Retention of hydrogen isotopes in diamond for applications in fusion energy

: A combined computational and experimental study

Abstract

Diamond is a material of interest for various applications within fusion energy, including windows, diagnostics and even as a plasma facing material. However, as a carbon material, concerns remain over the retention of hydrogen isotopes and chemical etching of the surface. As such, this work looks to further develop understanding of the interaction of diamond with low energy hydrogen isotopes. Retention mechanisms were explored for various diamond samples post exposure to deuterium ions. To develop understanding of experimental results, a series of molecular dynamics simulations were carried out in which diamond surfaces and grain boundaries are bombarded with hydrogen isotopes. Diffusion calculations were also performed for both bulk diamond and grain boundaries. From both experimental and computational results, it was concluded that the interaction between hydrogen and diamond is likely to be limited to very shallow depths, leaving bulk properties largely unaffected. Some grain boundaries demonstrated channelling effects leading to anisotropic diffusion but, in contrast to other crystalline materials, presented slower diffusion rates than the bulk. The formation of a disordered top layer during
continued bombardment means the presence of grain boundaries are unlikely to significantly impact total retention.

Lastly, a new, low energy ion source was developed. ExTEnD (Exposure to Low Energy
Deuterium) uses an electrical discharge to create a plasma from which ions can be extracted via a biased sample stage. The bias applied to the stage dictates the ion energy, whilst the current required to maintain this bias can be used to estimate ion flux. The design, assembly, and testing of ExTEnD is presented, alongside a comparison with equivalent samples tested in an established facility. The increased flux of ExTEnD resulted in a notable increase in retention and loss of clear peaks in desorption spectra, possibly because of ion damage to the surface.

Date of Award	17 Jun 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Thomas Bligh Scott (Supervisor) & Neil A Fox (Supervisor)