Advances in physics and applications of 3D magnetic perturbations on the J-TEXT tokamak

In the recent two years, three major achievements have been made on J-TEXT in supporting for the expanded operation regions and diagnostic capabilities, e.g. the 105 GHz/500 kW/1 s ECRH system and the poloidal divertor configuration. Especially, the 400 kW ECW has also been successfully injected into the diverted plasma. The locked mode (LM), especially the 2/1 LM, is one of the biggest threats to the plasma operation. Both the thresholds of 2/1 and 3/1 LM are observed to vary non-monotonically on electron density. The electrode biasing (EB) was applied successfully to unlock the LM from either a rotating or static RMP field. In the presence of 2/1 LM, three kinds of standing wave (SW) structures have been observed to share a similar connection to the island structure, i.e. the nodes of the SWs locate around the O- or X- points of the 2/1 island. The control and mitigation of disruption is essential to the safe operation of ITER, and it has been systematically studied by applying RMP field, MGI and SPI on J-TEXT. When the RMP induced 2/1 LM is larger than a critical width, the MGI shutdown process can be significantly influenced. If the phase difference between the O-point of LM and the MGI valve is +90° (or -90°), the penetration depth and the assimilation of impurities can be enhanced (or suppressed) during the pre-TQ phase and result in a faster (or slower) thermal quench. A secondary MGI can also suppress the RE generation, if the additional high-Z impurity gas arrives at the plasma edge before TQ. When the secondary MGI has been applied after the formation of RE current plateau, the RE current can be dissipated, and the dissipation rate increases with the injected impurity quantity, and saturates with a maximum of 28 MA/s.