The Study Of Electron Acceleration By Ultra-Intense Laser With Circular Polarization

Ultra-intense laser involves its intensity above 10¹⁸W cm^-2. At these intensities electrons swing in the laser pulse with relativistic energies. The laser electric field is already much stronger than the atomic fields, and any material is instantaneously ionized, creating plasma. In recent years, particles acceleration from such intense short laser pulse has been paid increasing attention. One of the very important applications of the relativistically laser beams accelerating particles is the fast ignition (FI) concepts for the inertial confinement fusion (ICF). Petawatt-class lasers may provide enough energy to isochorically ignite a pre-compressed target consisting of thermonuclear fuel. The FI approach would ease dramatically the constraints on the implosion symmetry work. The laser pulse cannot reach the dense core of the target directly. The laser energy must be converted into fast particles first and then transported through the overdense plasma region. So the energy spectra of laser-generatted particle beams, their emittance and transport problems are very important. On the other hand, the conventional microwave accelerators are currently reaching their energy gradient limit of about 100MeV m^-1 due to the breakdown of optical materials. While the laser accelerators have demonstrated gradients of 100GeV m^-1 over distances of order of a millimeter. So the new generation of high-energy laser-driven accelerators are prospective. In addition, studies of particles acceleration are significant in space and astrophysical physics.Presently, particles acceleration by ultra-intense laser mainly focuses on acceleration mechanisms and generation of quasi-monoenergetic particle beams. In this thesis, we concentrate our work on the electron acceleration by intense circularly-polarized (CP)laser pulse. Both the resonant acceleration of electrons in underdense plasma and the scattering acceleration of injected electrons by tightly-focused laser in vacuum are investigated.In the first part, the resonant acceleration of plasma electrons by intense CP laser is studied theoretically and numerically. The resonant condition is theoretically given and numerically testified. we emphasize that the resonant electron beam accelerated by intense CP laser has better collimation in comparison with the LP case due to the the guiding effect of the axial qausistatic magnetic field. On the other hand, in order to improve the energy gain of resonant electrons, we put forward a new acceleration scheme, which deals with an intense laser propagating in a density-attenuating plasma. In such a plasma, the phase velocity and Doppler-shifted frequency of the laser both decrease, so that the electrons can retain betatron resonance for a rather long time and effectively acquire the energy from the laser. The theoretical analysis is verified by test-particle numerical calculations. These works are arranged in chapter III and chapter IV, respectively.In the second part, acceleration of electrons injecting into a tightly-focused laser pulse with circular polarization is investigated. We have compared the results of sideways injected electrons accelerated by the tightly-focused CP laser and the linearly-polarized one, respectively, and found both cases give energy spreads larger than 100%. In view of this, we propose a new scheme involving an electron beam injected into a CP laser pulse along the axial direction. The calculation results show that the accelerated electrons have good qualities including extremely small energy spread and very low beam divergence. We also discuss the axially injected electrons are accelerated by a CP beat wave. The results show that beat wave acceleration can effectively improve the energy gain in comparison with the case of the single pulse. These works are arranged in chapter V and chapter VI, respectively.