Investigation of MHD instabilities and related dynamic interactions during the major disruption in the HL-2A tokamak

Avoiding the major disruption is of paramount importance in future reactor-level devices, for which understanding the disruption mechanism is essential. In this work, MHD instabilities and related dynamic interactions during the major disruption have been investigated in the ohmically heated plasmas in the HL-2A tokamak. It is reported that the interaction between a kind of edge-oscillating-mode (EOM) perturbation and a rotating m/n = 2/1 tearing mode (TM) inside the plasma plays an important role in inducing the mode locking and the subsequent disruption. The EOM perturbation is oscillating in the laboratory frame, which is proposed to be originally generated by the penetrated error field at the plasma edge and is modulated by the rotating 2/1 TM mode. Before mode locking, the 2D electron cyclotron emission imaging shows that the momentary coupling of the EOM and the 2/1 mode can be decoupled each other and the mode structure does not alter significantly. After the mode locking, the EOM and the 2/1 mode expand and couple each other and induce the heat transfer from the core to the edge. The influence of the TM instability and the EOM perturbation on surrounding plasmas prior to the mode locking has also been presented. The results deepen our understanding of the disruption dynamics related to the external field perturbations, especially in the presence of TMs inside the plasma.