Development of Laser Photodetachment Technique Using Heated Probe to Eliminate the Effect of Probe Surface Ablation Phenomena

Abstract

The characteristics of the phenomena caused by laser irradiation to an electrostatic probe in plasmas are studied to avoid the disturbance of the laser photodetachment signals for negative ion density measurement. In helium–hydrogen and hydrogen–methane plasmas, a probe surface ablation phenomenon was observed as an anomalous excess electron current in response to the laser irradiation to the electrostatic probe, while the phenomenon was not observed in pure hydrogen plasmas. Contaminations of the probe surface appear to be the mechanism causing the ablation phenomena. In order to clean the probe surface, a filament-type heated probe, which is the same type of conventional emissive probe, is applied to the laser photodetachment technique. When the surface is cleaned by heating the probe, the ablation phenomenon disappears, and the negative ion density can be evaluated at a sufficiently high laser pulse energy to saturate the photodetachment rate of negative ions. The method developed in this paper is useful for the measurement of negative ion density in plasmas where the probe surface is easily contaminated.