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The injection of a large amount of impurities is one of the possible ways for disruption mitigation on large-scale tokamaks. The deposition of impurities at the center of the plasma is the key for the radiation of plasma energy and runaway suppression. The interaction of the gas jet with the rational surfaces has been studied by scanning the plasma current. Experimental results show that the injection of a massive amount of argon can cool the plasma from edge to core region, and the cooling process is accompanied by different magnetohydrodynamic (MHD) modes when the gas jet reaches the corresponding rational surfaces. It is observed that with different edge safety factors and electron density, gas injection can induce different poloidal modes at first. Then the poloidal mode will traverse to lower m (m is the poloidal mode number) MHD activities until a 2/1 mode is initiated and a thermal quench (TQ) started. The experimental results show that the penetration of a gas jet across the rational surfaces is faster in the plasmas with pre-existing large 2/1 tearing modes, which indicates that the 2/1 mode plays an important role in the penetration process. Disruptions triggered by supersonic molecular beam injection (SMBI) display a slower cooling process compared with MGI, which can be divided into four stages. The dominant poloidal mode transition from m=3 to m=2 is associated with electron temperature recovery. Keyword: disruption, massive gas injection, magnetic perturbation, tokamak a Current address: Princeton Plasma Physics Laboratory, Princeton NJ 08543-0451, USA

Study of MHD mode and cooling process during disruptions triggered by impurities injection in J-TEXT