Laser Energy Absorption And Conversion During The Interaction Of Intense Laser Pulses With Solid Targets

With the development of laser technology,the interaction of intense laser pulse with solid targets offers more and more promising applications such as high harmonic generation,plasma-based particle acceleration,and high-energy electron production,etc.Knowing how much laser energy can be converted to thermal or radiation energy is of critical importance to these applications.Besides,laser-produced plasmas as intense x-ray sources have attracted much interest in recent decades due to their wide applications in many fields,such as advanced lithography,x-ray backlighter imaging,high energy density physics and inertial confinement fusion(ICF).One of the major goals in these applications is to enhance the x-ray conversion efficiency(CE)and therefore the x-ray emission intensity.In this dissertation,focused on the ICF,we investigated the laser energy absorption and conversion process during the interaction of intense laser pulses with solid targets theoretically and numerically:Firstly,an analytical model for laser-plasma interaction during the oblique incidence by an intense p-polarized laser on a solid target is proposed.Both the resonant absorption and not-so-resonant absorption are self-consistently included.Different from the previous theoretical works,the physics of resonant absorption is found to be valid in more general conditions.Even for a relativistic intensity laser,resonant absorption can still exist under certain plasma scale length.For shorter plasma scale length or higher laser intensity,the not-so-resonant absorption tends to be dominant.The laser energy absorption rates for both mechanisms are discussed in detail,and the difference and transition between these two mechanisms are presented.Secondly,an analytical model for energy absorption during the interaction of an intense laser with solid targets is proposed.Both the compression effect of the electron density profile and the oscillation of the electron plasma surface are self-consistently included,which exhibit significant influences on the laser energy absorption.Based on our model,the general scaling law of the compression effect depending on laser strength and initial density is derived,and the temporal variation of the laser absorption due to the boundary oscillating effect is presented.It is found that the compression effect of the electron density will reduce the laser energy absorption dramatically.Due to the oscillation of the electron plasma surface,the laser absorption rate will vibrate periodically at ? or 2? frequency for the p-polarized and s-polarized laser,respectively.Comparison of our model calculation with the experimental results is displayed,which shows good agreement.Thirdly,based on the MULTI-2D program,we developed a new laser energy deposition module: Ray_Tracing.Detailed numerical model and general algorithm of the MULTI-2D are presented.Compared with the crude ray-tracing approximation forlaser light propagation in the original code,the new module can handle the reflection and refraction of the laser beams simultaneously.The physical model and code structure of the new module are described,and three typical testing cases are presented,which proves the validity and efficiency of the new model clearly.Fourthly,to gain a deep insight of the temporal evolution of plasma density distribution in the New Ignition Target,detailed simulations using the code MULTI-2D are carried out.We investigated the influences of the target geometry and laser parameters on the plasma density distribution.Energy deposition of the laser beams is calculated by the new Ray_Tracing module.The simulation results give us a clear description of the plasma filling process in the hohlraum,and the temporal evolution of the laser energy deposition and conversion are presented.To avoid the plasma filling of the entrance hole and improve the laser energy deposition further,better target design is needed.Finally,a novel double-foil configuration is proposed to improve the laser to x-ray conversion efficiency from laser irradiating a solid target.The improvement is mainly due to the enhanced soft x-ray emissions.Influences of the target geometry parameters on the x-ray conversion efficiency are investigated.Detailed energy distributions and the individual contributions of the two foils to the thermal and kinetic energy terms are presented.It is found that the enhancement of radiation is attributed to the lower ion kinetic energy of the double-foil target.