Different representations of a partially ionized plasma

Abstract

It is economical to devise ways and means to simplify a multi-species particle system. A partially ionized plasma consisting of electrons, ions and neutral particles can be described as a three-fluid, a two-fluid or a single-fluid system by appropriately combining the dynamics of each of the species. Here, one of the three possible two-fluid descriptions is discussed wherein the electron fluid and the neutral fluid are combined into one fluid, christened as ENe fluid, and treat the ions 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. Hence, here is a plasma with heavy (with nearly the mass of the neutral particle) negatively charged particles along with the positively charged ions. After establishing the framework for the two-fluid, the ENe–ion, system, the characteristic electrostatic wave mode of this novel unmagnetized plasma is determined. A new mode of frequency x ¼ xEN emerges in the limit ð2cen þ ceiÞ x, where cen and cei are, respectively, the electron–neutral and the electron–ion collision frequencies. This is identified as a collective mode of the ENe fluid, a counterpart of the electron plasma mode in a fully ionized plasma.