Collisionless Shock Formation And Ion Acceleration In Intense Laser Interactions With Near-criticle-density Plasmas

The study of collisionless shocks is crucial to understand a number of astrophysical phenomena,such as supernova remnants and gamma ray bursts.With the development of modern laser technology,intense-laser-matter interactions can already create some astrophysical-like extreme conditions,and hence provide the unique opportunity to generate and study collisionless shocks in the laboratories.On the other hand,intense-laser-matter interactions can offer an accelerating field as strong as a few hundreds of GV/m,which is many orders of magnitude stronger than that in the conventional accelerators.This promises the compact size for laserdriven accelerators.In particular,the ion beams generated in laser-driven ion acceleration have the advantages of short bunch and high density in comparison with these from the conventional accelerators.Laser-driven ion beams have many great potential applications,such as proton beam imaging,cancer therapy,fast ignition fusion and so on.So laser-driven ion acceleration attracts growing attention in the last decades.Combining the advances in the high-power laser technology and the target fabrication,breathtaking progresses have been achieved in the various scenarios of laser-driven ion acceleration.Recently,the generation of near 100 Me V protons has been demonstrated in the target normal sheath acceleration(TNSA),which is still the most common scenario of laser-driven ion acceleration in the experiments.However,the ion beam from the TNSA usually has an exponential energy spectrum,which is a big obstacle to many important applications such as cancer therapy.In contrast,it is promising to generate the quasi-monoenergetic ion beams in the scenario of radiation pressure acceleration(RPA),which can be further classified as the“hole-boring”and“light-sail”modes according to the target thickness.In the“light-sail”RPA,it is predicted that a nanoscale solid foil could be continuously pushed by the radiation pressure of an ultraintense laser pulse.Correspondingly,a quasi-monoenergetic ion beam could be produced.Nevertheless,the laser and target conditions for the RPA are extremely rigorous.Moreover,the onset of transverse instabilities and the subsequent plasma heating may terminate the RPA prematurely.Therefore,the experimental results of the RPA are far away from the predictions.Until now,it is still a great challenge to improve the energy and quality of ion beams in laser-driven ion acceleration.With the development of the target fabrication technology,the laser interactions with near-critical-density targets gain more and more attention recently because of the efficient ion acceleration as well as the abundant physical phenomena in this regime.This thesis is focused to the theoretical and numerical study of the collisionless electrostatic shock formation and the ion acceleration in the laser interactions with the near-critical-density plasmas.The thesis is composed of the following parts:Firstly,we introduce some basic concepts,parameters and the main content of laser plasma physics.The numerical method of particle-in-cell simulation is also simply introduced.Then we present a simple review on some laser-driven ion acceleration mechanisms,including the target normal sheath acceleration,radiation pressure acceleration and collisionless shock acceleration.Secondly,we have studied the formation and evolution of collisionless electrostatic shocks using two counter-stream plasma flows.In the experiment,a pair of collisionless shocks that propagate in the opposite directions are observed during the interaction of two counter-streaming plasma flows.The counter-streaming plasma flows are generated by irradiating a pair of opposing copper foils with multi laser beams.One-dimensional PIC simulations show that a pair of strong electrostatic fields are gradually raised during the collision of two plasma flows.Consequently,some ions from the upstream will accumulate around the shock front and form a density jump,which greatly contributes to the shock formation.In addition,the simulation results indicate that the collisionless shock evolution is mainly governed by the thermal pressure gradient.This is because that the most kinetic energy of electrons will be converted into the thermal energy when they pass through the shock front.Therefore,the downstream temperature is obviously higher than that in undisturbed upstream.In this case,the shock is not in thermal equilibrium state and unstable.The pair of shocks will move to upstream due to the thermal pressure gradient.The simulation results are in good agreement with the experiment results.Thirdly,the collisionless electrostatic shock formation and the subsequent ion acceleration in the interactions of ultra-intense laser pulses with near-critical-density plasmas have been studied.Compared with the collisionless shock generated in the laser interactions with solid targets,the collisionless shock generated in the laser interactions with near-critical-density targets could have an ultra-high speed.To analyze the conditions for the generation of a high-speed collisionless shock,we have reviewed the fluid models for the solitary and shock wave generation at first.Then we have presented the Rankine-Hugoniot Relation,which defines the parameter relations between the downstream and the upstream of the shock.More importantly,the PIC simulations indicate that both the speed of laser-driven collisionless electrostatic shock and the energies of shock-accelerated ions can be greatly enhanced using a near-critical-density target.However,a response time longer than tens of laser wave periods is required for the shock formation in this scenario.Anomalously,we find that some ions can be reflected by the collisionless shock even if the electrostatic potential jump across the shock is smaller than the ion kinetic energy in the shock frame.It is because that the longitudinal electric field excited in a near-critical-density target is strongly time-oscillated,so the ion reflection condition based on the completely-static electric field is no longer appropriate in this case.At last,we proposed an efficient hybrid laser-driven ion acceleration scheme using a combination target of a solid foil and a density-tailored background plasma.In the first stage,a sub-relativistic proton beam can be generated by the radiation pressure acceleration in the intense laser interaction with the solid foil.In the second stage,these pre-accelerated protons are injected into the near-critical-density plasma,where they are further accelerated by the laser wakefield.The simulation results show that the wakefield phase velocity is approximate to the propagating velocity of laser front,and the latter slowly increases as the plasma density decreases.Thus,by properly decreasing the near-critical-density plasma density along the laser propagation direction,the wakefield can travel faster and faster as the accelerated protons,which postpones the dephasing between the accelerated protons and the wakefield.As a result,an efficient wakefield proton acceleration can be achieved.This hybrid laser-driven proton acceleration scheme can be realized by using a laser pulse with a duration of a few tens femtosecond and a peak power of 10 PW for the generation of multi-Ge V proton beams.