Observation of neutrals carrying ion-acoustic wave momentum in partially ionized plasma

TL;DR

This study experimentally investigates how neutral particles influence ion-acoustic wave propagation in argon plasma, revealing that increased neutral density can support longer wave propagation contrary to previous damping expectations.

Contribution

It provides new experimental evidence that neutrals can enhance ion-acoustic wave propagation in partially ionized plasma, supported by a simple analytical model.

Findings

Neutral density affects wave amplitude, velocity, and length.

Collisions can support longer wave propagation despite damping.

Experimental results align qualitatively with the analytical model.

Abstract

An experimental study of Ion Acoustic (IA) wave propagation is performed to investigate the effect of neutral density for argon plasma in an unmagnetized linear plasma device. The neutral density is varied by changing the neutral pressure, which in turn allows the change in ion-neutral, and electron-neutral collision mean free path. The collisions of plasma species with neutrals are found to modify the IA wave characteristics such as the wave amplitude, velocity, and propagation length. Unlike the earlier reported work where neutrals tend to heavily damp IA wave in the frequency regime {\omega}<{\nu}_in (where {\omega} is ion-acoustic mode frequency and {\nu}_in is ion-neutral collision frequency), the experimental study of IA wave presented in this paper suggests that the collisions support the wave to propagate for longer distances as the neutral pressure increases. A simple analytical model is shown to qualitatively support the experimental findings.