Intensity threshold in vacuum laser acceleration

Abstract

The dependence of the electron-energy gain on the on-axis laser intensity of a TEM00 light wave in vacuum, called the capture and acceleration scenario (CAS), has been studied. We found that there exists a laser intensity threshold for the CAS scheme to work. The physical meaning of the intensity threshold is that, when the intensity is strong enough, fast electrons injected into the Rayleigh zone where the phase velocity of the light wave is subluminous can be accelerated until they catch up with the phase velocity before they slip out. Thereby these electrons can receive a considerable amount of energy from the laser field. Analytical calculations based on this situation and simulation results show similar features in that the intensity threshold value, (a0T)2, is strongly dependent on the laser-beam width at focus, kw0. For example, kw0=40 corresponds to a0T∼5, which is available by present laser systems. Also, it has been proved that the maximal electron-energy gain in the CAS regime is linearly proportional to the laser intensity as well as to kw0.