Plasma Electrode Structure Suitable for H$^-$ Extraction from a Bernas Type Ion Source
Effect of the plasma electrode position upon the negative hydrogen (H$^-$) ion beam current extracted from a Bernas type ion source have been studied. In the previous study plasma electrode located inside of the arc chamber covered almost all length along the plasma column sustained in a liner confinement magnetic field. In this study the amount of H$^-$ beam current was measured as a function of the voltage to the plasma electrode, which was located outside of the arc chamber. The area of the plasma electrode exposed to the ion source plasma was reduced to 26 mm × 85 mm.
Effect of filament materials on the H$^-$ ion beam production efficiency of the Bernas type ion source has been also experimentally studied. Tungsten and tantalum are tested for comparison. Dependence of H$^-$ ion current and that of extracted electron current were studied by applying bias voltage on the plasma electrode for two different filament materials. H$^-$ beam current increased by about 200% with the present plasma electrode structure compared with the previous design. The amount of co-extraction electron current was reduced to two thirds of the original amount. More than 30% increase in H$^-$ ion beam current was observed with the tantalum filament. The amount of co-extraction electron current decrease by 40% with the tantalum filament as compared with that for a tungsten filament.