Ionization waves (striations) in a low-current plasma column revisited with kinetic and fluid models

TL;DR

This paper uses kinetic and fluid models to simulate ionization waves in low-current neon and argon plasmas, reproducing experimental striation characteristics and analyzing their physical mechanisms.

Contribution

It presents the first fully kinetic self-consistent simulations of ionization waves across various conditions, linking kinetic and fluid model insights.

Findings

Reproduces experimental properties of ionization waves

Identifies the kinetic origin of spatial resonances

Shows the importance of non-Mawellian electron energy distributions

Abstract

A one-dimensional Particle-In-Cell Monte Carlo Collisions (PIC-MCC) method has been used to model the development and propagation of ionization waves in neon and argon positive columns. Low current conditions are considered, i.e. conditions where stepwise ionization or Coulomb collisions are negligible (linear ionization rate). This self-consistent model describes the development of self-excited moving striations, reproduces many of the well-known experimental characteristics (wavelength, spatial resonances, potential drop over one striation, electron "bunching" effect) of the ionization waves called p, r and s waves in the literature and sheds light on their physical properties and on the mechanisms responsible for their existence. These are the first fully kinetic self-consistent simulations over a large range of conditions reproducing the development of p, r and s ionization waves. Although the spatial resonances and the detailed properties of the striations in the non-linear regime are of kinetic nature, the conditions of existence of the instability can be obtained and understood from a linear stability analysis of a three-moment set of quasi-neutral fluid equations where the electron transport coefficients are expressed as a function of electron temperature and are obtained from solutions of a 0D Boltzman equation. An essential aspect of the instability leading to the development of these striations is the non-Mawellian nature of the electron energy distribution function in the uniform electric field prior to the instability onset, resulting in an electron diffusion coefficient in space much larger than the energy diffusion coefficient.