Relative Contribution of Volume and Surface-Plasma Generation of Negative Ions in Gas Discharges

Surface-plasma generation of extracted H^-ions in gas discharges without the addition of cesium is analyzed. At the present time, it is common consensus that surface-plasma generation of extracted H^-ions dominates above volume processes in discharges with the admixture of cesium or other catalysts with low ionization potential. We will present evidence that surface-plasma generation can be enhanced in high density discharges without cesium after electrode activation by high temperature conditioning in the discharge. The diffusion of impurities with low ionization potential is the presumed cause of the observed H^-emission enhancement. For the effective generation of H^-ion beams in discharges without cesium, it is necessary to optimize surface-plasma generation of extracted H^-ions. Such optimization allows considerable improvement of H^-/D^-sources characteristics.