The Theoretical And Numerical Simulation Research For Ultra-intense Laser-driven Ion Acceleration

With the development of laser technology especially the invention of the chirped pulseamplification (CPA) technique, ultra-short and ultra-intense laser pulses with unprecedentedintense field strength can be realized by a table-top facility in laboratory, which greatlybroadens the research area of the interaction between light field and matter, entering theultra-relativistic nonlinear optics. The contents included in this area is conspicuously rich,such as laser electron acceleration, laser ion acceleration, high harmonic and attosecond pulsegeneration, laser nuclear physics, and even nonlinear vacuum physics and so on. Thecorresponding application prospect is also very broad. It not only can provide high energyparticle and high energy radiation sources, but also opens the door detecting the unchartedultra-fast physical phenomenon. Among the numerous research subdisciplines of therelativistic laser matter interactions, the laser-driven ion acceleration is one of the mostchallenging issues, depending heavily on the enhancement of the laser intensity due to theirlarge mass comparing with that of electron. There are a few mechanisms of ion accelerationproposed so far, such as target normal sheath acceleration (TNSA), radiation pressureacceleration (RPA), break-out afterburner (BOA), and collisionless shock wave acceleration,etc. However, there are a few problems related with these mechanisms such as the lowmaximum energy, poor energy spectra, low energy conversion efficiency, or excessively highrequests on laser parameters. All these factors put significant limitation of these mechanismsfor practical applications. Therefore, new and effective schemes are urgently demanded.Under this background, this thesis carries out theoretical and numerical investigation on thenew mechanisms and schemes of laser-driven ion acceleration in direct laser interaction withion beams and laser interaction with thin solid targets. Main results are given as follows.1. We proposed an exact solution of the Maxwell equations based on the plane-wave angular spectrum analysis. This solution is applied to calculate the fields of a radially polarizedLaguerre-Gaussian laser beam. It shows that the strength of the longitudinal electric fieldnear the focus obtained from our solution can be significantly larger than that of transverseelectric field as the beam waist size is less than a laser wavelength. Based on the abovemethod, a solution valid for tightly focused radially polarized few-cycle laser pulsespropagating in vacuum is presented. The resulting field distribution is significantlydifferent from that based on the paraxial approximation for pulses with either small or largebeam diameters (comparing with laser wavelength). We compare the electron accelerationsobtained with the two solutions and find that the energy gain obtained with our newsolution is usually much larger than that with the paraxial approximation solution.2. Based on the method proposed in the previous chapter, a solution for the field of a tightlyfocused radially polarized (RP) chirped laser pulse is presented. With this solution, directlaser acceleration of protons by this kind of RP laser pulses is investigated numerically. It isfound that a RP laser pulse with proper negative frequency chirps can lead to efficientproton acceleration, reaching sub-GeV at the laser intensity of1022W/cm2from itsinjection energy of45MeV.3. Currently, most laser ion acceleration experiments are baed on the TNSA mechanism, whilethe main problem of this mechanism is its low efficiency (the maximum proton energy isabout60MeV at present) and usually it can only obtain an exponentially decreasing energyspectrum. To solve the above problem, we proposed a two-stage proton accelerationscheme using present-day intense lasers and a unique target design. The target systemconsists of a hollow cylinder with conical inner wall, which is followed by the main targetwith a flat front and a dish-like flared rear surface. At the center of the latter is a taperedproton layer, which is surrounded by side proton layers at an angle to it. In the firstacceleration stage, protons in both layers are accelerated by target normal sheathacceleration. The center-layer protons are accelerated forward along the axis while the sideprotons are accelerated and focused towards them. As a result, the side-layer protonsradially compress as well as axially further accelerates the front part of the center-layerprotons in the second stage.2D PIC simulation show that a quasi-monoenergetic proton bunch with the maximum energy over250MeV and energy spread~17%can be generatedwhen such a target is irradiated with an80fs laser pulse with focused intensity3.11020W/cm2.3D PIC simulation gives the reduced maximum energy~112MeV buteven smaller energy spread~3%under the same laser conditions due to anisotropic electronacceleration with linearly polarized lasers.4. The realizing of the radiation pressure acceleration (RPA) usually needs laser intensitylarger than1023W/cm2which is beyond the currently available one. Meanwhile, this ionacceleration mechanism is subject to the Rayleigh-Taylor like instability which prematurelyterminates the acceleration process. To solve the above problem, we proposed a new targetdesign named as Dual Parabola Target (DPT) consisting of a side target (ST) with parabolicinner surface and a middle target (MT) with parabolic front surface to generate high qualityquasi-monoenergetic proton bunch. In the case of using a plane target, the transversedistribution of the Rayleigh-Taylor instability appears a Gaussian shape which is harmful tothe formation of a light sail in the central part of the target. While in the case of using theDPT target, the lateral part of the laser pulse is reflected and focused by the parabolic innerwall of the ST target and then obliquely incident at the surrounding area of the MT targetthat causes the proton kinetic energy thus the surface instability growth rate of the wingregion of the MT target is higher than that of its central region which protects this centralpart and enable it effectively to be a light sail. Furthermore, this effect significantly reducesthe requirement of the peak laser intensity.2D and3D PIC simulation results indicate that aquasi-monoenergetic proton bunch with peak energy262MeV and energy spread~13%can be generated when such a target is irradiated by a66fs circularly polarized laser pulsewith focused intensityI L9.9′1021W/cm2.5. Multi-dimensional effects on proton acceleration by linearly polarized intense laser pulseinteracting with a thin solid target have been investigated numerically. We checked thesimulation geometry effects by running one, two, and three dimensional (1D,2D,3D)particle-in-cell (PIC) simulations.3D simulation results show that, electrons spread almostuniformly along two transverse directions in the case of using a relatively thick target (inthe opaque regime). While electrons spread more quickly along the direction orthogonal to the laser polarization direction in the case of using an ultra-thin target (in the transparentregime) especially at the early stage. The transverse spreading of electrons stronglydecreases the electron density at the rear side of the target. Such effect makes differentestimation of electron temperatures in different simulation geometries. Usually1D and2Dsimulations overestimate the temperature and as a result, the maximum proton energyobserved in1D or2D simulation is about3or2times of that observed in3D simulations.Therefore, it appears that3D simulation is necessary if one wants to compare simulationand experimental results no matter thin or thick targets are adopted.