Study On Simulation Of Disruption Mechanism Triggered By Supersonic Molecular Beam Injection

Tokamak device is one of the most likely ways to realize the industrialization of nuclear fusion.However,the magnetohydrodynamic instabilities(MHD)and the possible operational errors on tokamak devices may lead to the major disruption.During the major disruption,the stored thermal energy and magnetic energy in the plasma will be released in a flash.This releasing will bring a large amount of thermal load to the first wall materials of the vacuum vessel.The electromagnetic stress generated by halo current will damage the device structure,and the energetic runaway electron will bombard the device wall.Large plasma loop current achieve a rotational transformation of magnetic field lines to ensure its confinement performance,but plasma current density gradients and pressure gradients often trigger some MHD.These characteristics result that plasma disruption cannot be completely avoided.So,the reliable disruption prediction and disruption mitigation is the preferred countermeasure for future fusion reactors.It demands the clear understanding of the disruption mitigation,so it is quite important to study the mechanism of disruption mitigation to mitigate and avoid the disruption.At present,the injection of impurity into plasma is the main measure to mitigate thermal energy and electromagnetic energy.Previous research shows that various MHD triggered by impurity injection play important roles in disruption mitigation.However,the detailed mechanism of disruption mitigation needs further investigation.Only the key physical parameters that cause the disruption can be used to more effectively achieve disruption mitigation.Previous experiments and simulations used massive gas injection(MGI)to trigger disruption,but the duration of those major disruptions were too short to analyze the detailed process.Therefore,supersonic molecular beam injection(SMBI)is used in this paper to trigger disruption,resulting in minor disruption with relative smaller amplitude and longer duration,which are easier for further study.SMBI experiments and the corresponding simulations on J-TEXT have been used to study the detailed evolution from minor to major disruption.The experimental results show that the injection of argon(Ar)impurity with SMBI can cool the plasma from the edge to the core,followed by the growth of various MHD when the cooling front reaches the rational surface.Finally,the multi-collapse of the core temperature occur.Numerical simulation of the disruption triggered by SMBI on the J-TEXT tokamak are performed with 3D MHD code NIMROD,and then compared in detail with experimental results.The SMBI simulation reproduces the observed phenomena in the experiment such as the destabilization of MHD during penetration process of the cold front,followed by the collapse of q ? 2 region and a sequence of partial temperature collapses in the core during minor disruption.The mode coupling between 2/1,3/2 mode and the core instability drives a successive impurity and heat transport events inside the q = 2 surface.The magnitude of impurity determines the amplitude of 2/1 mode and further takes an effect on the process of impurity spreading across q = 2 surface.The mass impurity injection triggers major disruption directly without obvious multi-step thermal collapses in the plasma core.In addition,the research also shows that each stage of minor disruption contains a rapid collapse phase and a slow recovery phase.During the recovery phase,MHD instabilities decrease and the stochastic region shrinks,so that partial plasma parameters have a certain degree of recovery.The toroidal rotation of plasma also plays an important role in the mechanism of recovery.The simulation results show that there is a temporal correspondence between the collapse of the temperature in the core and the steep drop of the toroidal rotation.The above numerical simulation and experimental research gives a clear understanding of the minor and major disruption triggered by impurity injection.And the volume of impurity injection has an important influence on the mitigation evolution process.Various MHD caused by different volumes of impurity affect the stochastic degree of magnetic surface,so that change the mixing ratio of impurities in the core.These studies have an important reference for understanding the physical process of impurity injection to mitigate the disruption,which lays a foundation to further study the toroidal radiation asymmetry generated by MGI.