Simulation of photo-detached electrons in negative ion plasmas

The evolution of laser photo-detached and background electrons in a glow discharge plasma containing negative ions was followed using a two-dimensional particle-in-cell simulation code. Emphasis was placed on the short timescale (20ns after photo-detachment for the parameters of a typical glow discharge plasma) behaviour because this is the timescale of the recently developed laser Thomson scattering technique combined with photo-detachment. A plasma consisting of background electrons, positive argon ions (Ar⁺) and negative oxygen ions (O^-) was treated. The loss of photo-detached electrons from the photo-detachment region quickly forms a potential structure in a short time of 2ns. The potential traps the remaining photo-detached electrons. The profile of the trapped photo-detached electrons is hollow. The loss of the photodetached electrons from the photo-detachment region is compensated by the bulk electrons coming from the ambient plasma; the sum of the photo-detached and bulk electron densities in the photo-detachment region remains unchanged for 20 ns because ions are almost immobile in this timescale. This analysis has also revealed the fractions of the photo-detached electrons remaining in the photo-detachment region during the 20 ns to be relatively insensitive to a change in laser beam size and very sensitive to the relative densities of negative ions against background electrons, and to the shape of the velocity distribution function at different angles with respect to the laser beam axis.