Characterization of ionization injection in gas mixtures irradiated by sub-petawatt class laser pulses

TL;DR

This paper investigates ionization injection in gas mixtures during laser wakefield acceleration using simulations, revealing how electrons are trapped and the efficiency of injection under various conditions.

Contribution

It provides a detailed analysis of ionization injection mechanisms and compares its efficiency with wave breaking injection in different plasma densities and laser intensities.

Findings

Electrons ionized at the laser pulse maximum are trapped in the first wake bucket.

Electrons produced at the front propagate backwards and can be trapped in subsequent buckets.

Ionization injection efficiency is several picoCoulombs per millimeter per bucket, competitive at lower densities.

Abstract

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 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 backwards; some of them are trapped in the second bucket, third bucket and so on. The efficiency of ionization injection is not high, several pC/mm/bucket. This injection becomes competitive with wave breaking injection at lower plasma density and over a rather narrow range of laser pulse intensity.