Speaker
Description
We have discussed in the previous meetings on the possible improvements for the negative ion to electron current ratio by depositing fresh coating of tantalum (Ta) on the plasma grid surface of a negative hydrogen (H-) ion source [1]. A fresh Ta evaporation by burning a thin Ta filament increased H- ion current by 12 % [2]. However, the enhanced H- production lasted only about one hour after burning off the Ta filament in a discharge maintained by the electron emission from a hot tungsten filament. To find out the mechanism responsible for improving H- ion to electron current ratio, a thin (0.1 mm thick) foil of Ta was attached to the area of the extraction hole opened at the center of the plasma grid. The attached Ta foil did not show enhancement as large as that observed by evaporation, while the dependence of H- ion current upon the plasma grid bias [3] showed a change in local plasma potential by putting the foil on the surface of a stainless-steel plasma electrode. When the plasma grid surface facing the hydrogen plasma was covered by a titanium foil, the H- to electron current ratio increased like the case of Ta, while the absolute amount of H- ion current decreased. Correlations among the H- ion to electron current ratio, plasma parameters and the local H- ion density measured by the photodetachment method have been observed.
[1] M. Bacal and M. Wada, AIP Conf. Proc. 1869, 030025 (2017).
[2] K. Maeshiro, M. Wada, S. Masaki, Proc. 34th Int. Conf. Phys. Ionized Gases, PO16PM-027.
[3] M. Bacal et al., Rev. Sci. Instrum. 87, 02B132 (2016).
