UNRAVELING THE EFFECTS OF MIXES OF IMPURITIES (O, LI, & B) ON TUNGSTEN AS A DIVERTOR MATERIAL AND THEIR INFLUENCE ON D RETENTION
Open Access
- Author:
- Sharkass, Meral
- Graduate Program:
- Nuclear Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 19, 2024
- Committee Members:
- Jon Schwantes, Program Head/Chair
Xing Wang, Major Field Member
Martin Nieto-Perez, Chair & Dissertation Advisor
Predrag Krstic, Special Member & Co-Dissertation Advisor
Mia Jin, Major Field Member
Adri van Duin, Outside Unit & Field Member - Keywords:
- Fusion
Retention
Sputtering
reflection
tungsten - Abstract:
- The performance of tungsten as a fusion material is highly influenced by its surface properties, which are significantly impacted by interactions with impurities, either originating from the plasma or present within the material's bulk. Plasma-facing components, such as the divertor and the first wall in current fusion reactors, as well as in many laboratory experiments, operate at or are cooled to room temperature. This dissertation examines the effects of oxygen, lithium, and boron on tungsten surfaces. Specifically, we investigate the behavior of various tungsten-based surface mixtures under deuterium irradiation within an impact energy range of 5–100 eV. Using the LAMMPS molecular dynamics simulation tool and a newly developed ReaxFF force field created by our research group, we analyze the retention, reflection, sputtering, and surface chemistry of oxidized layers with varying thicknesses at room temperature. Our first study, focused on studying the oxidation of tungsten and it revealed that oxygen atoms form an adlayer on the tungsten surface, reaching saturation as thickness and oxygen content increase at higher impact energies. Subsurface oxidation layers were thinner, with stoichiometry transitioning from WO3 in the adlayer to WO2 at the boundary region. Tungsten was sputtered as mainly WO4 molecules, with the yield increasing at higher impact energies. Deuterium irradiation (5–100 eV) of W-oxide layers revealed enhanced D retention at lower energies, particularly in thin oxide layers, with retention probabilities reaching 90% at 5 eV. At higher energies, retention in oxidized tungsten decreased below that of pure tungsten. Reflection and sputtering of D and oxygen differed significantly from pure tungsten, with preferential D binding to oxygen in oxide layers and tungsten in oxygen-poor regions. Thicker oxide layers reduced D reflection depths and shifted retention probability crossovers to higher energies. Additionally, our study highlights the significant role of lithium, boron, and oxygen concentrations in tungsten matrices in influencing deuterium (D) retention. Lithium enhances D retention, particularly at low energies, where Li-containing matrices show higher retention compared to pure W. At intermediate energies, boron introduces a distinct peak in retention, while the combination of Li and B produces a behavior similar to that of Li-W systems, with slight improvements at higher energies. Oxygen contributes to increased retention at low energies but becomes less effective as energy increases. At lower energies, matrices containing Li, B, and O demonstrate the highest retention, making them the most effective for trapping D in this regime. At higher energies, all modified matrices retain more D than pure W, although the retention differences between compositions diminish.