Surface production of negative deuterium ions from plasma-exposed boron doped diamond and graphite: work function measurements using photoemission yield spectroscopy

Abstract

Negative-ion sources are of considerable interest for applications such as materials processing and neutral beam injection for magnetic confinement fusion. The efficient production of negative ions in these sources often relies on surface production. Work function measurements are critical to enable a detailed understanding of the mechanisms that underpin this. In this study we used a combination of photoemission yield spectroscopy and the Fowler method to determine the work functions of boron doped diamond (BDD) and highly oriented pyrolytic graphite (HOPG) directly after exposure to a low-pressure inductively coupled deuterium plasma (150 W, 2 Pa). A magnetised retarding field energy analyser is used to measure the negative ion current from the samples. During plasma exposure, samples are biased at −130 V or −60 V and their temperature is varied between 50 ∘C and 750 ∘C. The results show that the increasing work function of the plasma exposed HOPG occurs over the same sample temperature range as the decreasing negative-ion current. In contrast, the work function of BDD does not show a clear relationship with negative-ion current, suggesting that different mechanisms influence the negative-ion production of metal-like HOPG and dielectric-like BDD. The necessity for an additional fitting parameter for the Fowler fits to BDD suggests that its electronic properties are changing under plasma exposure, unlike HOPG. For both materials, the maximum photocurrent measured from the samples displays a strong similarity with negative-ion current, suggesting they are driven by a common mechanism. The in-situ measurement of the work function using non-invasive techniques is of interest for the development of negative ion sources.