| Electron-positron(e-e+)plasma is important subject of plasma physics,the equal mass of electrons and positrons gives e-e+ plasmas abundant unique properties,which makes them different from electron-ion plasmas.For example,the well-known Faraday effect in electron-ion plasmas will disappear in e-e+ plasmas.e-e+ plasmas are ubiquitous in the universe,such as around active galactic nuclei and gamma-ray bursts,their interactions with ambient medium not only dissipate their energy but also provide ideal circumstances for particle acceleration.The collisionless shock in e-e+plasma can accelerate charged particles,which may partly explain the origin of high-energy cosmic rays.With the rapid development of laser technology,nowadays researchers have been able to generate high-density e-e+plasma in the laboratory,which allows us to study the unique properties of e-e+ plasmas in the laboratory.The understanding of e-e+ plasmas,especially the interaction of e-e+ plasmas with ambient medium(for example,electron-ion plasmas)will contributes to our comprehension of relevant phenomena which occur widely in astronomical environments or in laboratories.In this paper,we use the theoretical analysis and numerical simulation(PIC simulation)to study the shock formation and particle acceleration in e-e+plasmas.We first introduce the relevant background and research methods,and then we introduce our main content in three parts:In the first part,we study the ion acoustic shock wave(IASW)formation and consequent ion acceleration when cold e-e+ jets are injected into cold ambient electron-ion plasmas.It is found that the Buneman instability can be excited firstly,which induces the formation of an ion acoustic wave(IAW).As the amplitude of the IAW increases,its wave form is steepened and subsequently an IASW is formed.Some ions in the ambient plasmas will be reflected when they encounter the IASW,and thus can be accelerated to form an energetic ion beam.For an initial e-e+jet with the Lorentz factor γ0=100 and the ion-electron mass ratio mi/me=1836,the ions can be accelerated up to 580 MeV.This study deepens our understanding of the fireball model of gamma-ray bursts,the shock model of pulsar wind nebulae,the origin of cosmic rays and other related astrophysical processes.In the second part,we study the IASW formation and ion acceleration when e-e+jets interacts with ambient electron-ion plasma under the guiding of longitudinal background magnetic field.We found that when the initial temperature of jet and ambient plasmas are not zero,the guidance of the longitudinal background magnetic field is facilitates the formation of IASW,the formed IASW moving at high speed too,accelerating the ambient plasma ions.We also study the IASW formation and ion acceleration under different parameters,and found that the IASW can be formed in a wide range of parameters.The third part of this paper studies the relativistic-induced opacity effect when the intense laser interacts with e-e+plasma.It is found that an initial underdense e-e+plasma can become opaque under the irradiation of a relativistically intense laser pulse.The strong ponderomotive force of the relativistic laser pulse and the small mass density of the e-e+plasma can combine to induce the efficient pile-up of the electrons and positrons at the front of the laser pulse.Therefore,the local plasma density at the laser pulse front increases dramatically and finally the initial underdense e-e+plasma becomes opaque.This relativistic-induced opacity effect of e-e+plasmas is opposite to the well-known relativistic-induced transparency effect,in which an initial overdense electron-ion plasma can become transparent to a relativistically intense laser pulse.This relativistic-induced opacity effect in e-e+plasmas can be used to generate mid-infrared radiation and accelerate positrons.Finally,a brief summary of the paper and the prospect for future study are included. |
