Effect of filament material and area on the extracted current from a volume H(-) ion source

Experiments with a volume H(-) ion source are described in which the number of operating filaments and the refractory metal of which they were made were varied. The results obtained with tungsten and tantalum filaments indicate that at operating pressures above 10 mTorr the extracted H(-) ion current can be 50% larger when tantalum filaments are used. The simultaneously extracted electron current is reduced. Changing the number of heated filaments at constant discharge current showed that the highest extracted H(-) current was obtained with the lowest number of filaments. The evidence obtained supports the view that the filament material affects H(-) production via the fresh metal film deposited on the source chamber wall. It was also found that a freshly deposited metal film was better than an old contaminated one. The simulation of negative ion production indicates that the difference in H(-) ion yield between the cases of tantalum and tungsten wall films may be due to a higher atom-wall recombination coefficient in the case of tantalum.