Study On Filamentation And Anomalous Collective Stopping Of High Current Relativistic Electron Beams In Plasmas Based On Particle-in-Cell Simulations

The interaction of high current relativistic electron beams with plasmas plays an important role in many important applications,such as the plasma based accelerator,fast ignition in the inertial confinement fusion and the collision-less shock formation of the astrophysics.In the fast ignition,essential energy for the hot-spot ignition can be transported into the compressed plasma fuel with the effects of instabilities and Coulomb scattering of high current relativistic electron beams.Strong electromagnetic fields due to instabilities excited by the interaction of electron beam and plasma are associated with the formation of collision-less shocks.For the beam with radius much larger than plasma skin depth,Weibel-type current filamentation instability can be excited by the electron beam in the plasma,which breaks the beam into filaments,leads to the formation of strong magnetic fields and further attributes to anomalous energy deposition consequently.In this work,a self-consistent particle-in-cell simulation method is adopted to investigate the strong magnetic fields and anomalous energy deposition due to Weibel-type current filamentation instability excited by high current relativistic electron beams in background plasmas,aiming at the problem of background plasmas with inhomogeneous density in fast ignition and strong magnetic fields in collision-less shock.In Chapter 1,the research background and recent advances in the interactions of relativistic electron beams with plasmas are reviewed,and then the research purposes of the current work are given.In Chapter 2,a particle-in-cell(PIC)simulation code adopted in this work is introduced,such as particle moving,boundary conditions,the solver of electromagnetic field and numerical diagnosis.The electromagnetic field is solved by the explicit finite difference-time domain(FDTD);relativistic Boris rotation method is proposed to solve the particle equation of motion;the boundary conditions include periodic and absorbing boundaries;A moving window approach is adopted in the code to reduce the simulation cost.In Chapter 3,the filamentous structures and directional drift of relativistic electron filaments in transversely nonuniform plasmas are investigated with two-dimensional electromagnetic PIC simulations.The effects of plasma ion species are shown to be significant and asymmetric transverse magnetic fields are formed in plasmas with heavy-ion species due to the asymmetric neutralization of beam space charge by plasma ions.The asymmetric transverse magnetic fields contribute to the directional drift of beam electrons to lower plasma density regions,which may accelerate the filaments merger process and lead to highly localized beam-energy deposition in plasmas.Meanwhile,the influences of plasma ion dynamic and plasma density distributions on the drift velocity,merger rate and collective stopping are also investigated.In Chapter 4,two-dimensional electromagnetic PIC simulations are proposed to study the density evolution and collective stopping of electron beams with finite size in background plasmas.The coupling effect of the wakefield and Weibel-type current filamentation instability is considered here.The formation of the multi-layer structure of the relativistic electron beam in the plasma due to the different betatron frequency from the beam front to the beam tail is observed.Meanwhile,the nonuniformity of the longitudinal wakefield is the essential reason for the multi-layer structure formation in beam phase space.Weibel-type current filamentation instability is associated with the appearance of filamentous structures at the head of the beam and density oscillations at the beam axis.The influences of beam parameters(beam radius and transverse density profile)on the formation of the multi-layer structure and collective stopping in background plasmas are also considered.In Chapter 5,two-dimensional electromagnetic PIC simulations are performed to investigate the transport of relativistic electron beams co-moved with a proton beam in background plasmas.The proton beam is modulated strongly by the electromagnetic fields in plasmas,and a netlike structure in the beam profile is formed with a high density contrast,which can be provided as an effective tool to diagnose Weibel-type current filamentation instability.By choosing the appropriate plasma length,the number of beam filaments can be clearly identified from the proton beam profile.The dynamic effects of plasma ions are shown to be important and play a significant role in the dynamic evolution of the proton beam and electron beam.In Chapter 6,the propagation for relativistic positron beam and electron beam in plasmas are compared with two-dimensional electromagnetic particle-in-cell simulations.Special emphasis is laid on investigating asymmetrical magnetic fields generation and collective stopping,which are associated with the current filamentation instability arose for electron beam and positron beam respectively.The effects of beam charges are significant and asymmetric neutralizations of space charges and current for electron filaments and positron filaments are formed in ambient plasmas due to the asymmetric response for plasma ions and electrons to beam charges.The asymmetrical current neutralizations contribute to the asymmetry for magnetic field generation and beam energy.Besides,the dynamics of plasma ions attenuate asymmetrical collective stopping.In Chapter 7,the main conclusions and innovation points are summarized and the prospects of the future work are discussed.