Energy distribution of negative carbon ion beam extracted from a plasma-sputter-type negative ion source

The energy distributions of C− and C2− beams extracted from a plasma-sputter-type negative ion source with the concave graphite target were measured by a retarding field energy analyzer having 1.5% resolving power. The reduction in energy spread of the energy distribution functions was observed by adding Cs into the discharge. The observed energy spreads of C− beam for 650 V target bias were 15 eV without Cs and 13 eV with Cs, respectively. The shift toward the lower energy due to Cs supply was also observed. The observed negative energy shift was constant at about 3.7 eV for the C beam, the value corresponded to the work function reduction from a pure graphite to Cs covered graphite surface.