A molecular dynamics study of diamond as a plasma facing material for fusion

摘要

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 materialsudsuitable to withstand the intense plasma-material interactions. Carbon, mostly inudits graphitic form, is currently a favorite plasma facing material in many reactors.udDiamond, however, o�ffers many advantages over other materials but is not widelyudaccepted. Although diamond exhibits excellent structural and thermal properties,udtritium retention is a major concern for carbon. However, recent experimental evidenceudsuggests that diamond might fare better than other carbon structures as audplasma facing material.udThis thesis investigates the the cumulative eff�ect of exposing diamond to highudthermal shock and tritium bombardment using classical molecular dynamics simulations.udOf interest is diamond's resistance to graphitisation and the mechanisms behind tritium retention.udSurfaces of di�fferent lattice orientation and level of hydrogen termination wereudincrementally heated to temperatures in excess of 2000 K. Generally, these diamond structures appeared to be stable up to temperatures of about 1000 K. Orientationuddid play a large part in determining the temperature of phase change, as did the leveludof hydrogen termination. Greater hydrogen coverages mimicked bulk continuationudand increased resistance to graphitisation.udThese diamond surfaces, as well as a graphite and a diamond grain-boundaryudsurface, were bombarded at a range of temperatures (300-2100 K) with high fluxesud(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 ofudgraphitisation the diamond structure con�fined tritium, and thus further structural damage, to the upper surface. The graphitic surface allowed for deeper tritiumudpenetration and retention. The presence of a grain boundary in the diamond slab allowed small amounts of tritium to penetrate deep into the bulk.udDiamond surfaces were also bombarded at 300 K whilst independently varyingudincident 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 theudstructure 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 udfluence.