Erosion-deposition and fuel retention on plasma-facing components in fusion reactors

Research towards a fusion power plant encompasses numerous scientific and technological challenges. Plasma-material interactions are essential for energy and particle extraction, but present risks for the plasma-facing materials and impact plasma purity and performance. The so-called first wall needs to be able to withstand high energy and particle fluxes without the need for frequent maintenance. In sum, the plasma-materials interactions are crucial for the performance and safe operation of fusion reactors. This thesis focuses on fuel retention and erosion-deposition mechanisms, evaluating plasma-wall interactions with components from the JET tokamak equipped with the full-metal ITER-like wall (JET-ILW) and assessing the effects of fusion reactor environments on diagnostic mirror performance. The work includes the first study of the beryllium (Be)-coated inner wall cladding tiles from JET, revealing Be spalling-off mechanisms and establishing a lower limit for deuterium (D) retention in these tiles. These findings support an extrapolated global fuel retention estimate for JET-ILW plasma-facing components (PFCs) relevant in view of the subsequent D-T operation, totaling 4.19 ⋅ 1023 D atoms (0.19 % of the injected D fuel) after the ILW-3 campaign. This thesis also includes an initiative to improve inter-comparability of fuel retention studies for JET’s PFCs. Studies on bulk Be limiters and tungsten (W) divertor tiles allowed for detailed analysis of co-deposited species, confirming generally low fuel retention and demonstrating H-D isotope exchange on surfaces after hydrogen fueling in the ILW-2 campaign. Comprehensive retention analysis of noble, seeded, and tracer gases in the PFCs indicates that nitrogen (N) is retained during JET operations, with rates correlated to N seeding. Additionally, trace amounts of injected gases were found to persist on PFC surfaces, while carbon (C) retention was low, confirming an absence of open C sources in JET-ILW. The comprehensive analysis of JET PFC enhances understanding of materials migration and fuel retention within the JET tokamak with the ILW. The thesis further examines the impact of fusion environments on diagnostic mirrors. Experiments on ion irradiation compare single-crystal and polycrystalline molybdenum (Mo) mirrors, showing minimal performance differences. The effects of W and, for the first time, boron (B) on reflectivity are evaluated, with results indicating significant performance risks from deposits, especially from B in the short-wavelength range. Laboratory studies link surface damage to fuel retention, and an unique in situ JET experiment is introduced that assesses the retention on pre-damaged Mo mirrors under operational conditions.