Investigation of the temporal behaviour of the H- density in a low-pressure pulsed discharge using laser photodetachment

Abstract

Time-resolved photodetachment measurements have been carried out in a low-pressure, high-power modulated high-volume ion source. Extraction measurements from pulsed ion sources have previously been obtained which show large enhancements in the extracted negative ion current densities for low-pressure operation when modulated at high frequencies. Here we present the results of laser photodetachment measurements of the time-dependent H-density in such a source. These results show that the H- density behaves in a manner consistent with enhanced production due to electron temperature cooling and reduced fast electron losses in the post-discharge of a plasma. Large increases in the negative ion density in the post-discharge are observed with substantial increases in the time averaged H- density. The increases observed in the time-averaged negative ion density in a modulated discharge can be attributed to the increased production, via the dissociative attachment process, of H/sup /n the post-discharge. These measurements also indicate that the characteristic negative ion loss times are approximately= 10 mu s during the discharge and the negative ion loss times in the post-discharge are in the region of 30 mu s to 40 mu s. Even at the highest modulation frequency studied the negative ion density remains highly modulated.