Ion Acceleration Mechanism In The Interactions Of Ultra-short And Ultra-intense Laser Pulses With Solid Targets

Great advances in the ultra-short and ultra-intense laser-pulse technology o?ers anew possibility to accelerate ions by laser-material interactions, which plays a greatrole in developing e?cient, compact accelerators, proton-induced fast ignition forinertial-confinement fusion, cancer therapy, compact heavy ion-injection source andmany other applications. Based on the theory of the isothermal plasma expansion intoa vacuum, target normal sheath acceleration (TNSA) is a widely accepted mechanismof energetic-ion acceleration from a solid target shot by an ultra-intense laser pulse.However, until now, there is not an adequate theory to describe the non-quasi-neutralplasma expansion with non-Maxwellian electrons in the ultra-short time interval oftens of femtoseconds. Therefore, exploring new perspective acceleration regimes isstill challenging and comprehensive understanding of laser-ion acceleration physics isdesired.In this dissertation, comprehensive theoretical studies are performed to investigatethe process of isothermal plasma expansions into a vacuum based on the fluid equationsand Maxwell's equations. Main points are described below.Under the quasi-neutral assumption, a special solution for multi-species ions ac-celeration from a solid target shot by an ultra-intense laser pulse is obtained and then ageneral solution is achieved as well. It is shown that theoretical results agree well withexperimental data.To describe isothermal expansions of non-quasi-neutral plasmas with an energeticelectron tail into a vacuum, a half-self-similar solution is proposed by employing theequation of continuity, the equation of ion motion and Poisson's equation . Conse-quently, an analytic formula of maximum ion velocity is obtained. It is found that theion acceleration by an energetic electron tail is more efficient than that in quasi-neutralplasmas.A discrete model, called the Step Model, is proposed for describing hot-electron recirculation and show the influence of it on the ion acceleration from the laser-foil in-teractions. A time-dependent solution that describes neutral-plasma isothermal expan-sions into a vacuum is obtained in a special-transformation coordinate system. Com-bining the time-dependent solution and the quasi-linear increase of the electron densitydue to the hot-electron recirculation, an analytic model is proposed to reveal the in-fluence of the hot- electron recirculation on the increase of electric field and on theacceleration of ions of different masses and charges.A general time-dependent solution is obtained to describe laser-plasma isother-mal expansions into a vacuum. It is adequate for non-quasi-neutral plasmas, di?erenttypes of the scale length of the density gradient and different charge separations. Theprevious solutions are some special cases of our general solution.A two dimensional planar model is developed for describing self-similar isother-mal expansions of non-quasi-neutral plasmas into a vacuum when solid targets shotby ultra-intense laser pulses. The angular ion distribution and the dependence of themaximum ion velocity on laser parameters and target thicknesses are predicted. Con-sidering the self-generated magnetic field of plasma beams as a perturbation, the ionenergy on edge at the ion opening angle has an increase of 2% relative to that on thefront center. Therefore, the self-generated magnetic field of plasma beams is not largeenough to interpret for the ring structures.A two-phase model, where the plasma expansion is an isothermal one when laserirradiates and a following adiabatic one after laser ends, has been proposed to predictthe maximum energy of the proton beams induced in the ultra-intense laser-foil inter-actions. The hot-electron recirculation in the ultra-intense laser-solid interactions hasbeen accounted in and described by the time-dependent hot-electron density continu-ously in this model. The dilution e?ect of electron density as electrons recirculate andspread laterally has been considered. With the model, the scaling laws of maximumion energy have been achieved and the dependence of the scaling coefficients on laserintensity, pulse duration and target thickness have been obtained.Based on theoretical results involved in this dissertation, the influence of variousphysical parameters on laser-ion acceleration is discussed. A cone-shape multilayer target composed by high Z metal and Hydrocarbon foil is designed to optimize thegeneration of the quasi-monoenergetic ions with ultra-intense laser pulse, which isschemed in future experiments.