Destabilization of low-frequency modes (LFMs) driven by a thermal pressure gradient in EAST plasmas with q min ⩽ 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 ⩽ ρ ⩽ 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 Alfvénic 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 τ = T e / T i with larger η i = L ni / L Ti are required, namely the normalized pressure gradient α ∝ (1 + τ )(1 + η 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.