The Numerical Study Of The Effect Of Pre‐existing Magnetic Island On Generation Of Runaway Current

During the discharge operation of the tokamak device,major disruption of plasma can occur as a result of magnetohydrodynamic instability,misoperation and other factors.High energy and high flux runaway electrons may appear during the disruption,pose a fatal threat to the device wall,and the suppression of runaway current or mitigation of damages of runaway current have become the key issue for the safe operation of future fusion reactors.Thus,suppression of runaway current or mitigation of damages of runaway current is of great importance.A potential method is the use of external resonant magnetic perturbation to increase the level of the magnetic perturbation of plasma during the thermal quench phase and to enhance the vertical transport of runaway electrons,so as to achieve the runaway electrons colliding with the wall before the current quench,consequently,the suppression or mitigation of runaway electrons are realized.This method has also achieved very good results on some small size devices.However,for large-size tokamak,it is difficult for RMP to provide deep enough penetration of strong magnetic perturbations.This method may be insufficient to suppress runaway electrons.To overcome this limitation,the use of RMP to create seed islands,large magnetic islands are eventually formed during the disruption and provide a large enough magnetic perturbation to expel the runaway electrons,thereby suppressing the generation of runaway current.J-TEXT experimental results show that the use of RMP to create large enough seed magnetic islands achieves full suppression of the runaway current.However,there are still some uncertainties.To in-depth understanding the related mechanism,macroscopic magnetohydrodynamic three-dimensional NIMROD code is used in this paper to simulate the disruption process triggered by massive gas injection,and with the background of disruption evolution,the trajectory calculation of runaway electrons by the use of test particle model was carried out.Through the characteristics that runaway electrons transport in the plasma region during the disruption process,the effect of pre-existing magnetic island widths on generation of runaway current during the disruption triggered by massive gas injection is studied.The simulation results reveal that the suppression effect of runaway electrons does not change monotonously with the increase of the width of the pre-existing magnetic island.The runaway electrons suppression is realized when the size of the magnetic island is large enough,and the mitigation of runaway electrons is also achieved when the width of the magnetic island is small,ranging from 0.07 to 0.11 times the minor radius.Analysis of several typical cases of different magnetic island widths in the simulation found that the loss of runaway electrons is closely related to the degree of stochasticity of the magnetic surface.In the case of large island,strong and long enough magnetic perturbation lead to a strong stochasticity of the magnetic surfaces,which facilitates the loss of runaway electrons,thus achieving suppression of runaway electrons.A comparative study of the small magnetic island case and the medium magnetic island case found that the small island case has higher amplitude of toroidal high-order mode,and the degree of stochasticity of the magnetic surfaces for small island case is also stronger.These are the key reasons for the difference of runaway electrons loss between small and medium island cases.The magnetic perturbation is related to the MHD instability during the disruption,and the MHD instability might be correlated with the spreading and radiation cooling of impurities.The impurity transport are significant different under the several widths of islands.The relevant experimental results on the J-TEXT device also show that different magnetic island widths have a significant effect on the suppression of runaway current.When the mode penetration duration is greater than 50 ms,full runaway current suppression can be achieved.The longer the penetration duration,the larger the magnetic island structure formed at the time of gas injection.The runway current can also be suppressed when the penetration duration is less than 10 ms,indicating that when the 2/1 magnetic island is relatively small,it may have a better suppression effect on the runaway electrons.The results of this study reveal that appropriate pre-existing 2/1 magnetic island width is of great importance to REs suppression in J-TEXT tokamak.These results provide a reference for the application of RMP in the suppression of runaway electrons of future large-scale tokamak devices.