# Particle-in-Cell Simulations of Density Peak Formation and Ion Acceleration from Short Pulse Laser-Driven Ponderomotive Steepening

## TL;DR

This study uses particle-in-cell simulations and analytic models to explore how short laser pulses induce density peaks and accelerate ions in plasmas through ponderomotive steepening, revealing high electric fields and ion energies.

## Contribution

It provides new insights into ion acceleration mechanisms and electric field strengths during ponderomotive steepening with short laser pulses, supported by simulations and theoretical analysis.

## Key findings

- Ion populations reach a few keV in energy.
- Longitudinal electric fields of 200 GV/m are generated.
- Ion density peaks develop on a specific timescale.

## Abstract

We use particle-in-cell (PIC) simulations and simple analytic models to investigate the laser-plasma interaction known as ponderomotive steepening. When normally incident laser light reflects at the critical surface of a plasma, the resulting standing electromagnetic wave modifies the electron density profile via the ponderomotive force, which creates peaks in the electron density separated by approximately half of the laser wavelength. What is less well studied is how this charge imbalance accelerates ions towards the electron density peaks, modifying the ion density profile of the plasma. Idealized PIC simulations with an extended underdense plasma shelf are used to isolate the dynamics of ion density peak growth for a 42 fs pulse from an 800 nm laser with an intensity of 10$^{18}$ W cm$^{-2}$. These simulations exhibit sustained longitudinal electric fields of 200 GV m$^{-1}$, which produce counter-steaming populations of ions reaching a few keV in energy. We compare these simulations to theoretical models, and we explore how ion energy depends on factors such as the plasma density and the laser wavelength, pulse duration, and intensity. We also provide relations for the strength of longitudinal electric fields and an approximate timescale for the density peaks to develop. These conclusions may be useful investigating the phenomenon of ponderomotive steepening as advances in laser technology allow shorter and more intense pulses to be produced at various wavelengths. We also discuss the parallels with other work studying the interference from two counter-propagating laser pulses.