed during the oscillation of annular/central collapse events: (I) the upward sweeping frequency of LFMs; (II) the upward frequency jumpsof LFMs. The annular/central events are triggered by the m/n = 2/1 double tearing modes with different q-profiles, while the LFMs are characterized by higher mode numbers m/n = 4/2, 6/3, ..., where m and n are the poloidal and toroidal mode numbers, respectively. The maximum radial coverage of the LFMs is located in the annular region of 1.97 <= R <= 2.07 m with the normalized minor radius 0.2 <= rho <= 0.4, while the higher-frequency (or upward sweeping frequency) branch is more localized to the radial position of 2 <= R <= 2.02 m (q(min)). The frequency characteristics of upward sweeps or upward jumps of the LFMs are mainly attributed to the change in the q-profile, e.g. the upward sweeping frequency in stage I is caused by q(min) decreasing. Accordingly, the linear wave properties of LFMs in EAST with weak/reversed magnetic shear are studied numerically and analytically based on a general fishbone-like dispersion relation. Without considering the contribution of energetic ions, it is shown that the LFM with Alfvenic polarization is an MHD-unstable kinetic ballooning mode with frequency of the order of the ion diamagnetic drift frequency. Several important factors for the excitation of LFM instability are analyzed: (1) the role of energetic ions is unimportant, and the LFMs can be excited under the two conditions of with/without energetic ions; (2) the higher tau = T-e/T-i with larger eta(i) = L-ni/L-Ti are required, namely the normalized pressure gradient alpha proportional to (1 + tau)(1 + eta(i)) should be large enough to overcome the stability effect of finite field line bending; (3) the weak/reversed shear q-profile with q(min) <= 2 and suitable S (r/q)(q '')(1/2) are required.

Automated summary

Mode structures and excitation conditions for LFMs were investigated in EAST plasmas, revealing two stages of instabilities and analyzing the frequency characteristics and factors affecting their excitation.