Phase space consideration of low energy electron injection for Direct Laser Acceleration

TL;DR

This paper analytically investigates ionization injection for Direct Laser Acceleration, identifying optimal phase space trajectories and plasma conditions that enable electrons to gain hundreds of MeV energy from near-zero initial energies.

Contribution

It introduces a simplified analytical model using phase portraits to determine effective injection criteria and optimal electron trajectories in DLA.

Findings

Optimal phase space trajectory enables ~10^9 energy gain

Electron density should be a few percent of critical density

Model predictions agree with numerical simulations

Abstract

Feasibility of ionization injection for Direct Laser Acceleration (DLA) of electrons up to hundreds of MeV was studied analytically. Criteria for effective injection determining range of background and in-channel plasma parameters, laser intensity, etc. were found using phase portraits of the system deduced from the simplified analytical model. The found optimal trajectory in the phase space corresponds to the electron with low (few eV) initial energy experiencing $\sim$10$^9$ times energy gain. For this to occur, electron density should be a few percent of the critical density, while the in-channel electron density should be $\sim$3 times lower. The analytically obtained dependence of the energy gain on the initial electron longitudinal and transverse momenta corresponds well to the results of exact numerical simulations of an electron motion in the plasma channel. Developed approach can form the basis for further studies of electron injection in DLA varying plasma and laser parameters as well as initial electron energies.