| In the last few decades,intense laser pulse technologies develop rapidly and laser intensities have reached 1018～1022 W/cm2.Laser-plasma interaction has entered relativistic and even ultra-relativistic regime which involves various nonlinear effects,such as relativistic light solitons,energetic particle acceleration and coherent radiation sources.To our knowledge,relativistic soliton formation is mainly studied in underdense plasmas before,and its formation mechanism in overdense plasma is not clear yet.Meanwhile,vector light technologies make it possible to produce intense vector laser pulses in the laboratories.Relativistic vector-light solitons in plasmas have still not been investigated yet.In this Thesis,we successfully develop a numerically dispersionless particle-in-cell code JPIC3D.The JPIC3D solves Maxwell’s equations in three-dimensional space by use of a direct-splitting(DS)algorithm,which can accurately solve electromagnetic dynamics with less computation consume.Combined with the calculation of relativistic motions of electrons and ions in plasmas,JPIC3D can self-consistently simulate complex nonlinearities and relativistic physics between electromagnetic waves and plasmas.By JPIC3D,we study the formation mechanism and physical conditions of relativistic solitons.in overdense plasmas.Two new electron direct acceleration schemes are proposed,one is to use the plasma-focused radially-polarized laser to accelerate electrons directly.And the other scheme is that electrons are injected by a plasma mirror into a radially-polarized laser harmonics and get accelerated.We also found that the angularly-polarized laser can form doughnut-shaped solitons in plasmas.Details are as follows:1.We comprehensively simulate on relativistic soliton formation during the interaction of tightly-focused laser pulses with collisionless plasmas.By varying the background plasma density and the laser amplitude,significant differences are found in terms of energy trapping efficiency and formation process.For underdense plasmas,solitons are formed in the wake of laser pulses,after a series of nonlinear processes such as self-focusing and plasma dispersion.For plasmas of near or over critical density nc,solitons are directly evolved from electron cavities which are formed by the ponderomotive forces at the plasma interface.The most favorable plasma density for soliton formation is found to be about 0.8nc.For the density higher or lower than this value,the solition formation needs higher laser intensities.2.We simulated the interaction between angularly-polarized laser and underdense plasmas,and found that this kind of light can be easily trapped by the plasma and forms a doughnut-shaped soliton.During the self-focusing propagation of the angularlypolarized laser in the plasma,its frequency can be reduced to the local plasma frequency,and then the laser is trapped by the plasma and forms a doughnut-shaped soliton.The trapping distance is related to the intensity gradient of the laser and the plasma density.3.We propose two new electron direct acceleration methods with radially polarized laser.In the first method,laser first pushes electrons inside the plasma and creates a large charge-separation field.Both the longitudinal component of the laser fields and the charge-separation field pull an electron bunch out of the plasma.The laser is then reflected by the plasma and focused into a stronger and shorter laser pulse.The laser catches up with the running electron bunch and accelerate it to a higher energy.The second method is to use the self-focused laser and other nonlinear effects to accelerate electrons in underdense plasma.Using these two method,we can obtain ultrashort relativistic electron beams within several microns. |
