Influence of Ion Streaming Instabilities on Transport Near Plasma Boundaries

TL;DR

This paper investigates how ion streaming instabilities near plasma boundaries affect transport processes, analyzing stability conditions and reviewing experimental evidence for these effects in different plasma compositions.

Contribution

It characterizes the stability phase-space for ion streaming instabilities and discusses their impact on transport, including recent experimental validation and research needs.

Findings

Ion-acoustic instabilities occur at low pressure and high electron-to-ion temperature ratios.

Ion-ion two-stream instabilities can be excited in plasmas with two ion species.

Instabilities can lead to rapid thermalization and enhanced ion-ion friction.

Abstract

Plasma boundary layers are susceptible to electrostatic instabilities driven by ion flows in presheaths and, when present, these instabilities can influence transport. In plasmas with a single species of positive ion, ion-acoustic instabilities are expected under conditions of low pressure and large electron-to-ion temperature ratio ($T_e/T_i \gg 1$). In plasmas with two species of positive ions, ion-ion two-stream instabilities can also be excited. The stability phase-space is characterized using the Penrose criterion and approximate linear dispersion relations. Predictions for how these instabilities affect ion and electron transport in presheaths, including rapid thermalization due to instability-enhanced collisions and an instability-enhanced ion-ion friction force, are also briefly reviewed. Recent experimental tests of these predictions are discussed along with research needs required for further validation. The calculated stability boundaries provide a guide to determine the experimental conditions at which these effects can be expected.