MHD instabilities and heat transfer across the major disruption have been investigated using various diagnostics in the ohmically heated plasmas in the HL-2A tokamak. During the phase of pre-thermal quench, in the core region inside the q=2 surface the m/n = 1/1, 3/2 and 2/1 modes are all rotating, while in the outer cold region the m/n=5/2 mode doesn’t rotate and it interacts with the 2/1 island periodically. The dynamic interaction of multiple modes gradually brakes down the plasma rotation and eventually results in mode locking of the islands. Before mode locking, the 2D electron cyclotron emission imaging (ECEI) shows that the coupling of m/n=5/2 and 2/1 modes can be decoupled each other and the island size does not alter significantly. However, after mode locking both 5/2 and 2/1 islands expand and merge together, leading to massive heat transfer and enormous energy losses (i. e., thermal quench) in the low field side, and consequently, a major disruption.
 

MHD instabilities;major disruption;ECE imaging