Hydrodynamic model for expansion and collisional relaxation of x-ray laser-excited multi-component nanoplasma

TL;DR

This paper introduces a hydrodynamic model for simulating the expansion and collisional relaxation of multi-component nanoplasmas created by femtosecond x-ray laser pulses, enabling efficient analysis of their picosecond dynamics.

Contribution

The paper develops a hydrodynamic framework based on Boltzmann moments to model nanoplasma dynamics, including ionization and recombination processes, for the first time.

Findings

Model successfully simulates spherical argon nanoplasma expansion.

Hydrodynamic approach offers computational efficiency for nanoplasma dynamics.

Framework can be extended to other collisional and relaxation processes.

Abstract

The irradiation of an atomic cluster with a femtosecond x-ray free-electron laser pulse results in a nanoplasma formation. This typically occurs within a few hundreds femtoseconds. By this time the x-ray pulse is over, and the direct photoinduced processes no longer contributing. All created electrons within the nanoplasma are thermalized. The nanoplasma thus formed is a mixture of atoms, electrons and ions of various charges. While expanding, it is undergoing electron impact ionization and three-body recombination. Below we present a hydrodynamic model to describe the dynamics of such multi-component nanoplasma. The model equations are derived by taking the moments of the corresponding Boltzmann kinetic equations. We include the equations obtained, together with the source terms due to electron impact ionization and three-body recombination, in our hydrodynamic solver. Model predictions for a test case: expanding spherical Ar nanoplasma are obtained. With this model we complete the two-step approach to simulate x-ray created nanoplasmas, enabling computationally efficient simulations of their picosecond dynamics. Moreover, the hydrodynamic framework including collisional processes can be easily extended for other source terms and then applied to follow relaxation of any finite non-isothermal multi-component nanoplasma with its components relaxed into local thermodynamic equilibrium.