Understanding the spectrum during the tearing mode locking by rotating RMP in J-TEXT

The rotating resonant magnetic perturbation (RMP) could lock the tearing mode (TM) and prevent disruption. It is found that the TM instantaneous frequency oscillates during TM locking on the RMP. In this work, the TM frequency oscillation was derived analytically based on the toroidal torque balance, which was verified experimentally. It is found that two peaks with higher frequency (2 fTM0- fRMP) and lower frequency ( fRMP) occurred in the spectrum when the window width of FFT was shorter than the TM frequency oscillating period. Those two peaks were explained analytically based on the description of TM instantaneous frequency. The independence of the temporal information and the spatial information of the TM was explained analytically and verified by the spatial structure analysis of each peaks. This work is important for mode structure identification and provides a scheme for mode coupling control.