Numerical Research Of Influences Of External Non‐resonant Magnetic Perturbations On MHD Instabilities Of Plasma

In the process of tokamak plasma discharge,the MHD instability driven by pressure gradient and current density gradient will not only reduce the confinement performance of plasma,but also lead to plasma disruption,which will cause fatal damage to the device.Therefore,the control of MHD instability is a key challenge for ITER,CFTER and tokamak fusion reactors in the future.Up to now,radio frequency wave heating,radio frequence current drive and external resonant magnetic pertubation are used to control the MHD instability.However,at this stage,whether the accuracy of deposition of radio frequency wave heating and radio frequence current drive or the reliability of control of resonant magnetic perturbation,still need to be further studied.In order to solve the above problems,this paper proposes an innovative scheme to control the MHD instability by using the external non-resonant magnetic perturbation(non-RMP).Based on the J-TEXT tokamak,the numerical simulation of the influence of external non-RMP on the MHD instability is carried out by using the 3D nonlinear MHD program,NIMROD.In this paper,based on the nonlinear MHD model and the understanding of the physical model of MHD instability,a new scheme to control the MHD instability by enhancing the stability term of magnetic field curvature is proposed,which proves the feasibility of numerical simulation of the scheme by using NIMROD.Based on the understanding of the mechanism of the non-RMP on MHD instability,a new process of NIMROD for numerical calculation of non-RMP is developed from grid construction of tokamak,spatiotemporal discretization of MHD equations,and initial condition settings,which lays a solid foundation for subsequent numerical simulation research.According to the requirements of the control scheme,helical coil is designed which is based on Fourier decomposition.The non-RMP generated by helical coil has the characteristics of strong magnetic field amplitude,simple magnetic spectrum and flexible mode adjustment,which is very suitable for the study of the influence of non-RMP on plasma instability.In this paper,a series of numerical simulation studies are carried out on J-TEXT by using helical coil.The results of the effect of non-RMP on MHD instability under different dominant modes and different amplitudes are obtained.The simulation results show that the suppression effects of the non-RMP under different dominant modes and different amplitudes are different.When the toroidal mode number of external non-RMP matches with plasma’s,the suppression effect is obvious;in a certain range,with the increase of the amplitude of non-RMP,the suppression effect is more obvious;the q_a can be increased by about 3.6%when the 4k A non-RMP with m/n=-1/1 as the dominant mode is applied.At the same time,the application of non-RMP to mitigate the damage of plasma rupture is further explored.It is found that non-RMP can not only significantly reduce the amplitude and rate of instability growth in the process of disruption,but also promote the emission of runaway electrons and reduce the damage of disruption.In addition,new experimental results on J-TEXT show that the non-RMP can suppress the growth of tearing mode and avoid the generation of runaway current platform.The feasibility of controlling the MHD instability and mitigating the damage of disruption by non-RMP is proved experimentally.The simulation and experimental results show that the non-RMP can suppress the plasma instability to a certain extent,which provides a new solution for the suppression of plasma instability in tokamak.