Ionization waves (striations) in low-current DC discharges in noble gases obtained with a hybrid kinetic-fluid model

TL;DR

This study uses a hybrid kinetic-fluid model to analyze ionization waves in low-current noble gas discharges, revealing nonlocal effects as key to striation formation and comparing wave properties with experimental data.

Contribution

It introduces a coupled kinetic-fluid modeling approach to explain plasma stratification and ionization wave dynamics in noble gases, including Helium stability insights.

Findings

Nonlocal effects cause ionization wave formation.

Calculated wave properties match experimental data.

Stratified plasma is more efficient than uniform plasma.

Abstract

A hybrid kinetic-fluid model is used to study ionization waves (striations) in a low-current plasma column of DC discharges in noble gases. Coupled solutions of a kinetic equation for electrons, a drift-diffusion equation of ions, and a Poisson equation for the electric field are obtained to clarify the nature of plasma stratification in the positive column and near-electrode effects. A simplified two-level excitation-ionization model is used for the conditions when the nonlinear effects due to stepwise ionization, gas heating, and Coulomb interactions among electrons are negligible. It is confirmed that the nonlocal effects are responsible for the formation of moving striations in DC discharges at low plasma densities. The calculated properties of self-excited waves of S, P, and R types in Neon and S type in Argon agree with available experimental data. The reason for Helium plasma stability to stratification is clarified. It is shown that sustaining stratified plasma is more efficient than striation-free plasma when the ionization rate is a nonlinear function of the electric field. However, the nonlinear dependence of the ionization rate on the electric field is not required for plasma stratification. Striations of S, P, and R types in Neon exist with minimal or no ionization enhancement. Effects of the column length on the wave properties have been demonstrated in our simulations.