A New Electrostatic mode in Two-Fluid (ENe-Ion) Formalism of a Partially Ionized Plasma

TL;DR

This paper introduces a novel two-fluid plasma model combining electrons and neutrals into a single fluid, revealing a new electrostatic mode with frequency influenced by collision rates.

Contribution

It proposes a new ENe-Ion two-fluid formalism for partially ionized plasmas and analyzes its unique electrostatic wave mode.

Findings

Discovery of a new electrostatic wave mode in the ENe-Ion plasma

Wave frequency depends on collision frequencies and plasma parameters

Neutral particles acquire a negative charge in this formalism

Abstract

It is economical to devise ways and means to simplify a multi-species particle system. The single fluid MHD description of a plasma is sufficiently adequate to study plasma phenomena . A plasma containing multi-species of particles can also be described by representing each particle species by the corresponding fluid. A partially ionized plasma consisting of electrons, ions and the neutral particles can be described as a single fluid by appropriately combining the dynamics of each of the species. The three component plasma can also be reduced to a two-fluid system. The most obvious way is to combine the electron and the ion fluids to obtain a single fluid in the MHD limit and the neutral particles to form the second fluid. The other two possibilities are 1) to combine electron fluid and the neutral fluid into one fluid, here, christened as ENe fluid, and treat the ions as the second fluid and 2) to combine the ion fluid and the neutral fluid into one fluid, here, christened as INe fluid, and treat the electrons as the second fluid. It is found that the process of combining the electrons and the neutrals endows the neutrals with a negative electric charge. So, here is a new plasma with heavy ( with mass of the neutral particle assuming the electron mass to be negligible) negatively charged particles along with the positively charged ions. One could in principle, similarly treat the second possibility. However , since there can be a huge variation between the relative masses of the neutrals and the ions, the system turns out to be more complex and its discussion is deferred until later. Here, after establishing the framework for the two-fluid ENe-Ion system, the characteristic electrostatic wave mode of this novel two-fluid plasma is determined. The wave frequency is found to be a function of the collision frequencies in addition to the dependence on usual parameters.