Molecular Dynamics Study of the Interactions between Hydrogenous Plasma and Beryllium First Wall Materials

During the operation of nuclear fusion reactors, plasma-facing components lining the reactor vessel are continuously subjected to high thermal loads and bombardment by hydrogenous ions, resulting in erosion of the first-wall material and sputtering of atomic and molecular species into the edge plasma. The management of reactor component lifetimes and control of sputtered plasma impurities remain open problems in fusion research, where quantifying the sputtering yields and understanding the sputtering processes of each impurity species is pivotal for planning an effective maintenance schedule and ensuring optimal plasma performance. Additionally, the penetration and subsequent trapping of these bombarding plasma ions has implications for component damage as well as in-vessel inventory. Accurately predicting the expected ion penetration depth profiles at a range of plasma ion and surface temperatures typical of fusion reactor operating conditions will inform the scrape-off layer design to limit particle radiation damage and tritium trapping in order to prolong the lifetime of the plasma-facing components and satisfy the DT fuel cycle requirements. In this thesis, a molecular dynamics model is implemented and used to study the bombardment of beryllium lattices with surface temperatures of up to 1000 K by 5 eV - 150 eV deuterium and tritium ions, with the aim of investigating relevant sputtering processes and quantifying ion penetration depth profiles. Firstly, although the sputtering of atomic species is historically well-established, the sputtering of molecules from metallic first-wall materials such as beryllium has only recently been confirmed to be possible and relevant for fusion reactors. The first part of the thesis focuses on the sputtering of atomic beryllium, Be₂ dimers, Be₃ trimers, beryllium hydrides, and hydrogenous dimers. The sputtering yield of each species is reported, and the observed ion trajectory preferential sputtering behaviour is presented; beryllium lattice thermal effects as well as bombarding ion isotope effects are discussed where relevant. A theoretical description of the universal sputtering yield relation and its various corrections is explored in detail, and the molecular dynamics model is benchmarked against analytical, computational, and experimental data. Secondly, by defining a statistical distribution for ion penetration depth and describing the evolution of its parameters across the fusion parameter space of interest, the expected ion deposition depth profiles can be calculated for any subset of ion and surface temperature ranges as needed. In the second part of the study, the distributions of two penetration depth quantities, considered from the perspectives of lattice damage and hydrogen retention, are defined and their distribution parameter dependence on surface and ion temperature is identified. The expected positive correlation between penetration depth and ion temperature is observed, where the non-linear relationship between these quantities indicates the expected form of the velocity dependence of nuclear stopping power at low bombardment energies. Isotope effects on the distributions are also investigated, with results suggesting that heavier ions have comparably lower mobility within the sample and will generally accumulate closer to the surface. A short study on ion deposition rates is also performed; a non-linear increase of deposition rate with increasing bombarding ion energy has been observed, and evidence of a weak positive surface temperature correlation has been noted.