Studies Of Laser Plasma Expansion In Magnetized Ambient Plasma

Magnetic fields and plasmas are both widely distributed in the known universe,many phenomena in universe can be found related with the interaction between magnetic field and plasma,such as plasma instability in Crab Nebula,collimated jet in accretion disk and particle acceleration in space.In astrophysics and space physics,due to the limit of remote sensing and spacecraft observations,the research of these phenomena are still incomplete.In past decades,strong pulsed magnetic field generators and high-power laser facilities were built along with the progress of pulsed power technology,these facilities provided laboratory platforms for studying the interaction between magnetic fields and plasmas.This thesis focus on laser plasma expansion in magnetized ambient plasma via experimental and computer simulation.Laser plasma expansion in magnetized ambient plasma can be categorized as sub-Alfvenic expansion and sup-Alfvenic expansion according to Alfvenic velocity of magnetized ambient plasma.We carried out sub-Alfvenic expansion experiment on magnetized laser plasma device in University of Science and Technology of China,plasma was generated by laser irradiation of a planar target and magnetic field were provided by Helmholtz coils.The evolution of laser plasma in magnetic field were diagnosed by optical imaging of plasma self-luminescence and optical interferometry.The magnetic field pressure made the plasma decelerated and accumulated at the plasma-field interface.It lead to a local density increase at the interface and formed a low-density plasma diamagnetic cavity.The saturation size of cavity depend on confinement of magnetic field.We changed laser energy E and magnetic field strength B in experiment,and found that the saturation size of cavity satisfied a scaling law Vmax?E/B2.Moreover,MHD simulations were used to examine the scaling law on a broader laser energy range of 0.3-1000 J that covered the parameter in hohlraum fusion experiments,in which magnetic field can confine hohlraum wall plasma expansion.Meanwhile,effective gravity drove flute instability at plasma-field interface as plasma decelerated.The growth rate of flute instability at linear stage exceeded 0.4/ns which was larger than ion gyrofrequency.Hence,electrons were magnetized but ions were not during the instability development,which means that the flute instability should be kinetic.Base on the growth rate in linear stage and fishbone structure in nonlinear stage,we confirmed that flute instability in experiment was Large Larmor radius instability.These phenomena in experiment agreed well with AMPTE and CRRES space experiment in the 80's to the 90's,and dimensionless parameters also showed good agreement.Our experiment reveals a new section of parameter space with stronger magnetic field than previous work.We also carried out sub-Alfvenic expansion experiment at SGIIU facility,and we observed flute instability at early stage using proton radiography.We found that the wavelength of flute instability became longer as magnetic field elevates,which is a nonlinear feature of Low Hybrid Drift instability and we observed the mode cascading of flute instability which was the first time capturing flute instability using proton radiography.Sup-Alfvenic expansion experiment was carried out at SGII facility,high density ambient plasma was produced by precursor laser beam ablating planar target and plasma produced by drive laser beam expanded in magnetized ambient plasma Sup-Alfvenic expansion could drive collisionless shock in ambient plasma.We observed clear shock structure by optical interferometry and schlieren imaging and found that shock velocity was about 400km/s,Mach number was about MA?7.A large number of ions were accelerated to 900km/s as diagnosed by Faraday cup when ambient plasma is magnetized,and the amount of high velocity ions increase about 1.5-3 times than that in unmagnetized ambient plasma.The portion of electrons whose energy was 20-150keV increased obviously when ambient plasma was magnetized according to electron spectrum from electron spectrometer.The results demonstrate that sup-Alfvenic expansion can drive high Mach number magnetized shock and collisionless shocks can effectively accelerate particle,the laboratory experiment can complement,and in some cases overcome,the limitations of similar measurements undertaken by spacecraft missions.