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Locked mode unlocking by rotating resonant magnetic perturbations in J-TEXT tokamak

This study aimed to unlock the m/n = 2/1 locked mode (LM) performed in J-TEXT tokamak by using rotating resonant magnetic perturbations (RMPs), where m and n are the poloidal and toroidal mode numbers, respectively. In the experiments, to maintain the LM, mode locking occurs by using static RMPs generated by a set of saddle coils. After mode locking, another rotating RMP with frequency of several kilo-Hz is applied to drive the static LM to rotate. The unlocking of LM is realized by using rotating RMP with different frequency and amplitude. It is found that the unlocking process contains two stages, i.e. the oscillating stage and the unlocking stage. In the oscillating stage, the rotating RMP with amplitude that is not strong enough causes the LM to oscillate around its locked phase and produces magnetic fluctuation to behave as a standing wave-like structure in poloidal direction. When the amplitude of the rotating RMP is strong enough, it first causes the LM to oscillate and then transforms to mode unlocking quickly in less than 1 ms, namely the unlocking stage. Further analysis shows that the unlocking of LM is determined by the torque balance between the viscous torque and the electromagnetic torques exerted by both the static and the rotating RMP. In addition, the unlocking process is sensitive to both the amplitude and the frequency of the rotating RMP as well as the amplitude of static RMP. Nonlinear numerical modeling based on reduced MHD equations is also performed to understand the unlocking process, and numerical results qualitatively agree with the experimental ones.