Simulations of ion heating due to ion-acoustic instabilities in the presheath

TL;DR

This study uses particle-in-cell simulations to demonstrate that ion-acoustic instabilities in presheaths can cause significant ion heating, especially at low neutral pressures and high electron temperatures, revealing a wave reflection heating mechanism.

Contribution

It provides the first detailed simulation-based analysis of ion heating due to ion-acoustic instabilities in presheaths, highlighting the conditions for wave-driven heating.

Findings

Ion-acoustic instabilities cause ion heating when T_e/T_i exceeds ~28.

Ion heating extends from the sheath into the bulk plasma due to wave reflection.

Higher neutral pressures damp ion-acoustic waves, reducing ion heating.

Abstract

Particle-in-cell direct simulation Monte Carlo simulations reveal that ion-acoustic instabilities excited in presheaths can cause significant ion heating. Ion-acoustic instabilities are excited by the ion flow toward a sheath when the neutral gas pressure is small enough and the electron temperature is large enough. A series of 1D simulations were conducted in which neutral plasma (electrons and ions) was uniformly sourced with an ion temperature of 0.026 eV and different electron temperatures (0.1 - 50 eV). Ion heating was observed when the electron-to-ion temperature ratio exceeded the minimum value predicted by linear response theory to excite ion-acoustic instabilities at the sheath edge ($T_e/T_i\approx28$). When this threshold was exceeded, the temperature equilibriation rate between ions and electrons rapidly increased near the sheath so that the local temperature ratio did not significantly exceed the threshold for instability. This resulted in significant ion heating near the sheath edge, which also extended back into the bulk plasma; presumably due to wave reflection from the sheath. This ion-acoustic wave heating mechanism was found to decrease for higher neutral pressures, where ion-neutral collisions damp the ion-acoustic waves and ion heating is instead dominated by inelastic collisions in the presheath.