Thermodynamic State Variables in Quasi-Equilibrium Ultracold Neutral Plasma

TL;DR

This paper uses molecular dynamics simulations to evaluate thermodynamic variables in ultracold plasmas, introducing a method to distinguish bound states from free charges and analyzing their contributions to pressure and energy.

Contribution

It presents a novel approach to separate bound and free charge contributions in ultracold plasma thermodynamics using a pseudo-potential model.

Findings

Free charge contribution to pressure is independent of core length-scale.

Partial pressure of free charges follows one-component plasma model, becoming negative at strong coupling.

Total system pressure remains positive despite negative partial pressures.

Abstract

The pressure and internal energy of an ultracold plasma in a state of quasi-equilibrium are evaluated using classical molecular dynamics simulations. Coulomb collapse is avoided by modeling electron-ion interactions using an attractive Coulomb potential with a repulsive core. We present a method to separate the contribution of classical bound states, which form due to recombination, from the contribution of free charges when evaluating these thermodynamic state variables. It is found that the contribution from free charges is independent of the choice of repulsive core length-scale when it is sufficiently short-ranged. The partial pressure associated with the free charges is found to closely follow that of the one-component plasma model, reaching negative values at strong coupling, while the total system pressure remains positive. This pseudo-potential model is also applied to Debye-H\"{u}ckel theory to describe the weakly coupled regime.