Particle-in-cell Simulations Of Magnetic Reconnection In Laboratory And Space Plasmas

Magnetic reconnection is a fundamental physical process which is believed to be related to many explosive phenomena in space,astronomy,and laboratory plasma en-vironments.During magnetic reconnection,magnetic energy is rapidly converted into the kinetic and thermal energy of plasma along with the topology change of magnetic field,energetic particles are also generated.Since the concept of magnetic reconnection was proposed in the 1940s,it has been studied using satellite observations,numerical simulations,and laboratory experiments by many researchers.Although significant progresses are achieved,there are still many unsolved problems.In this dissertation,we study magnetic reconnection in space and laboratory plasmas using two and three dimensional particle-in-cell simulations,and our main results are shown in below.1.Particle-in-cell simulation of magnetic reconnection in laser-produced plasmaExperiments about the flow-driven magnetic reconnection in high-energy-density(HED)laser-produced plasmas have recently been conducted on different platforms of giant laser facilities.We perform two-dimensional(2D)particle-in-cell(PIC)simula-tions to study the interaction of two colliding laser-produced plasma bubbles with a self-generated toroidal magnetic field.Two cases are investigated:in one case,the two plasma bubbles have an anti-parallel magnetic field(AP-case)in the colliding region,and in the other case,the two interacting parts of the magnetic field are configured parallel to each other(P-case).By comparing the results of the two cases,we study the evidence of magnetic reconnection and the formation of electron energy spectra in HED system.It is found that the evolution of such an interaction between the two plasma bub-bles in AP-case can be separated into two distinct stages:squeezing and reconnection stages.In the squeezing stage,when the two plasma bubbles expand quickly and col-lide with each other,the magnetic field in the inflow region is greatly enhanced.In the second stage,a thin current sheet is formed between the two plasma bubbles,and then,magnetic reconnection occurs therein.While in P-case,there is only squeezing between the two bubbles.In both cases,the quadrupole structure of the out-of-plane magnetic field is observed,as well as the Hall electric field,the electron energization,and the enhancement of electron agyrotropy in the colliding region.However,only in the AP-case,three well-collimated in-plane electron jets are observed.We demonstrate that besides the annihilation of the magnetic field in the colliding region between the two laser-produced plasma bubbles approaching each other,the three well-collimated electron jets can also be considered as the evidence for the magnetic reconnection.During the squeezing stage,electrons are heated in the perpendicular direction by Betatron acceleration due to the enhancement of the magnetic field around the plasma bubbles.Meanwhile,non-thermal electrons are generated by the Fermi mechanism when these electrons bounce between the two plasma bubbles approaching quickly and get accelerated mainly by the convective electric field associated with the plasma bub-bles.During the reconnection stage,electrons get further accelerated mainly by the reconnection electric field in the vicinity of the X line.When the expanding speed of the plasma bubbles is sufficiently large,the formed electron energy spectra have a kappa distribution,where the lower energy part satisfies a Maxwellian function and the higher energy part is a power-law distribution.Moreover,the increase in the expanding speed will result in the hardening of formed power-law spectra in both the squeezing and reconnection stages.2.Particle-in-cell simulation of magnetically driven reconnection using laser-powered capacitor coilsMagnetically driven reconnection using laser-powered capacitor coils is a new ex-perimental scheme to study reconnection in low ? regimes.Here,two laser beams are focused on a capacitor-coil target and then strong currents are wired in two parallel cir-cular coils.Magnetic reconnection occurs between the two magnetic bubbles created by the currents in the two parallel circular coils.A 2D PIC simulation model in the cylindrical coordinate is used to investigate such a process,and the simulations are per-formed in the(r,z)plane.The results show that with the increase of the currents in the two coils,the associated magnetic bubbles expand and a current sheet is formed between the two bubbles.Magnetic reconnection occurs when the current sheet is sufficiently thin.A quadrupole structure of the magnetic field in the ? direction(B?)is generated in the diffusion region and a strong electron current along the r direction(Jer)is also formed due to the existence of the high-speed electron flow away from the X line in the center of the outflow region.Because the X line is a circle along the ? direction,the convergence of the plasma flow around r=0 will lead to the asymmetry of Jer and B?between the two outflow regions of magnetic reconnection.3.Spontaneous growth of the reconnection electric field during magnetic recon-nection with a guide fieldReconnection electric field is a key element of magnetic reconnection.It quantifies the change of magnetic topology and the dissipation of magnetic energy.In this work,2D PIC simulations are performed to study growth of the reconnection electric field in the electron diffusion region(EDR)during magnetic reconnection with a guide field.At first,a seed electric field is produced due to the excitation of the tearing mode instability.Then,the reconnection electric field in the EDR,which is dominated by the electron pressure tensor term,suffers a spontaneous growth stage and it grows exponentially until it saturates.A theoretical model is also proposed to explain such kind of growth.The reconnection electric field in the EDR is found to be directly proportional to the electron outflow speed.The time derivative of electron outflow speed is proportional to the reconnection electric field in the EDR because it is formed after the inflow electrons are accelerated by the reconnection electric field in the EDR and then directed away along the outflow direction.This kind of reinforcing process at last leads to the exponential growth of the reconnection electric field in the EDR.4.Scaling of 3D magnetic reconnection with a limited X line extentPrevious simulations and models of magnetic reconnection are usually limited in 2D condition,where the reconnection X line extent is infinitely long.However,many observations indicate that reconnection with a limited X line extent is a universal phe-nomenon.We use 3D PIC simulations to study magnetic reconnection with a limited X line extent,and propose a model of reconnection rate and outflow speed as a function of the X line extent.We find that reconnection is more active on the dawn side(electron drifting side),while on the dusk side(ion drifting side)reconnection is suppressed,this internal X line asymmetry along the current direction is developed because the magnetic flux is transported to dawn side by electrons.The length of the suppression region is around 10di.The average reconnection rate drops because of the limited active region,and the outflow speed reduction is associated with the decrease of the J x B force,which is caused by the phase shift between the J and B profiles,also as a consequence of this flux transport.