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The general features of the small-amplitude oscillations of a two-component ionized gas are discussed. The effects of the random thermal motions of ions and electrons are described by one-particle distribution functions. When collisions are neglected, there are two types of waves for a given wavelength. One is a high-frequency electron plasma oscillation, slightly modified by ionic motions. The other is the Tonks-Langmuir positive-ion oscillation which is shown to be undamped when the electron temperature is considerably greater than the ion temperature. The effects of collisions are treated by a kinetic model which satisfies the conservation laws, and provides for energy and momentum exchange between components. The low-pressure waves are damped primarily because of the decreasing electron temperature with increasing collision frequency. The validity of transport treatments is investigated. At high density and high frequency one finds the correct sound wave for a mixture of gases. Study of the frequency as a series of inverse powers of the collision frequency shows that the first-power term yields absorption independent of the electric charge. Higher powers give contributions to the absorption and dispersion which depend on electrical polarization as well as on diffusion, viscosity, and heat conductivity. The behavior at the more frequently realized case of low frequency and high pressure depends on the electric charge more directly.
Gross et al. (Tue,) studied this question.