Boron and phosphorus doped diamond: change in the electronic structure post exposure to low pressure deuterium plasma and its effect on the production of negative ions

Abstract Negative ion production is of significant interest for materials processing and neutral beam injection systems for magnetic confinement fusion reactors. The surface production mechanism can be required for high current applications. Dielectric materials, including doped diamond, are of interest for increasing surface production as a potential alternative to low-work function metals and further understanding of the underpinning mechanisms is needed. In this study we use photoemission yield spectroscopy, in conjunction with the Fowler model and mass spectrometry, to measure the negative ion yield and ionization threshold of micro-crystalline diamond ( µ c-D), micro-crystalline boron doped diamond ( µ c-BDD) and single crystal phosphorus doped diamond (PDD) under deuterium plasma exposure at sample temperatures between 30 ∘ C and 700 ∘ C. We observe that deuterium plasma exposure of PDD at 400 ∘ C reduces its ionization threshold from approximately 4.0 eV to 2.1 eV, which is similar to the work function of cesium. While the ionization threshold of diamond is observed to be sensitive to sample temperature and the dopant used, this appears to have negligible effect on negative ion production. Developing a better understanding of which material properties are most significant for negative ion production will assist the development of improved negative ion sources.