Instability-Enhanced Friction in the Presheath of Two-Ion-Species Plasmas

TL;DR

This paper investigates how two-stream instabilities in the presheath of two-ion-species plasmas can enhance friction forces, affecting ion entry speeds into the sheath, and reconciles conflicting experimental and simulation results.

Contribution

It introduces a revised theory accounting for electron-ion temperature ratio effects and confirms the role of instability-enhanced friction through numerical solutions and particle-in-cell simulations.

Findings

Instability-enhanced friction depends on electron-ion temperature ratio.

Two-stream instabilities can generate significant ion-ion friction.

Theory predictions align with simulation results when including this friction.

Abstract

The speed at which ions enter a sheath is a fundamental property of a plasma that also provides a useful boundary condition in modeling. A recent theory proposed that this can be significantly influenced by an instability-enhanced friction force arising from two-stream instabilities in the presheath when multiple ion species are present. Although experiments appeared to confirm this theory, recent particle simulations have brought it into question. We reconcile this controversy using direct numerical solutions of the dispersion relation, which show that there is a dependence on the electron-ion temperature ratio that was not considered previously. In addition, particle-in-cell simulations are used to show that ion-ion two-stream instabilities can arise near the sheath edge and generate an enhanced ion-ion friction force. Only by accounting for the instability-enhanced friction force can theory predict the simulated ion speeds at the sheath edge.