H− production on poly- and monocrystalline converters in a surface-plasma ion source

Monocrystalline (110) and polycrystalline tungsten and molybdenum converter surfaces have been tested in a surface negative hydrogen-ion source, both with and without cesium added to the plasma. With cesium added to the discharge, no significant difference in H− yield is measured between the four converter surfaces. Energy spectra reveal that the contribution to the total H− yield from the backscattered hydrogen is negligible compared to the ion-impact desorbed H− ions. The converter voltage required for optimum H− production increases with the amount of cesium added to the plasma. The measurements without cesium show roughly equal fractions of backscattered and desorbed H− ions in the self-extracted beam. The presence of oxygen on the surfaces enhances the ion-impact desorbed (sputtered) H− yield with respect to the reflected H− yield. This is attributed to the formation of OH on the surfaces.