Acoustic Wave Propagation in a Gas Discharge

When ordinary sound propagates in a weakly ionized gas, the wave motion of neutral particles controls that of charge carriers. The ion‐density, electric‐field, and electron‐temperature responses to a given gas‐density perturbation are calculated by means of linearized hydrodynamic equations for the case of wave propagation along the axis of a discharge. The equations include ionizing collisions, ambipolar diffusion, wall recombination, and collisions leading to intercomponent momentum and energy transfer. Because of competing effects of ions and electrons, no observable wave amplification can result from intercomponent momentum transfer. In addition to previously reported consequences of elastic collisions, the calculated dispersion relation contains an amplification term characterized by a time constant τν which is approximately the reciprocal ionization frequency (per electron) times the gas‐to‐plasma pressure ratio.