Determining absolute VUV fluxes for assessing the relevance of photon-surface interaction in ion sources

Abstract

A portable device was developed to quantify VUV fluxes flexibly at ion source setups. It consists of a VUV sensitive photodiode and optical filters for wavelength selection and is calibrated against a VUV spectrometer down to 46 nm for a variety of discharge gases, including Ar, N2, O2 and H2. It was applied to the negative hydrogen ion source at BATMAN Upgrade to quantify the VUV radiation emitted by the driver as well as in front of the extraction surface (plasma grid, PG). The combined VUV fluxes impinging on the PG with photon energies larger than 6.6 eV has a comparable magnitude as the ion flux. It could be shown that the recently confirmed influence of the ion source plasma on the surface work function of the PG can at least partly be ascribed to the VUV radiation from the driver and that photo-emitted electrons from the PG should not play a role in the sheath physics.