# Anomalous collisional absorption of laser light in plasma using particle-in-cell simulations

## TL;DR

This study uses particle-in-cell simulations with Monte Carlo collisions to investigate anomalous collisional laser absorption in under-dense plasma, revealing a non-monotonic dependence on laser intensity at low electron temperatures.

## Contribution

First to demonstrate anomalous collisional laser absorption using PIC simulations with Monte Carlo modeling, connecting experimental observations with simulation results.

## Key findings

- Absorption increases with laser intensity up to a critical point at low temperatures.
- Absorption then decreases beyond the critical intensity, showing non-monotonic behavior.
- The results bridge the gap between experimental findings and theoretical models.

## Abstract

Collisional absorption of laser light in a homogeneous, under-dense plasma is studied by a new particle-in-cell (PIC) simulation code considering one-dimensional slab-plasma geometry. Coulomb collisions between charge particles in plasma are modeled by a Monte Carlo scheme. %[J. Comput. Phys. {\bf 25}, 205 (1977)]. %Both PIC and MC parts are individually benchmarked. For a given target thickness of a few times the wavelength of 800~nm laser of intensity $\I0$, fractional absorption ($\alpha$) of light due to Coulomb collisions (mainly between electrons and ions) is calculated at different electron temperature $\Te$ by introducing a total velocity $v = \sqrt{\vth^2 + \v0^2}$ dependent Coulomb logarithm $\ln\Lambda(v)$, where $\vth$, and $\v0$ are thermal and ponderomotive velocity of an electron. It is found that, in the low temperature regime ($\Te\lesssim15$~eV), fractional absorption of light anomalously increases with increasing $I_0$ up to a maximum corresponding to an intensity $I_c$, and then it drops when $I_0>I_c$. %(approximately) obeying the conventional scaling, i.e., %$\alpha \propto I_0^{-3/2}$ when $I_0>I_c$. Such an anomalous variation of $\alpha$ with $I_0$ in the low intensity regime was demonstrated earlier in experiments, and recently explained by classical and quantum models [Phys. Plasmas {\bf 21}, 013302 (2014); Phys. Rev. E {\bf 91}, 043102 (2015)]. % using the total velocity dependent cut-offs. %Here, we report anomalous nature of laser absorption by Here, for the first time, we report anomalous collisional laser absorption by %PIC simulations assisted by Monte Carlo collisions, PIC simulations, thus bridging the gap between models, simulations, and experimental findings.