Review of models for collisionless one-dimensional plasma flow to a boundary

Four models of collisionless one-dimensional plasma flow to a boundary are compared with regard to their predictions of particle and heat fluxes to the boundary for a given plasma density and temperature far from the boundary. The models include two kinetic treatments, that of Emmert et al. [Phys. Fluids 23, 803 (1980)], and that of Bissell and Johnson [Phys. Fluids 30, 779 (1987)], an isothermal fluid model, Self and Ewald [Phys. Fluids 9, 2486 (1966)] and Stangeby, [Phys. Fluids 27, 2699 (1984)], and an adiabatic fluid model, Zawaideh, Najmabadi, and Conn [Phys. Fluids 29, 463 (1986)]. The fluid models do not explicitly include collisions; however, the adiabatic closure condition employed, namely, neglect of ion heat conduction, implies a degree of ion self-collisionality. It is found that the particle and heat fluxes to the boundary differ very little among the four models: spanning a range of about +- 10%. It is therefore concluded that, with regard to modeling of such important practical quantities as outfluxes, a simple and convenient formulation, such as the isothermal fluid model, is adequate. Substantial differences among the models are found for certain other predicted quantities, namely, the spatial variation of ion temperature along the flow and the magnitude of the electric field near the boundary