Abstract
A one-dimensional particle-in-cell simulation has been conducted in order to improve the physical understanding of the laser-pulse photo-detachment diagnostics of negative ions, and thus to assess the accuracy of the measurements. An emphasis is placed on a short time scale (20 ns after photo-detachment for parameters of a typical glow discharge plasma) behaviour, where the laser Thomson scattering technique combined with the photo-detachment has recently been developed. The plasma consisting of background electrons, positive argon ions (Ar+), and negative oxygen ions (O-) is treated. It is assumed that all of the negative ions in the laser-irradiated region (photo-detachment region) are replaced by the photo-detached electrons in the time scale much faster than in the time characteristic of the electron plasma oscillation. The loss of photo-detached electrons from the photo-detachment region quickly forms a potential structure in 2 ns. The potential traps the remaining photo-detached electrons. Due to the thermal fluctuation of the electrostatic electric field, the slow decay of the photo-detached electrons follows the initial fast decay. Because of these motions of photo-detached electrons, the measured negative ion density was found to underestimate the actual value by 25 %.
