Characteristics of electron ponderomotive acceleration by a laser pulse in vacuum

The ponderomotive acceleration scenario (PAS) for slow-electron acceleration by an intense laser pulse has been studied in detail using three-dimensional test-particle simulations. The relativistic Newton-Lorentz equation has been solved numerically for a linearly polarized laser. The results show that the PAS can accelerate slow electrons to an energy close to 1 MeV for a laser intensity a(0) = 3 (around 10(19) Wcm(-2) for a Ti : sapphire laser) and 24MeV for a(0) = 20. These data are consistent with the experimental observation. The net gain comes from an asymmetry in the fields experienced by the electron during its acceleration and deceleration stages. It is found that electrons are scattered isotropically in the transverse direction for moderate laser intensity. The output bunches exhibit relatively wide angular and energy dispersions. We have also discussed an interesting bifurcation phenomenon in the energy-angle correlation spectra. According to our simulations, the maximal electron-energy gain in the PAS regime is approximately proportional to the laser intensity and the laser beam width and inversely proportional to the laser pulse duration. Physical interpretations based on the ponderomotive potential model as well as Lorentz-Newton force analyses are presented.