Transport properties of a weakly coupled, partially ionized argon plasma

Abstract

Accurate transport properties, such as viscosity, diffusion, and thermal conductivity, are essential input for computational fluid dynamics simulations of plasmas. These properties are difficult to obtain experimentally at high temperatures and are therefore most often calculated using kinetic theory. Molecular dynamics (MD) offers an alternative route, in which transport coefficients are extracted directly from particle trajectories without relying on the assumptions underlying kinetic theory. While MD is commonly applied to liquids, its use for dilute gases and partially ionized plasmas has received little attention.

In this work, transport coefficients are computed using both Chapman-Enskog kinetic theory and molecular dynamics for two model systems. The first is a high-temperature, non-ionized argon gas, used to develop and validate the MD framework in a dilute regime. The second is a partially ionized argon plasma, modelled as a multicomponent extension of the Yukawa one-component plasma, in which ions and neutrals are treated explicitly while the effect of electrons is incorporated implicitly through Debye screening. The kinetic theory is evaluated using reduced collision integrals computed for the Lennard-Jones, Morse, and Yukawa potentials, and the MD simulations are performed in LAMMPS using the OCTP plugin to extract transport coefficients through Green-Kubo and Einstein relations.

For the non-ionized gas, both methods reproduce the transport properties with good accuracy. The MD results show excellent agreement with kinetic theory for self-diffusion, and consistent agreement for viscosity, confirming that the framework operates correctly in a regime significantly more dilute than those typically considered in MD simulations. For the partially ionized plasma, the framework extends successfully using a combined Yukawa-Morse potential. The neutral self-diffusivity is well described by a pressure-corrected Chapman-Enskog prediction, the ion self-diffusivity converges toward the fully ionized prediction as the degree of ionization increases, and the viscosity reproduces the drop associated with the onset of ionization.

The validity of the modelling choices is assessed in detail. Treating electrons implicitly is justified for the weakly coupled conditions considered here, as the plasma coupling parameter remains at or below the boundary of the weakly coupled regime and the electron--argon cross section is negligible compared to the heavy-particle cross sections. The neglect of resonant charge exchange has no influence on viscosity, but a non-negligible influence on Ar self-diffusion. Consequently, the viscosity is found to be the most reliable output of this work, the self-diffusion coefficients are reliable within limited ranges, and the thermal conductivity cannot be interpreted as physical once ionization sets in, as the dominant electron contributions are absent from the simulations.