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

Abstract

The behaviour of a boundary layer containing fast isotropic primaryelectrons has been investigated using a modified Tonks-Langmuir model for theone-dimensional collisionless plasma, both using the full space-charge description and thequasi-neutral approximation. The plasma solutions are compared directly tothose 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 formfor all primary electron densities, with a two-stage rise in the boundarypotential approaching the wall. This single double-layer exists at a pointcorresponding to the initiation of oscillatory solutions (periodic doublelayers) in the potential, particle concentrations and space charge in thefluid model. In the quasi-neutral approximation of the kinetic model, forprimary to thermal electron densities above a ratio of about 0.3 thesheath-edge potential jumps from a value close to the voltage equivalent ofthe electron temperature to a value just under the isotropic electron energy.This is in very close agreement with the previously obtained fluidquasi-neutral solutions.