Evidence for neutrals carrying ion-acoustic wave momentum in a partially ionized plasma

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 (where is ion-acoustic mode frequency and 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.

Ion-acoustic waves in weakly ionized plasmas with charge-exchange collisions

“…Although this may appear counter-intuitive given that one may expect a combination of collisional and collisionless damping mechanisms, neutral pressure has an effect 3 , and whether a pressure threshold exists at which instability can be excited or suppressed depends on the collision frequency, or equivalently, on the mean free path. An experimental study 4 has also verified that ion-neutral collisions support the ion-acoustic wave propagation for longer distances at higher neutral gas pressure. These earlier studies indicate that the dynamical behavior of neutrals in weakly ionized plasmas must be properly treated in kinetic theory to understand wave phenomena.…”

Existence of Electrostatic Ion Cyclotron Waves in a Laboratory Created E Region Ionospheric‐Like Plasma

“…However, most previous works on EIC waves have been performed in collisionless or weak collisional plasma similar to that in the F region ionosphere, which is different from that in the E region ionospheric plasma (Liu et al., 2021). A significant characteristic of the E region ionospheric plasma, where molecular ions are dominant, is the collisional effect induced by the partially ionized plasma (Sharma et al., 2020). Ion‐neutral collisions introduce new physical effects and processes, such as charge‐exchange can lead to a retarding force, which subsequently influences the dynamic evolution of plasma waves (Okamoto et al., 2003; Saleem et al., 2016).…”

Plasma–neutral coupling allows electrostatic ion cyclotron waves to propagate below ion cyclotron frequency

“…10 A recent experiment by Sharma et al has verified this by showing that the IA waves excited with a grid exciter can propagate farther at higher neutral gas pressure. 11 These previous studies demonstrate that the behavior of neutral particles needs to be treated correctly in order to understand wave phenomena in partially ionized plasmas. On the other hand, the effect of neutral particles on wave phenomena in magnetized plasmas has not been fully discussed.…”