Experimental Study On The Pentration Process Of Massive Impurity Gas During Disruptions In J-TEXT

The damage of plasma disruption to the tokamak device is very serious,such as local thermal deposition,runaway electrons and halo current,etc.,which will damage the device.Moreover,the damage of plasma disruption to the device increases with the increasing of plasma discharging parameters.The main reason of plasma disruption is the increasing of MHD instabilities.In future high-parameter tokamak devices,95%～99% accuracy of disruption mitigation must be achieved.The injection of large amounts of impurities is one of the possible ways to mitigate the damage of disruption and is currently the most widely used method.The injection of a large number of impurities will activate the MHD instabilities in the plasma,and the complex MHD instabilities in the plasma will affect the process of impurity penetration into the plasma and the effect of disruption mitigation.Therefore,this paper carried out an experimental study on the MHD activities and the accompanying cooling process induced by the injection of a large amount of impurity gas into the J-TEXT(Joint Texas Experimental Tokamak).In this paper,a series of disruption mitigation experiments were carried out by MGI system and SMBI system on J-TEXT tokamak,and the magnetic perturbations and cooling process of plasma after impurity injection have been studied in detail.By changing plasma parameters,injected impurity gas type and injected impurity gas amounts,the effects of plasma edge safety factor,plasma electron density,type of impurity injection and the amounts of injected impurities on plasma MHD behavior and cooling process after impurity injection were studied in detail.Experimental results show that a large amount of argon gas injection can cool the plasma from the edge to the core,and the cooling process is accompanied by different magnetohydrodynamic instabilities.When the impurity crosses the rational surface,the corresponding MHD mode will be destabilized.The poloidal mode number of MHD first excited after impurity injection is related to plasma edge safety factor,plasma electron density,and gas injection amount.The higher the plasma edge safety factor,the lower the electron density,and the less the impurity injection amount,the larger the poloidal mode number of first excited MHD mode after impurity injection.Otherwise,the fist excited MHD mode maybe 2/1 tearing mode.By comparing the experimental results of the pre-existed 2/1 tearing mode and the 2/1 tearing mode excited by impurity injection,the important role of 2/1 tearing mode on the impurity penetration process and plasma cooling process was studied.The cooling processes of low Z(charge number)impurity gas,high Z impurity gas and mixing gas injected into plasma by MGI were also studied.It is found that the appearance of MHD mode is reated to the fast cooling of plasma.In addition,MHD modes and two stage cooling process in plasma after a large amount of argon gas injected by SMBI are also given.The above results provide important reference for understanding the physical process and mechanism of disruption mitigation by impurity injection.