On electrostatic modes in multi-ion and pair-ion collisional plasmas

TL;DR

This paper explores electrostatic wave modes in multi-ion and pair-ion plasmas, analyzing effects of density gradients, electron presence, and magnetic fields, revealing conditions for backward modes and instabilities relevant to recent experiments.

Contribution

It provides a theoretical analysis of plasma modes in pair-ion and multi-ion plasmas, including the effects of density gradients and electron dynamics, explaining experimental backward modes.

Findings

Electron plasma mode can become backward with density gradient.

Instability driven by electron collisions and density gradient in hot electron plasmas.

Pure pair-ion plasma modes can exhibit backward behavior similar to experiments.

Abstract

The physics of plasmas containing positive and negative ions is discussed with special attention to the recently produced pair-ion plasma containing ions of equal mass and opposite charge. The effects of the density gradient in the direction perpendicular to the ambient magnetic field vector, observed in the experiment, are discussed. In addition, the possible presence of electrons is discussed in the context of plasma modes propagating at an angle with respect to the magnetic field vector. The electrons may either be added to the plasma or enter the plasma attached to negative ions and then become released from the ions as a result of collisions. It is shown that the electron plasma mode in a cold plasma may become a backward mode in the presence of a density gradient, and this behavior may be controlled either by the electron number density or the mode number in the perpendicular direction. In plasmas with hot electrons an instability may develop, driven by the combination of electron collisions and the density gradient, and in the regime when the ions' response is similar to a sound mode, i.e., for un-magnetized ions, implying mode frequencies much above the ion gyro-frequency or mode wave-lengths shorter than the ion gyro-radius. In the case of a pure pair-ion plasma, for lower frequencies and for parameters close to those used in the recent experiments, the perturbed ions may feel the effects of the magnetic field. In this case the plasma mode also becomes backward, resembling features of an experimentally observed but yet unexplained backward mode.