Fusion fuel retention (trapping) and release (desorption) from plasma-facing components are critical issues for ITER and for any future industrial demonstration reactors such as DEMO. Therefore, understanding the fundamental mechanisms behind the retention of hydrogen isotopes in first wall and divertor materials is necessary. We developed an approach that couples dedicated experimental studies with modelling at all relevant scales, from microscopic elementary steps to macroscopic observables, in order to build a reliable and predictive fusion reactor wall model. This integrated approach is applied to the ITER divertor material (tungsten), and advances in the development of the wall model are presented. An experimental dataset, including focused ion beam scanning electron microscopy, isothermal desorption, temperature programmed desorption, nuclear reaction analysis and Auger electron spectroscopy, is exploited to initialize a macroscopic rate equation wall model. This model includes all elementary steps of modelled experiments: implantation of fusion fuel, fuel diffusion in the bulk or towards the surface, fuel trapping on defects and release of trapped fuel during a thermal excursion of materials. We were able to show that a single-trap-type single-detrapping-energy model is not able to reproduce an extended parameter space study of a polycrystalline sample exhibiting a single desorption peak. It is therefore justified to use density functional theory to guide the initialization of a more complex model. This new model still contains a single type of trap, but includes the density functional theory findings that the detrapping energy varies as a function of the number of hydrogen isotopes bound to the trap. A better agreement of the model with experimental results is obtained when grain boundary defects are included, as is consistent with the polycrystalline nature of the studied sample. Refinement of this grain boundary model is discussed as well as the inclusion in the model Nuclear Fusion
Thin tungsten oxide layers with thicknesses up to 250 nm have been formed on W surfaces by thermal oxidation following a parabolic growth rate. The reflectance of the layers in the IR range 2.5 -16 µm has been measured showing a decrease with the layer thickness especially at low wavelength. Raman microscopy and X-ray diffraction show a nanocrystalline WO3 monoclinic structure. Low energy deuterium plasma exposure (11 eV/D + ) has been performed inducing a phase transition, a change in the sample colour and the formation of tungsten bronze (DxWO3). Implantation modifies the whole layer suggesting a deep diffusion of deuterium inside the oxide. After exposure a deuterium release due to the oxidation of DxWO3 under ambient conditions has been evidenced showing a reversible deuterium retention.
We report for the first time on the ability of Raman microscopy to give information on the structure and composition of Be related samples mimicking plasma facing materials that will be found in ITER. For that purpose, we investigate two types of material. First: Be, W, Be 1 W 9 , and Be 5 W 5 deposits containing a few percents of D or N, and second: a Mo mirror exposed to plasma in the main JET chamber (in the framework of the first mirror test in JET with ITER-like wall). We performed atomic quantifications using ion beam analysis for the first samples. We also did atomic force microscopy. We found defect induced Raman bands in Be, Be 1 W 9 , and Be 5 W 5 deposits. Molybdenum oxide has been identified showing an enhancement due to a resonance effect in the UV domain.
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