# Particle-in-Cell Simulations of Electron Beam Production from Infrared Ultra-intense Laser Interactions

## TL;DR

This study uses high-resolution Particle-in-Cell simulations to explore electron acceleration mechanisms during mid-infrared ultra-intense laser interactions with dense targets, extending previous research to longer wavelengths and different intensity regimes.

## Contribution

It extends prior work by analyzing electron acceleration at mid-IR wavelengths and identifies conditions for super-ponderomotive electron acceleration at normal incidence.

## Key findings

- Super-ponderomotive electron acceleration occurs at mid-IR wavelengths when $a_0
ot\gg 1$.
- Electron angular and energy distributions match analytic models under certain conditions.
- Mid-IR simulations show ponderomotive steepening with multiple density peaks.

## Abstract

Advances in ultra-intense laser technology are enabling, for the first time, relativistic intensities at mid-infrared (mid-IR) wavelengths. Anticipating further experimental research in this domain, we present high-resolution two dimensional Particle-in-Cell (PIC) simulation results using the Large- Scale Plasma (LSP) code that explore intense mid-IR laser interactions with dense targets. We present the results of thirty PIC simulations over a wide range of intensities (0.03 < $a_0$ < 39) and wavelengths ({\lambda} =780 nm, 3 {\mu}m, and 10 {\mu}m). Earlier studies, limited to {\lambda} =780 nm and $a_0 \sim$ 1 [1, 2], identified super-ponderomotive electron acceleration in the laser specular direction for normal- incidence laser interactions with dense targets. We extend this research to mid-IR wavelengths and find a more general result that normal-incidence super-ponderomotive electron acceleration occurs provided that the laser intensity is not highly relativistic ($a_0 \lesssim 1$) and that the pre-plasma scale length is similar to or longer than the laser wavelength. Under these conditions, ejected electron angular and energy distributions are similar to expectations from an analytic model used in [2]. We also find that, for $a_0 \sim 1$, the mid-IR simulations exhibit a classic ponderomotive steepening pattern with multiple peaks in the ion and electron density distribution. Experimental validation of this basic laser-plasma interaction process will be possible in the near future using mid-IR laser technology and interferometry.