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Oxygen (O), as a common medium-Z impurity in vacuum vessels, is one of the main concerns for magnetic confinement fusion devices, in part due to its high chemical reactivity and its potential to act as seed for sputtering of high-Z materials. For devices with full tungsten (W) walls for which the adsorption of O is low relative to other plasma-facing materials (PFM), wall conditioning techniques such as boronization become necessary for successful plasma startup. Continuous re-deposition steps during operation will lead to the formation of mixed W and boron (B) layers on plasma-facing surfaces, in which the presence of O can be expected [1].
The aim of this study is to provide a deeper understanding of the trapping of O in plasma-deposited thin films of W, B as well as in W:B mixtures under the effects of thermal annealing. Magnetron sputter deposition was employed to grow thin-films of B, W and W:B mixed layers. The films were irradiated by 1 keV O$_2$, equivalent to 500 eV per O, to a nominal fluence of 4×10$^{16}$ O/cm$^2$ followed by in-situ stepwise thermal annealing. Ion-beam analysis (IBA) was used to monitor the presence of O before, during and after irradiation and annealing of the samples.
It was found that the O-content following implantation is significantly higher in pure boron when compared with W, and mixed W:B layers. Annealing reduced the O-content in all compositions but had the strongest effect for pure B, which was heated to a maximum temperature of 500°C and for which the retained amount of O was reduced by close to 50%, corresponding to 4×10$^{15}$ O/cm$^2$. Results highlight that the oxygen affinity to pure boron layers can be potentially reduced for mixed layers containing tungsten. Finally, the effect of pre-loaded deuterium in boron layers on the subsequent trapping of oxygen (as expected for the use of diborane gas used in boronization) will also be presented and discussed.
- A. Marin et al., J. Nucl. Mater. 604 (2025) 155525, doi: 10.1016/j.jnucmat.2024.155525.