Simulations For Some Issues On Plasmoid Instability In Plasmas

Magnetic reconnection,a fast event in space and laboratory plasmas,has been frequently observed in the nonlinear interaction between solar wind and magnetosphere,the substorm near Earth's magnetic tail,disruption during the tokamak experiment,etc.Magnetic reconnection can rearrange the topological structure of magnetic field,which is accompanied by the release of magnetic energy of plasma and the increase of kinetic energy of plasma.In recent years,many theoretical and simulation research have been done on the reconnection theory.Researchers have provided many possible physical mechanisms for fast magnetic reconnection in space plasmas and laboratory plasmas.The plamoid instability is one of the mechanisms.When the Lundquist number exceeds a critical value,the super-Alfvénic instability can occur in a thin current sheet,the current sheet breaks and produces a plasmoid chain.Then the plasmoids grow and coalesce in the nonlinear phase,which results in rapid magnetic reconnection.Recently,the structure of plasmoids observed in space plasmas and magnetically confined laboratory plasmas also proves that the plasmoid instability can result in the rapid magnetic reconnection.Aiming at understanding the plasmoid instability,a two-dimensional three-component resistive MHD model is adopted to numerically study the effect of out-of-plane driving flow,guiding field and asymmetric magnetic field structure on the plasmoid instability.The mechanism of rapid growth of plasmoid is explained by numerical simulation.In the first chapter,a review on magnetic reconnection and important models in space plasmas is introduced.We also introduce the research progress of plasmoid instability in recent decades.In the second chapter,three main models of plasma simulation,including magnetohydrodynamic model,particle simulation model and hybrid simulation model,are introduced.The physical equations,applicable conditions,advantages and disadvantages of each model are briefly described.In the third chapter,the resistive MHD model is used to simulate the evolution of magnetic field under the effect of out-of-plane driving flow.First,the simulation method and numerical algorithm are described in detail.Then the effect of the width and strength of the driving flow on the plasmoids growth is disscussed.It is found that the plasmoids are easily formed in the case of strong and wide out-of-plane driving flow.The reconnection rate and the number of the plasmoids increase with the driving flow width and/or driving flow strength increasing.In the fourth chapter,it is proved that the plasmoids can be stabilized by guiding field in the simulation.The nonlinear evolution of the plasmoids under the influence of the out-of-plane driving flow and guiding field are studied by using a resistive MHD model.In the presence of guiding field.It is found that the symmetry of the plasmoids is broken in the reconnection plane.In addition,for the fixed guiding field,the growth rate of plasmoids increases much faster as the strength of driving flow increases.In the fifth chapter,the resistive MHD model is employed to investigate the dynamic process of plasmoids in the sheet pinch configuration with asymmetric magnetic field.The effect of asymmetric degree on the growth of plasmoids is studied.It is found that the magnetic island grows faster on the side with lower magnetic field strength.The more asymmetrical the magnetic field,the more stable the reconnection process.Moreover,it is found that it is difficult to produce plasmoids in the current sheet with strongly asymmetrical magnetic field.A brief summary is given in the last chapter.