Plasma major disruption is one of the most critical issues to be solved for a tokamak fusion reactor. Experiments to prevent mode locking and subsequent disruptions have been carried out on J-TEXT tokamak using rotating resonant magnetic perturbations (RMPs). The tearing modes, to be locked and to lead to disruptions without applying RMPs, can be accelerated by rotating RMPs to the RMP frequency. As the result, the mode locking and subsequent disruptions are delayed or prevented. The effects of RMP amplitude and frequency on disruption prevention have been investigated.

The spectrum effect on the penetration of resonant magnetic perturbation (RMP) is studied with upgraded in-vessel RMP coils on J-TEXT. The poloidal spectrum of the RMP field, especially the amplitudes of 2/1 and 3/1 components, can be varied by the phase difference between the upper and lower coil rows, Δϕ = ϕtop – ϕbottom, where ϕtop and ϕbottom are the toroidal phases of n = 1 field of each coil rows. The type of RMP penetration is found to be related to Δϕ, including the RMP penetration of either 2/1 or 3/1 RMP and the successive penetrations of 3/1 RMP followed by the 2/1 RMP. For cases with the penetration of only one RMP component, the penetration thresholds measured by the corresponding resonant component are close for various Δϕ. However, the 2/1 RMP penetration threshold is significantly reduced if the 3/1 locked island is formed in advance. The changes in the rotation profile due to 3/1 locked island formation could partially contribute to the reduction of the 2/1 thresholds.

The coupling of multiple MHD modes can lead to mode locking and major disruption in tokamak plasmas. In the J-TEXT tokamak, the coupling between two small modes, i.e., m/n = 2/1 and 3/1 modes (m and n are poloidal and toroidal mode numbers, respectively), appears when the edge safety factor is reduced to the vicinity of 3. After the mode coupling, the toroidal phase difference between the 2/1 and 3/1 modes equals 0 in the low field side midplane. This phase relation of coupled modes leads to mutual destabilization and even major disruption. A control scheme to avoid disruption caused by coupled modes by resonant magnetic perturbations (RMP) is presented. It is found that the application of RMP significantly changes the evolution of the coupled modes. The coupling of the 2/1 and 3/1 modes occurs earlier as the RMP amplitude increases. The RMP with moderate amplitude can suppress the growth of 2/1 and 3/1 coupled modes and hence avoid disruption. These results provide a possible strategy for the suppression of neoclassical tearing mode (NTM) seed islands on ITER or future fusion reactors.

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