Investigation Of Generation And Radiation Of Sub-femtosecond Electron Pulses Driven By Radially Polarized Laser Pulses

Laser technologies have been developing rapidly,since the advent of Chirped Pulse Amplification.At present,the peak intensity of ultrashort ultraintense laser pulses has al-ready been far exceeding the relativistic intensity.As a kind of cylindrical vector beams,radially polarized laser pulses have circularly-symmetric laser electric field,and stronger longitudinal electric field component.Therefore,radially polarized laser pulses have many applications in a variety of areas,especially in electron acceleration.The experi-mental generation of high-quality radially polarized laser pulses with relativistic intensity can also be conducive to its application in electron acceleration.Sub-femtosecond electron pulses have diverse applications in a variety of areas,such as relativistic electron mirrors,free-electron laser and ultrashort radiation sources.These applications require high-quality sub-femtosecond electron pulses that can transport sta-bly.However,it is still challenging to generate electron pulses simultaneously with high beam charge,narrow energy spread and low divergence angle due to the beam loading ef-fects,the Coulomb repulsion between electrons and the pondermotive force of the laser,which limits the applications of sub-femtosecond electron pulses.In this dissertation,we propose new schemes to generate high-quality sub-femtosecond electron pulses,based on the technical foundation provided by the development of radially polarized laser pulses.The main content is as follows:Firstly,we propose a novel direct laser acceleration scheme by irradiating a radially polarized laser pulse on a nanofiber.It is demonstrated via three-dimensional（3D）PIC simulations that sub-femtosecond electron pulses simultaneously with high beam charge,narrow energy spread and low divergence angle can be generated.The energy spread,the center divergence angle and the full width at half maximum（FWHM）of the divergence angle of the selected electron slice at t=50T₀are 14%,0.75°and 1.75°,respectively.More importantly,the relative energy spread and the divergence angle can gradually de-crease as electrons transport stably with the laser pulse.Furthermore,an analytical model of electron acceleration is presented which interprets well by the 3D simulation results.Secondly,we propose a new scheme to generate an isolated sub-femtosecond elec-tron pulse and an isolated sub-femtosecondγray pulse by irradiating a radially polarized laser pulse on an underdense nanofiber.Using the Single Particle Code and Particle-in-cell codes,we study the effects of the electron initial injection position on the acceleration mechanism in detail.It is found that most of the electrons accelerated by the ponder-motive force can be removed by reducing the radius of the target.Only the electrons directly accelerated by the longitudinal electric field are retained,thereby improving the quality of the electron beam.It is demonstrated via 3D PIC simulations that isolated sub-femtosecond electron pulse can be generated using a radially polarized laser pulse with the pulse width of 10T₀by changing the parameters of the target.Furthermore,an isolated sub-femtosecondγray pulse can also be generated with the collision of a back-propagating laser with the generated isolated sub-femtosecond electron pulse.The density of the generated isolated sub-femtosecond electron pulse is about 4n_c.The bunch dura-tion of the isolated electron pulse is 600 as,and the duration of the isolatedγray pulse is about 300 as,with the maximum energy of 1.7 Me V.The results can provide references for the generation and application of isolated sub-femtosecond electron pulses andγray pulses in experiments.