The Effect Of Resonant Magnetic Perturbation On The Runaway Current Formation On J-TEXT Tokamak

The generation of runaway electrons(REs)during disruptions can cause severe damage to the plasma facing component,which poses a potential threat to the safety operation of large tokamaks.Thus,the avoidance and suppression of REs generation during disruptions is one of the key issue in tokamaks.The external applied resonant magnetic perturbations(RMP)provide an alternative methods for REs suppression during disruptions.It enhances the magnetic perturbations to destroy the magnetic surfaces and worsen the REs confinement before the REs are amplified.Many studies have been carried out for understanding the effect of RMP on runaway suppression during disruption.However,the REs suppression result is different in different tokamaks.The mechanism of the RMP on the REs suppression is so complexity that many factors should be considered.And the plasma response to the magnetic perturbation will be different in different tokamak parameters and RMP mode spectrum.Therefore,it requires further research to confirm whether the external magnetic perturbation is available for avoiding the runaway current formation during disruptions.In this thesis,the effect of external resonant magnetic perturbation with different mode,amplitude and phase on the runaway current formation have been systematically studied on the J-TEXT tokamak.For better design a reliable scenario to suppress REs generation,investigation of the REs generation region during disruption is highly essential.On J-TEXT,it presents that the REs generation is strongly depend on the toroidal magnetic fields B_T,the edge safety factor q_a,the electron density n_e,the electron temperature T_e and magnetic perturbations/.The runaway current can be generated in discharges with a high BT or a high q_a.However,it can be easy to suppress in discharges with a high ne or a low Te.And the strong magnetic perturbations can significantly suppress the REs generation during disruption.For discharges with a fixed BT,a high n_e threshold is required to suppress REs generation.This n_e threshold is decreased with the application of resonant magnetic perturbation which is applied before the thermal quench.The n_e threshold required for REs suppression is reduced with the increase of RMP strength and the m/n=2/1 mode RMP is more efficiency than the m/n=3/1 mode RMP for REs loss during disruption,where m and n are the poloidal and toroidal mode number,respectively.With low n_e,the strength and phase of 2/1 modes RMP can significantly impact on the REs generation during disruption.When the strength of 2/1 mode RMP is weak for mode penetration,there is no significant effect on the REs suppression in experiments.However,when the RMP strength is strong enough,the 2/1 mode island produced by RMP can significantly affect the REs generation.It was found that when the X-point of 2/1 island align to the MGI,the runaway current is still generated during disruption.But when the O-point of2/1 island align to the MGI,the runaway current can be significantly avoided.And this result is related to the islands width which is considered be large than 4 cm for REs suppression.The NIMROD simulation results also verify the effective of islands phase and size on the REs suppression.All results above provide a way to prevent runaway current formation by optimizing 2/1 mode RMP with phase and strength.When the X-point of 2/1 island align to the MGI,the runaway suppression can be optimized with the 3/1 mode phase.It is shown that the runaway current can survive when the X-point of 3/1 island align to the MGI,but can be full suppressed when the O-point of 3/1 island align to the MGI even though the amplitude of 3/1 mode RMP is small.And the 3/1 mode RMP just play a large auxiliary function on the REs suppression comparing to the 2/1 mode RMP.It is because the 2/1 mode RMP can impact more REs in the region close the core.The NIMROD simulation result indicates that the coupling modes of 2/1 and 3/1 islands can create a high stochasticity during disruption,resulting in the enhancement of RE loss,which will be helpful for avoidance the generation of runaway current.This result demonstrates that multi-modes is effective for REs suppression.