The Numerical Study Of Runaway Suppression By External Resonant Magnetic Perturbation

Plasma disruption,which causes the generation of a large amount of high-energy runaway electrons(REs)and then a huge damage to device,is a critical safety problem for the reliable operation for the future tokamaks.Therefore,it’s vital important to study the mechanism of the generation of REs,runaway suppression and runaway dissipation.During the process of plasma disruption triggered by massive gas injection(MGI),the use of mode penetration induced by resonant magnetic perturbations(RMPs)to suppress RE generation has been investigated on the J-TEXT tokamak.The experimental results show a discrepant effect of runaway suppression appear in different devices.In order to get a further understanding of the physical mechanism that RMP affect RE behavior,the processes of both mode penetration and plasma disruption have been performed with the 3D magnetohydrodynamic(MHD)code NIMROD.The modelling results show magnetic topology structure can be changed by magnetic perturbation induced by RMP directly or indirectly.A more stochastic flux surface is beneficial for RE loss,which might suppress runaway current plateau due to a weak avalanche.Both experiments and simulation show different phases of 2/1 islands seeded by RMPs have a significant effect on RE loss during plasma disruption.It’s found that the curve of the remaining rate of RE versus relative toroidal phase between pre-seeded m/n=2/1 islands and MGI valve approximates a sine function dependence.The remaining rate of RE is lowest when the relative phase difference is-90~°.Simulations indicate that different phases of 2/1 magnetic islands can drastically affect impurity spreading.A faster impurity spreading will result in a corresponding cooling,which might have an influence on magnetic perturbation.At the same time,RE loss greatly depends on magnetic perturbation.When there is a most appropriate 2/1island phase increasing RE loss in MGI shutdown,a reverse phase will reduce RE loss.Modeling results demonstrate that the trajectories of low-energy REs depend significantly on the trails of the magnetic field lines(MFLs).RE confinement is certified to be relevant to stochasticity of flux surface.Magnetic surfaces will become more stochastic with a stronger magnetic perturbation during disruption phase,which results in more short MFLs in the plasma region.These short MFLs will help a large amount of REs to escape.What’s more,for a sufficiently long mode penetration duration,robust runaway suppression has been reached during the disruptions.Experiments and simulations demonstrate large 2/1 mode structure after a long mode penetration duration will induce strong magnetic perturbation during TQ phase,which can increase RE loss.It has been demonstrated that the phase of pre-existing 2/1 magnetic islands have a significant effect on runaway suppression in MGI shutdown.An appropriate phase of 2/1magnetic islands excited by RMP is beneficial for runaway suppression.Nonetheless,a larger magnetic island will result in a more stochastic magnetic surface and increase the loss of RE seed,which might fully suppress runaway current.These results might be an alternative method to suppress runaway current for the future ITER.