Source-flow expansion of a partially ionized gas into a vacuum.

The purpose of this investigation is to provide some theoretical results for use in predicting the flow conditions which could be expected in freejet plasma expansions. The model investigated is a one-dimensional spherically symmetric, supersonic source flow of partially ionized argon, the assumption heing that this model represents a reasonably good approximation for the centerline flow conditions of a freejet. The analysis utilizes kinetic theory equations of change for the three species (ions, electrons, and neutrals) and a relaxation-type collision model. An ellipsoidal Maxwellian distribution function for each of the species is postulated, and using this distribution function equations for the macroscopic flow variables are derived. The reactions considered are two-body ionization and collisional-radiative recombination, with rate constants being those computed by Bates and co-workers. Both the optically thick and optically thin cases are investigated. The equations are integrated numerically for the case of argon. Two types of source flows are investigated: 1) a simple supersonic source flow, with the initial supersonic state assumed to be an equilibrium one; and 2) a nozzle expansion from an equilibrium stagnation state to slightly supersonic conditions, followed by a source flow. In the second case, both equilibrium and nonequilibrium nozzle flows are considered.