Measurements of work function and surface conditions in cesiated negative ion sources

Negative hydrogen (H^-) ion sources are operated with Cs injection to reduce coextracted electron current. Injected Cs atoms adhere on the surface of the electrode with extraction holes and increase the local density of H^- ions. The surface work function of the extraction electrode is the key parameter determining electron transfer from the electrode to hydrogen atom leaving the surface. Thus, the work functions of the target surfaces for obtaining fundamental process data were measured with the work function diode, Kelvin probe, and photoelectric method. The measurement of the work function of the extraction electrode accompanies difficulties as probe insertion blocks the plasma flow and photoelectric signals are small. Attempts made to monitor the surface conditions for realizing better ion source operations are summarized.