Study Of Cold Front Propagation Mechanism Of Massive Gas Injection By Fast Frame Camera On J-TEXT

For avoiding the negative influence on all the major disruption, pellet injection, massive gas injection(MGI) and external magnetic perturbation(EMP) are the main means. The pellet injection can reduce the heat deposition and the electromagnetic force, but will lead to runaway electrons generation. The external magnetic perturbation plays a role on inhibition of runaway electrons, but has no obvious effect on reducing the thermal deposition and electromagnetic force. Massive gas injection can effectively relieve the heat deposition, halo current, and suppress partial of runaway electrons. Therefore, it’s worth to study the disruption mitigation. It needs to inject hundreds of times plasma particle number into the tokamak device for suppressing runaway electrons completely, which means putting forward higher requirements to the vacuum system and part of the diagnosis systems. How to improve the mixing efficiency of impurity gas and inject gas to the plasma core are the main subject in the MGI experiment. So studying the cold front propagation mechanism in MGI experiment has very important significance to achieve the above purpose.It is found that gas injection depth is limited to a certain extent. Several experiments showed that the injected gas can only achieve the q=2 surface, rather than the plasma core. Effective penetration is the key to realize the mitigation of disruption. If the gas can be injected into the plasma core, less number of gas particles will be needed. Therefore, it is necessary to study the propagation process of cold front and its interaction with the q=2 surface, explore the methods of optimizing the mixing efficiency and injection depth.On J-TEXT tokamak, massive gas injection system is used to shutdown of plasma. At the same time, fast frame camera is used to record the shutdown process. By analyzing the experimental results, found out the main factors affecting the cold front propagation. The experimental results show that, both the injection from the top of the device and from the bottom, the cold front always, penetrate towards the plasma core gradually along the high field side, eventually stays near the q=2 surface.However, for the future tokamak like ITER, the current MGI system is not suitable. Because of huger damage accompanied by disruption on ITER, the MGI system must provide greater numbers of gas particles and higher gas injection efficiency. So it is necessary to improve the MGI system, and the efficiency of gas injection.