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Description
The replacement of the originally planned beryllium first wall with tungsten in the current ITER baseline makes boronization an important strategy for reducing impurities such as oxygen in the plasma fuel. Consequently, the effects of boron on plasma-material interactions during and after plasma operations are of high importance. Although hydrogen isotope retention in tungsten has been widely studied because of tritium inventory and safety concerns, the influence of boron layers on hydrogen isotope retention and removal in tungsten is still not well known.
Hydrogen isotope exchange effect has been shown to successfully decrease near-surface tritium retention in tungsten by replacing tritium with lighter isotopes. The exchange process is the strongest near the material surface, where incoming light isotopes diffuse in and displace heavier isotopes from the trap sites. This near-surface effect makes the role of any surface coating critical for the overall efficiency.
This study experimentally investigates how thin boron layers will affect hydrogen isotope exchange in tungsten. Boron films with fusion-relevant thicknesses of 50 nm and 250 nm were deposited on tungsten substrates by magnetron sputtering. The 50 nm case corresponds to a typical layer formed by a single boronization, while the 250 nm thickness reflects deposition-dominant regions, allowing the influence of coating thickness on isotope exchange to be measured. Samples were implanted with deuterium at two energies (5 keV/D and 20 keV/D) to investigate trapping within the boron layer and deeper in the tungsten. Isotope exchange was promoted by subsequent annealing in H2 gas which was chosen to introduce hydrogen gently without energetic high-flux ion sputtering of the boron film. The results were compared to vacuum-annealed and uncoated tungsten references. The resulting deuterium and hydrogen depth profiles were measured by elastic recoil detection analysis (ERDA) to quantify isotope replacement as a function of depth, coating thickness and implantation energy.
By investigating hydrogen isotope exchange efficiency in the boron layer, across the boron-tungsten interface, and within the underlying tungsten bulk with a boron surface layer, this work clarifies the role of boron in modifying near-surface tritium removal by isotope exchange. A key hypothesis of the study is that thin boron layers may still permit efficient isotope exchange, while thicker layers could increasingly limit hydrogen permeation and thus reduce the exchange efficiency at the tungsten surface. The findings are relevant for tritium inventory management and wall-conditioning strategies in future fusion devices and reactors with tungsten-based first walls.