Relevance of Volume and Surface Plasma Generation of Negative Ions in Gas Discharges

The relative contribution of volume and surface‐plasma generation in emission of H− ions in gas discharge sources is analyzed. At the present time, it is generally accepted that surface‐plasma generation of extracted H− ions dominates over volume processes in discharges with admixture of cesium or other catalysts with low ionization potential. We will attract attention to the evidence, that surface‐plasma generation can be significantly enhanced in high density discharges without cesium after electrode activation by high temperature conditioning in discharge. With this optimization of conditions for surface‐plasma generation of emitted H−, an emission current density was increased up to ∼1 A/m2 in discharges without cesium. Diffusion of impurities with low ionization potential can be the reason for the observed enhancement of H− emission. Such optimization allows considerable improvement of H−/D− source characteristics. Volume generation of extracted H− in high density discharges is suppressed by the high...