The Numerical Study Of The Suppression Of Pre-existing Magnetic Island On Runaway Current On J-TEXT

In the process of tokamak discharge,a variety of magnetohydrodynamic(MHD)instabilities may cause disruption,which will terminate the plasma discharge suddenly.Once the plasma is broken down and not mitigated,a lot of heat and magnetic energy will be rapidly lost in milliseconds,which will cause serious damage to the device.The damage includes three aspects: the heat loads of vacuum vessel wall and divertor target plates,electromagnetic force of vacuum vessel wall and runaway current.In the experiment of actively triggering plasma disruption by massive gas injection(MGI),the method of suppressing runaway current by external resonant magnetic perturbation(RMP)has been applied by many devices.Due to the difference of runaway current in different devices,we need to further study the influence of perturbation field mechanism on plasma magnetic perturbation.Based on the J-TEXT Tokamak,the three-dimensional magnetohydrodynamics(MHD)code NIMROD is used to simulate the plasma disruption by active MGI triggering,and provides the time evolution of electric field and magnetic field during disruption.Then,the influence of RMP on the runaway electrons(REs)transport is studied under the single particle model.The simulation results show that in the process of plasma discharge,the magnetic perturbation level is increased by adding perturbation field,so as to destroy the magnetic surface and promote the loss of REs,which can effectively suppress the runaway current.The simulation results show that REs in MGI triggering disruption process is obviously suppressed by different relative phases in mixed mode of pre-existing island.When the O point phase of the m/ n =2 /1magnetic island is fixed at 330 ° in the toroidal direction(align to the MGI),the runaway suppression can be optimized with the m/ n =3/1magnetic island phase of 330 °.The correlation curves between the confined fraction of the REs and different relative phases show a sine-like function dependence.When the magnetic topological structure changes,the magnetic surface will become stochastic and the magnetic lines will quickly hit the vacuum vessel wall,which is conducive to the rapid transport of RE.Therefore,the transport process of REs is closely related to the degree of randomization of magnetic topology caused by magnetic perturbation,and the size of magnetic perturbation is also greatly affected by the deposition effect and diffusion of impurity gas in the plasma region,especially inside the q = 2 rational surface.The relative phase difference between pre-existing 2/1 magnetic island and 3/1 magnetic island will lead to different impurity diffusion processes,which will greatly affect the evolution of the stochastic degree of the magnetic surface during disruption,leading to the difference of the radial transport of REs from the core to the boundary.In addition,in the experiment,when the X point of the2/1 magnetic island align to the gas injection port,the 3/1 RMP with different phase in the mixed mode has suppression effects on REs: when the X point of the 3/1 magnetic island align to the gas injection port,the REs still exists;when the O point of the 3/1 magnetic island align to the gas injection port,the REs can be completely suppressed.In this paper,it is proved that the relative phases of 2/1 and 3/1 magnetic islands have an important influence on the suppression of runaway current during MGI triggering plasma disruption.Therefore,in the case of multi-mode coexistence,the realization of specific phase of mixed mode can provide an effective way for RMP to suppress runaway current.