Work function behavior of a biased C12A7 electride in low temperature hydrogen plasmas

Abstract

The development of ion sources for NNBI systems at future fusion devices like ITER or DEMO is based on the surface production of negative hydrogen ions. Thus, low work function converter surfaces are mandatory. Besides the state-of-the-art technique of continuous Cs injection during ion source operation, alternative materials are desirable to overcome the drawbacks of volatile Cs coatings. In this work, a C12A7 electride material is studied regarding the work function behavior in a hydrogen and deuterium plasma environment at ion source relevant conditions. The minimum measured work function obtained during the campaign is 2.9 ± 0.1 eV, with an optimization potential to lower values at better vacuum conditions and higher annealing temperatures. Biasing the sample during plasma operation exhibits a strong impact on the work function performance, which is dependent on the polarity and the applied bias potential. The minimum work function obtained for the C12A7 electride used in this experiment is substantially higher than what is achieved with in situ caesiation (~ 2 eV), but the sample has demonstrated promising properties in terms of plasma resilience.