The plasma boundary containing fast isotropic electrons: a comparison between kinetic and fluid ion models

The behaviour of a boundary layer containing fast isotropic primary electrons has been investigated using a modified Tonks-Langmuir model for the one-dimensional collisionless plasma, both using the full space-charge description and the quasi-neutral approximation. The plasma solutions are compared directly to those obtained from a fluid model of the same system developed by Schott in 1987 ( Phys. Fluids 30 1795). In the kinetic ion model a single double-layer structure is observed to form for all primary electron densities, with a two-stage rise in the boundary potential approaching the wall. This single double-layer exists at a point corresponding to the initiation of oscillatory solutions (periodic double layers) in the potential, particle concentrations and space charge in the fluid model. In the quasi-neutral approximation of the kinetic model, for primary to thermal electron densities above a ratio of about 0.3 the sheath-edge potential jumps from a value close to the voltage equivalent of the electron temperature to a value just under the isotropic electron energy. This is in very close agreement with the previously obtained fluid quasi-neutral solutions.