Electron Acceleration Mechanism And Nonlinear Propagation Of Relativistic Vortex Laser Pulses In Plasmas

With the rapid development of the ultra-intense vortex laser pulses technol-ogy,ultra-intense vortex laser with high angular momentum has attracted more interest for its advantages in laser based particle accelerator,laser based radi-ation source and fast ignition scheme.It is of great significance to investigate the electron acceleration mechanism and nonlinear propagation of ultra-intense vortex laser pulses in plasmas.This thesis is focused on the theoretical and numerical studies on the nonlin-ear propagation of relativistic vortex laser pulses in underdense plasmas,angular momentum evolution and transfer mechanism of ultra-intense laser in underdense plasmas and the electron acceleration mechaninm of strong vortex laser in plasma bubble.This thesis is mainly composed of the following four parts:Chapter 2 summarizes the history and the development of vortex beam,and gives a brief introduction of the generating technique of non-relativistic and relativistic vortex laser pulses.Then the features of intense vortex laser plasma interactions are given.Finally the views on the unsolve problems among intense vortex laser plasma interactions are put forward.Chapter 3 studies the filamentation dynamics of relativistic optical vortex beams(OVBs)propagating in underdense plasma.Based on the relativistic Schrodinger equation that takes into account the relati,vistic ponderomotive ef-fects,it is shown that the optical vortex beam with a finite orbital angular mo-mentum(OAM)would exhibit a much robust propagation behavior in under-dense plasmas compared with the normal Gaussian laser beam without OAM.The growth rate of azimuthal modulational instability would be much decreased with the increase of topological charge of the optical vortex beams.As a result,the relativistic optical vortex beams in underdense plasmas can maintain its pro-file for a longer distance before the filamentation occurs.In addition,during the filamentation stage,the optical vortex beam breaks up into regular filamentation patterns under the constraint of OAM conservation,in contrast to the Gaussian laser beam that experiences a random filamentation process.Chapter 4 studies angular momentum evolution and transfer mechanism of ultra-intense laser in underdense plasmas.Based on the equtions of electron motion,the relation of electron angular momentum between laser and plasma parameters is proposed.It was shown that in the interactions of ultra-intense circularly polarized laser pulse with the near-critical plasmas,the angular mo-mentum can be transferred efficiently from the laser beam to electrons through the resonance acceleration process.The transferred angular momentum increases almost linearly with the acceleration time ta when the electrons are resonantly accelerated by the laser field.In addition,it is shown analytically that the av-eraged angular momentum of electrons is proportional to the laser amplitude aL and the total angular momentum of the accelerated electron beam is proportion-al to the square of the laser amplitucde aL2 for a fixed parameter of ne/ncaL.These results are verified by three-dimensional particle-in-cell simulations.Chapter 5 studies the electron acceleration mechanism of strong Laguerre-Gaussian laser in plasma bubble.Based on the 3D simulation methods,the process of intense Laguerre-Gaussian laser interacting with underdense plasma is simulated.It is shown that electrons are accelerated by the longitudinal charge-separation electric field and also the transverse laser electric field.Meanwhile,the LG laser pulse effectively transfers its angular momentum to the accelerated electron beam and manipulates the topological structure of the electron beam.Our three-dimensional particle-in-cell simulations show that in this regime,the accelerated electron beam is spatially separated and consists of multi-slice helical bunches.A theoretical model based on the electron phase-space dynamics is also proposed to explain the emergence of multi-slice helical electron bunches.It is clarified that the topological charge of the LG laser pulse determines the number of fixed points in the system,which in turn determines the number of electron bunches that are accelerated in different phases of the laser electric field.In this way,the topological structure of the electron beam can be well controlled by adjusting the laser's topological charge.