Studies On Proton Beam Accelerated By Relativistic Laser-Solid Interaction

With the rapid development of laser technology,high energy particle and radiation from ultra-short ultra-intense laser plasma interaction have attracted broad interests.The high-energy proton beams driven by relativistic lasers(??10^??W/cm^?)have a wide range of potential applications,such as plasma state diagnostics,warm dense matter production,fast ignition of ICF and radiative therapy.In pratical experiments,laser prepulse plays a key role in the acceleration process due to preplasma formation at front surface,target deformation and preplasma excitation at target rear side.Therefor,It is key important to study the prepulse's effects on laser proton acceleration and develop approches to control the proton beam quality.This thesis reports on study of proton acceleration via ultra-short ultra-intense laserinteracting with solid-density foil targets,aiming at optimizing and controlling the spectrum and spatial distribution of laser driven protons through laser pulse shaping and target manipulations.The results presented in this thesis are mainly focused on experimental study of proton acceleration conducted on the 200TW laser system established at Shanghai Jiao Tong University.The main work is listed as below.Stable plateau structures are observed in proton spectrum in low laser contrastcondition.It is proposed that such proton spectra are attributed to the combined effect of laser driven collisionless shock acceleration?CSA?and target normal sheath acceleration?TNSA?.Simple analytic estimation and two-dimensional particle-in-cell simulations are performed,showing that protons from target front surface are accelerated via CSA mechanism and transport to target rear surface,where sheath field could further accelerate the beam and broaden the spectrum.The obtained plateau-shape spectrum may also serve as an effective tool to diagnose the plasma state and verify the ion acceleration mechanisms in laser-solid interactions.Laser prepluse could also affect the lateral transportation of hot electrons,which inturn change the performance of proton acceleration.The effects of laser contrast on protons emitted from targets of two different lateral sizes are comprehensively studied.Intrinsic correlation between the proton beam spatial profile and spectral shape is identified.When the laser pulse contrast is low,large beam divergence and near-plateau shape energy spectrum are observed for both the millimeter-sized foil targets?F-target?and the hundred-micrometer-sized targets?S-target?.With high contrast laser irradiation,proton beam divergence is remarkably reduced and the energy spectral shape is changed to exponential for F-targets.In comparison,a similar large divergence and the near-plateau spectral shape remain for S-targets.The results could be explained by the hot electron refluxing and the modified accelerating sheath field evolution in S-target,which were supported by the 2D hydrodynamic and PIC simulations.Results of proton beam spatial distribution from laser irradiated S-targets are also presented.Multiple line structures,which are more profound in low contrast case,are observed in the spatial profile of proton beam from S-targets.The line structures are TNSA protons emitting from S-target edges and mounting stalks.The line emissions represent the feature of lateral transportation and propagation of hot electrons and provide information of magnetic field evolution at the target rear surface.The thesis depends on the SJTU experimental platform for laser solid-density plasma interaction,which is mainly established by the author.The author designed and built up the SJTU 200TW solid target area.Ion diagnostics specialized for high-repetition shots have been developed,and a femtosecond probe beam for diagnosing the dynamics of plasma has been set up.The author also contributed to the design,set up and characterizing of a plasma mirror system,which reduces the intensity of prepulses and ASE pedestals of 200TW laser by 2 orders of magnitude.