Collisional transport treatment of surface plasma structures

Calculations are shown of the structure of plasmas in equilibrium with solid surfaces that reemit incident plasma ions as relatively cold neutral gas. A numerical transport model that includes a Fokker–Planck treatment of ion–ion collisions obtains the distribution function for ions in a phase space of one spatial coordinate and two velocities. This is done self-consistently with an electrostatic potential, a Maxwell–Boltzmann description of electrons, and electron impact ionization of the reemited neutrals. Solutions are obtained from a higher temperature kinetic regime where Coulomb collisions are nearly negligible to a lower temperature regime where plasma behavior is approximately fluidlike. A result of these calculations is the resolution of an ambiguity posed by previous kinetic regime calculations that omitted ion–ion collisions and obtained a family of solutions for each set of physical parameters [Phys. Rev. Lett. 49, 650 (1982); Phys. Fluids B 1, 448 (1989)]. The physically correct solution for semi-infinite surface plasmas is shown to be the member of each family that maximizes the ion thermal conduction to the surface and the magnitude of a maximum in the electrostatic potential that is found in these and the previous calculations. Further results are in agreement at lower temperatures with solutions obtained from a fluid model and the identification of the correct boundary condition on normal flow velocity to be used in fluid models.