Dunn, A.; (2011) A molecular dynamics study of diamond as a plasma facing material for fusion. Doctoral thesis , UCL (University College London).

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Abstract

Fusion power off�ers a promising source of clean energy for the future, however, one of the greatest challenges in tokamak reactor design is developing materials suitable to withstand the intense plasma-material interactions. Carbon, mostly in its graphitic form, is currently a favorite plasma facing material in many reactors. Diamond, however, o�ffers many advantages over other materials but is not widely accepted. Although diamond exhibits excellent structural and thermal properties, tritium retention is a major concern for carbon. However, recent experimental evidence suggests that diamond might fare better than other carbon structures as a plasma facing material. This thesis investigates the the cumulative eff�ect of exposing diamond to high thermal shock and tritium bombardment using classical molecular dynamics simulations. Of interest is diamond's resistance to graphitisation and the mechanisms behind tritium retention. Surfaces of di�fferent lattice orientation and level of hydrogen termination were incrementally heated to temperatures in excess of 2000 K. Generally, these diamond structures appeared to be stable up to temperatures of about 1000 K. Orientation did play a large part in determining the temperature of phase change, as did the level of hydrogen termination. Greater hydrogen coverages mimicked bulk continuation and increased resistance to graphitisation. These diamond surfaces, as well as a graphite and a diamond grain-boundary surface, were bombarded at a range of temperatures (300-2100 K) with high fluxes (1029 m-2s-1) of 15 eV tritium atoms in studying relative tritium retention at and below the surface as well as sputtered hydrocarbon yields. Below temperatures of graphitisation the diamond structure con�fined tritium, and thus further structural damage, to the upper surface. The graphitic surface allowed for deeper tritium penetration and retention. The presence of a grain boundary in the diamond slab allowed small amounts of tritium to penetrate deep into the bulk. Diamond surfaces were also bombarded at 300 K whilst independently varying incident ion energy (7.5-30 eV) and incident interval time (0.3-1.2 ps). Greater ion energies caused proportionally greater damage as well as reducing the ability of the structure to disperse incident thermal energy. At these extremely high fluxes sputter yield appeared to not vary with flux but was found to be proportional to fluence.

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