The rapid response of 2/1 tearing mode to electrode biasing in J-TEXT experiments

Electrode biasing (EB) has been applied to modulate the m/n = 2/1 tearing modes (TM) in the J-TEXT tokamak, where m and n are the poloidal and toroidal mode numbers. According to the response time, the response of 2/1 TM frequency to EB can be divided into two processes, the rapid response and the slow response. In the rapid response, experimental results show that the variation of mode frequency is ahead of the variation of plasma rotation, which is analogous to the result mentioned in paper (Liu et al 2017 Nucl. Fusion 57 016003). Statistical results indicate that the rapid response coefficient of TM frequency to EB current is a constant. The detailed analysis of rapid response obtained by auto-conditional average shows that the mode frequency varying lags about 13 mu s behind the variation of current. Additionally, the mode frequency derivative increases in proportion with the increase of the EB current value. A heuristic mechanism of the rapid response of 2/1 TM to EB is presented, which is relevant to the rotation difference between magnetic island and edge plasma layer rather than the flow shear around the magnetic island. The experimental results suggest that the application of EB can modulate the TM frequency in a small amount of time, which is a possible method for the avoidance of mode locking and disruption.