Features of ion acoustic waves in collisional plasmas

TL;DR

This paper investigates how collisions and friction affect ion acoustic waves in plasmas, revealing damping, evanescence, and growth phenomena depending on plasma composition, magnetic fields, and wave properties.

Contribution

It provides a comprehensive analysis of collisional effects on ion acoustic waves, including damping, evanescence, and growth, with new insights into behavior in inhomogeneous and magnetized plasmas.

Findings

In quasi-neutral plasmas, IA waves propagate without damping.

Collisions with neutrals can damp, suppress, or re-enable IA waves depending on neutral density.

In magnetized inhomogeneous plasmas, collisions can cause IA wave growth.

Abstract

The effects of friction on the ion acoustic (IA) wave in fully and partially ionized plasmas are studied. In a quasi-neutral electron-ion plasma the friction between the two species cancels out exactly and the wave propagates without any damping. If the Poisson equation is used instead of the quasi-neutrality, however, the IA wave is damped and the damping is dispersive. In a partially ionized plasma, the collisions with the neutrals modify the IA wave beyond recognition. For a low density of neutrals the mode is damped. Upon increasing the neutral density, the mode becomes first evanescent and then reappears for a still larger number of neutrals. A similar behavior is obtained by varying the mode wave-length. The explanation for this behavior is given. In an inhomogeneous plasma placed in an external magnetic field, and for magnetized electrons and un-magnetized ions, the IA mode propagates in any direction and in this case the collisions make it growing on the account of the energy stored in the density gradient. The growth rate is angle dependent. A comparison with the collision-less kinetic density gradient driven IA instability is also given.