Theoretical Studies On The Electron Acceleration In Laser Wakefield Acceleration And The High Frequency Radiation In Laser-plasma Interaction

Along with the development of ultra-short ultra-intense laser technology,the tabletop particle sources and radiation sources which can be generated by laser-plasma interaction have been studied widely.These sources have the advantage of short size,collimation and low cost,which were widely used in inertial confinement fusion,particle accelerator,med-ical diagnosis and national defense.My thesis is focused on the laser wakefield electron acceleration and the generation of radiation in laser-plasma interaction by means of the combination of theoretical analysis and numerical simulations.The thesis is composed of three parts:In the first part?chapters 2 to 4?,the laser wakefield electrons acceleration is inves-tigated.Firstly,the enhancement of electron injection in the laser-driven bubble acceleration by ultra-intense laser irradiating foil-gas targets is studied.In this scheme,a thin foil target is placed in front of a gas target.Upon interaction with an ultra-intense laser pulse,the foil emits electrons with large longitudinal momenta,allowing them to be trapped into the transmitted shaped laser-excited bubble in the gaseous plasma target.The foil target has two effects in the acceleration:First,the foil provides additional electrons accelerated by the laser ponderomotive force,which can be trapped by the wakefield.Second,the shaped laser pulse resulting from the relativistic self-induced transparency effect with a longitudinally steepened rising front enhances the electrons number of self-injection in the bubble.Comparing with the case of without foil target,the total number of the trapped electrons can be enhanced.Secondly,the effect of near-infrared and mid-infrared lasers on the electron accel-eration in laser wakefield acceleration is studied.For a fixed laser power,a mid-infrared laser pulse can deliver a larger ponderomotive force.Such a strong ponderomotive force can drive an intense plasma wave,and the electron self-injection can occur more effi-ciently.As a result,electron self-injection occurs earlier and the plasma density threshold for injection is lower as the laser wavelength becomes longer.Additionally,the trapped electron charge is enhanced by 60 times as the laser wavelength increases from 0.8?m to4?m by using P=100TW laser pulses.We also obtain two quasi-monoenergetic electron beams with narrow energy spreads of 2.4%and 4.3%using the laser wavelength of 2?m corresponding to a planned experiment at our laboratory.Thirdly,the acceleration and evolution of a hollow relativistic electron beam in a laser wakefield driven by a Laguerre-Gaussian laser pulse are studied.The Laguerre-Gaussian laser pulse,which has transverse ring intensity-distribution,can produce a ring-shaped wakefield.Part of the angular momentum of the Laguerre-Gaussian laser is transferred to the ionization-injected electrons.The angular momentum will result in rotation of the electrons in the wakefield.However,electrons from different positions of the hollow ring experience different rotation orientations because of the laser polarization and wakefield focusing forces.Due to the small initial transverse momenta,most of the ionization in-jected electrons cannot make multi cycle helical motion in the wakefield.Electrons in the ring move along the direction of the laser polarization piling up at the end and form an electron ring with non-uniform density distribution.Scaled simulations show that the ring size and the trapped electron charge of the hollow electron beam can be well controlled by tuning the laser spot size.However,both the laser intensity and plasma density have a weak effect on the ring size.Finally,the generation and transition of multiple transverse wakefields by using Laguerre-Gaussian pulse are studied.We find that,under suitable conditions?lower plasma densityor small laser spot size?,the inner electron sheaths of the donut-like wake will overlap in the front part of the wake,and the sheath electrons pile up on-axis.The trajectories of the background electrons cross each other near the end of the donut-like bubble forming a closed central bell-like wake.The wake has a longitudinal acceleration field and an inverted triangle-like transverse focusing field.The multiple transverse wakefields can accelerate both on-axis and ring-shaped electron beams simultaneously.One can control the transition process from a central on-axis electron beam to a ring-shaped electron beam by tuning the plasma and laser parameters.In the second part?chapter 5?,the Betatron radiation in laser gas target interaction is investigated.The tunable X-ray can be obtained by using the two-color laser ioniza-tion injection in laser wakefield acceleration.By controlling the injection laser offset distance and polarization direction,the motion trajectories of accelerated electron beam can be adjusted,which lead to a highly tunability of the radiation intensity shape and polarization distribution.When the suitable injection laser parameters are adopted,the X-ray radiation with an annular intensity shaped and symmetrical polarization distribu-tion can be obtained.The peak brilliance of the annular radiation can achieve 1.3×10¹⁹photon/s/mm²/mrad²/0.1%bandwidth.Meanwhile,the parameters of laser and plasma are realizable in the current experimental facilities.In the third part?chapter 6?,the high order harmonic generation in laser solid target interaction is investigated.The directional enhancement of selected high-order-harmonics can be realized using blazed grating targets.Such targets can select harmonics with fre-quencies being integer times of the grating frequency.Meanwhile,the radiation intensity and emission area of the harmonics are increased.The emission direction is controlled by tailoring the local blazed structure.Theoretical and electron dynamics analysis for harmonics generation are carried out,the selection and directional enhancement are due to the coherent addition effect of radiation from the interaction between multi-cycle laser and grating target and the enhancement effect from individual blazed protuberance which gives larger energy gain of the electrons.These studies will benefit the generation and application of laser plasma-based high order harmonics...