Diamond as a Plasma-Facing Protective Coating in Fusion Reactors

The plasma-facing material (PFM) is the material that lines the inside of the first wall and divertor of a nuclear fusion reactor. The primary role of a PFM is to protect more delicate components of the reactor from high thermal loads and low-energy ion irradiation. Such materials will also need to endure high fluences of fast neutrons, but typically will do little to protect the other components of a wall.

Diamond’s intrinsic hardness, excellent thermal properties, resistance to radiation damage, semiconducting properties and transparency, make it a material of interest for numerous applications within fusion energy. These applications include PFMs, windows, hydrogen permeation barriers, as well as neutron and tritium detectors. All of these potential applications involve the interaction of diamond with hydrogen isotopes on some level.

To assess the suitability of different diamond materials for this purpose, diamond’s response to fusion-relevant conditions needs to be understood. This has been done at Bristol through computer simulations using LAMMPS, and experiments using an in-house low-energy ion source (ExTEnD), as well as Temperature Desorption Spectroscopy (TDS). Polycrystalline diamond structures representative of different grain boundary types have been evaluated by irradiation with deuterium ions to assess damage and diffusion effects. Sample results are shown left.

The figure shows defect analysis of 300 repeated simulations for five different diamond structures bombarded with single 60 eV deuterium atoms. All atoms were incident normal to the (110) surface; the bottom four structures contained a tilt grain boundary (GB) in the middle and at the edge of the simulation cell. The GB plane is indicated by the Miller indices in the legend. When a GB was present, only this region was bombarded. Carbon and deuterium atoms are given in red and green, respectively, vacant sites (blue) and resulting carbon interstitials (black) were determined by a Wigner-Seitz analysis.