Electron bunching induced by betatron resonance in a picosecond laser driven plasma channel

Abstract

In direct laser acceleration, the spatial distribution of electrons will become bunched periodically. This bunching has been regarded to be caused by forced oscillations driven by the laser field. In this paper we report on the generation of electron bunching in a picosecond laser driven plasma channel. In our case, the bunched electrons are caused by betatron resonance, not forced oscillations. Due to the resonance, electrons are kept in phase with the laser field, which leads to similar trajectories in the laser field for the most energetic electrons. Furthermore, these electrons will cross the laser axis with an equal slope induced by the equal electron transverse velocity in the direct laser acceleration regime, which also aids the formation of the bunched electrons. We find that a quasi-static longitudinal electric field exists in the second half of the plasma channel, which provides a pre-acceleration process for the electron before the resonance condition is matched. The electron transverse velocity decreases dramatically during the pre-acceleration process and the decrease of electron transverse velocity is the key point to match the resonance condition.