Minor disruptions triggered by supersonic molecular beam injection on J-TEXT tokamak

Abstract

Disruption mitigation experiments with supersonic molecular beam injection of argon gas on J-TEXT were conducted to explore the evolution process during multistage minor disruption. The corresponding numerical simulation was performed using a 3D magnetohydrodynamic (MHD) code, namely, NIMROD (Sovinec et al 2004 J. Comput. Phys. 195 355). The experimental and numerical simulation results are shown in the following. The central plasma temperature decreases to less than tens of electovolts after a relatively long period of multistage thermal collapse. Different MHD instability modes appear when the impurity cold front propagates toward the q = 2 surface. During the burst of MHD activity and central plasma thermal collapse, the heat flux is transferred from the core region to the plasma boundary. Further numerical simulation results show that the MHD process is accompanied by a significant increase in the core-oriented spread of impurities through the q = 2 surface. During the transition stage from the minor disruption to the major disruption, the high order modes play an important role and the magnitude of the n = 3 growth rate is even larger than that of the n = 1 or n = 2 mode. What seems to separate major from minor disruptions is that in a minor disruption, a single MHD crash is not sufficient to cause a full radiative collapse.