Characterization of ionization injection in gas mixtures irradiated by subpetawatt class laser pulses

The effects of ionization injection in low- and high-Z gas mixtures for the laser wake field acceleration of electrons are analyzed with the use of balance equations and particle-in-cell simulations via test probe particle trajectories in realistic plasma fields and direct simulations of charge loading during the ionization process. It is shown that electrons appearing at the maximum of the laser pulse field after optical ionization are trapped in the first bucket of the laser pulse wake. Electrons, which are produced by optical field ionization at the front of laser pulse, propagate backward; some of them are trapped in the second bucket, the third bucket, and so on. The efficiency of ionization injection is not high, several pC/mm per bucket. This injection becomes competitive with wave breaking injection at lower plasma density and over a rather narrow range of laser pulse intensity.