Strong Laser Driven Collision-free Shock Wave Research

The purpose of this thesis is to study how to generate collisionless shocks in laboratory using intense laser facilities and also study the phenomena caused by collisionless shocks.The main methods have been used in this thesis are intense laser experiments and numerical simulations.Some main and/or innovative results are summarized as follows:1.Collisionless electrostatic shocks generated in laboratory are focused of the fields of laboratory astrophysics and high energy ions acceleration.In laboratory conditions,collisionless electrostatic shocks can be generated by dense plasma expansion into rarefied medium.However,so far,it is not clear that how the collisionless electrostatic shocks are affected by the initial parameters of plasmas.In this thesis,the collisionless electrostatic shocks velocity formula and the critical condition of collisionless electrostatic shocks formation were investigated by solving two-fluids equations and particle-in-cell simulations.At first,the particle-in-cell simulation shown that the expanding dense plasma cannot penetrate but pushes the rarefied plasma when the electrostatic instability between ions in dense and rarefied plasmas has been developed.The pushing of expanding dense plasma generates a collisionless electrostatic shock in the rarefied plasma.Then we obtained the collisionless electrostatic shocks velocity formula which is related with plasma electron temperature and initial plasma electron density by solving the two-fluids equations.Finally,according to the collisionless electrostatic shocks velocity formula and critical Mach number of collisionless electrostatic shocks formation,we found that the collisionless electrostatic shocks exist only when initial plasma electron density ratio is ranging from 4 to 25.2.Laser-driven helium ion source,with multi-MeV energy,has important application in the field of fusion reactor material irradiation damage.At present,generating high energy helium ions by relativistic ultraintense laser interacting with helium gas jet is the main scheme of laser-driven helium ion source.However,so far,this scheme is hard to generate the helium ion beam with the characteristics of forward,quasi-monoenergetic,multi-MeV and high yield.These helium ion beam characteristics are important for the application of material irradiation damage study.In this thesis,we propose a new scheme that is using ultraintense laser interacting with foil-gas complex target to generate helium ions.Using this method,we performed an experiment that laser with intensity 5×1018 W/cm2 and duration 0.8ps interacts with foil-gas target.The foil-gas target is composed by copper foil with 7?m thickness and helium gas nozzle which is behind the copper foil.The experiment results show that the obtained helium ions is forward and quasi-monoenergetic(the peak energy is 2.7MeV),and the total energy of helium ions that its energy larger than 0.5MeV is about 1.1 J/sr,the corresponding helium ions yield is about 1013/sr.The helium ions spectrum and hot electron temperature given by PIC simulation in experimental conditions are consistent with experimental results.In addition,the PIC simulations also show that helium ions are accelerated by CSA-like mechanisms,and the maximum helium ion energy is proportion to the hot electron temperature.3.Instabilities associated with collisionless shocks are crucial for the shocks evolvement and high energy cosmic ray generation.Here we report the experimental results of streak structures of electric field driven by ion-ion acoustic instability in laser produced collisionless electrostatic shock.At first,the rarefied pre-plasma with low speed and large scale is generated by low intensity laser amplified spontaneous emission(ASE)ablates the thick foil.Then the collisionless electrostatic shock is driven by hot dense plasma produced by the high intensity laser ASE and expands into the pre-plasma.The ion-ion acoustic instability is excited by ions reflected by collisionless shock penetrate through the shock upstream.The electric field of the collisionless electrostatic shock and the associated ion-ion acoustic instability are observed by proton radiography.Corresponding numerical results agree well with the experimental data.Both experimental and numerical results show that the ion-ion acoustic instability causes collisionless electrostatic shock turbulence.4.Relativistic collisionless shock charged particle acceleration is considered as a possible origin of high energy cosmic rays.However,it is hard to explore the nature of relativistic collisionless shock due to its low occurring frequency and remote detecting distance.Recently,there are some works attempt to solve this problem by generating relativistic collisionless shock in laboratory conditions.In laboratory,the scheme of generation of relativistic collisionless shock is that two electron-positron pair plasmas knock each other.However,in laboratory,the appropriate pair plasmas have been not generated.The ten-PW laser pulse maybe generates the pair plasmas that satisfy the formation condition of relativistic collisionless shock due to its ultrahigh intensity and energy.In this thesis,we study the positron production by ultraintense laser high Z target interaction using numerical simulations which consider quantum electrodynamics effect.The simulation results show thatthe forward positron beam up to 1013/kJ can be generated by ten-PW laser pulse interacting with Pb target.The estimation of relativistic collisionless shock formation shows that the positron yield satisfies formation condition and the positron divergence needs to be controlled.Our results indicate that the generation of relativistic collisionless shock by ten-PW laser facilities in laboratory is possible.