Collisionless Shock Waves And Ion Acceleration In Laser-Driven Plasma Flows

The development of high power laser facilities has provided new opportuni-ties in the study of collisionless shock waves,which is one of the most important problems in plasma physics.In this thesis,several important problems in col-lisionless shock physics in relevance to laser-driven inertial confinement fusion(ICF)have been studied numerically and theoretically.Some important results have been obtained which have both theoretical and practical significance.In chapter one,we first introduce the physics of both collisional and colli-sionless shocks,as well as the classification of collisionless shocks based on their basic properties.We then discuss the application and implication of collision-less shocks in astro/space physics,ion acceleration driven by super-intense laser pulse,and laser-driven inertial confinement fusion(ICF).Finally,several common methods in collisionless shock researches have been introduced,with emphasis on the recent progress in laser-driven collisionless shocks.The collision and interpenetration of the hohlraum-wall-ablator plasma flows can take place in the indirect-drive ICF.In chapter two,the kinetic and nonlinear effects that occur during the collision and interpenetration process are studied in details by particle-in-cell(PIC)simulation.It is found that a high Mach collisionless electrostatic(ES)shock can be driven during the collision and in-terpenetration of the hohlraum-wall-ablator plasma flows.The ES shock can accelerate the deuterium(D)and carbon(C)ions to energies of?25 keV and?150 keV,respectively.Beam-target neutrons with an amount of?107 can be generated as the high energy deuterium ions penetrate into the CD ablator plasma.These findings offer an cexplanatiorn for the anomalous innutron yield ob-served in the indirect-drive ICF experiments performed at the SG-? prototype laser facility.Further experiments have been performed on the SG-? Upgrade laser facility to study the collision and interpenetration of plasma flows driven by laser-planar-target interactions.A direct measurement of the structure and development of ES shocks is obtained through proton imaging and neutron di-agnosing,which further confirms the above statement.For indirect-drive ICF,the super-thermal ions accelerated by the ES shock could be a new source of the preheating effect of the compressed pellet.During the evolution of collisionless shocks,the excitation of unstable waves due to plasma instabilities and wave-particle interactions play important roles in the energy dissipation of collisionless shocks.In chapter three,the evolution of collisionless ES shocks in two symmetric counter-streaming plasma flows,as well as the effect of plasma instabilities on this process,are investigated by PIC simulations.It is found that when the flow velocity is low(about twice the ion acoustic speed),two ES shocks propagating in opposite directions are generated due to the steepening of electrostatic waves at the boundary of the plasma over-lap layer.With the propagation of ES shocks,longitudinal and oblique ion-ion acoustic instabilities are triggered by the transmitted ions in the downstream region and the shock accelerated ions in the upstream region,respectively.These instability modes lead to the deceleration and thermalization of ions through ion trapping or turbulent scattering,thus providing effective energy dissipation mechanism for ES shocks.In addition,it is found that when the flow velocity increases to about three times of the ion acoustic speed,oblique ion-ion acoustic instability is triggered in the overlap layer.The resulted oblique unstable waves destroy the longitudinal electrostatic wave at the boundary of the overlap layer,and thus destroy the ES shock formation.As a result,the maximum Mach num-ber that an ES shock can achieve is limited by plasma instabilities and should be less than the theoretical predictions based on the one-dimensional model.In the collisionless shock acceleration(CSA)of heavy ions driven by super-intense laser pulses,the quality of the accelerated ion beam(ion energy,flux,etc)is decided by the strength of the collisionless shock.While the shock strength is decided by the velocity of the laser-driven 'piston' plasma flow.In chapter four,a method aiming at improving the collisionless shock strength by applying perpendicular external magnetic field is proposed.Due to the external magnetic field,the fast electrons generated during laser-plasma-interaction(LPI)process is confined in the LPI region.The accumulated fast electrons result in a high pressure gradient between the LPI region and the unperturbed plasma,which drives a high velocity 'piston' plasma in conjunction with the laser light pressure.Using two-dimensional PIC simulations,it is found that the external magnetic field can effectively improve the shock strength and ion beam quality.For the interaction of a super-intense laser pulse(5 × 1019W/cm2)with near relativistic cut-off density(NRCD)plasma,the energy of shock accelerated ions significantly increase from?50 MeV in the unmagnetized case(where ES shock is driven)to?150 MeV in the magnetized case(where magnetized shock is driven).While the energy transfer efficiency from laser to shock accelerated ions is increased from?0.6%to?10%.