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International audience; We present measurements of the sticking probability of ammonia on two metals, tungsten and 316L stainless steel, covered with natural surface impurities as they will be used for the international nuclear fusion experimental reactor ITER. Using a collimated supersonic molecular beam at two different kinetic energies (55 and 255 meV), varying the sample temperature in the 130–425 K range, and characterizing the surface composition with Auger electron spectroscopy, we observe similar sticking features on both surfaces, consistent with a nondissociative adsorption mediated by two precursors having different trapping probabilities. First, the initial sticking probability decreases with increase in the surface temperature. Second, the sticking probability increases with the surface coverage up to near-saturation coverage, where it declines. Both features cannot be described together with the Kisliuk model (intrinsic + extrinsic precursors with identical trapping probabilities) or the modified Kisliuk model (direct adsorption + extrinsic precursor). Thus, we derive a generalized and separable Kisliuk (GSK) model that is able to reproduce quantitatively these two experimental observations thanks to intrinsic and extrinsic precursors having different trapping probabilities. The GSK model assumes a negligible transfer from the intrinsic precursor to the extrinsic precursor, which allows one to extract precursors kinetic parameters in a two-step analysis. The GSK analysis indicates that the ammonia trapping probability is lower on the bare surface (intrinsic precursor) than on the NH3-covered surface (extrinsic precursor). Furthermore, the barriers between the two precursor wells and the deep adsorption well are found below the vacuum level. Finally, we measure that the sticking probability does not decline to zero, i.e., steady-state sticking is observed with a probability up to 0.15 at a beam energy of 55 meV and a surface temperature of 220 K. This observation is consistent with NH3 multilayer adsorption. These experimental results and their fitting with the GSK model offer the starting point to a predictive determination of the fusion fuel inventory related to ammonia in the international experimental reactor ITER. more...

International audience; During H-mode plasma in ITER, type-I Edge Localized Modes can occur naturally leading to transient heat load (

Measurement of recovery and recrystallization kinetics of tungsten at high temperature is a key issue for many applications, such as plasma facing units in the framework of thermonuclear fusion. These kinetics are mostly derived from Vickers hardness and EBSD measurements, which can lead to some inaccuracies due to the competition between recovery and recrystallization mechanisms. A complementary/alternative approach based on statistical grid nanoindentation is proposed in this paper. The basic idea is to assume that the fraction recrystallized can be deduced using the hardness probability density function measured on a fully recrystallized sample. The hardness probability density function of the set of non-recrystallized grains can then be analyzed. The methodology was applied to rolled tungsten samples annealed at high temperature. It was clearly observed that recovery and recrystallization overlapped in terms of softening fraction in the investigated time&ndash, temperature range. Activation energy of the static recovery mechanism is in the correct order of magnitude compared to bulk self-diffusion in tungsten. High-throughput nanoindentation analysis appears as a promising way to investigate recrystallization/recovery mechanisms in metals. more...

International audience; Estimating the lifetime of W-armoured divertor components is of great importance for ITER. During heat loading, one of the mechanisms inducing tungsten microstructure modification is recrystallization. As tungsten recrystallization induces a decrease of the mechanical properties, the knowledge of the recrystallization kinetics of the ITER tungsten material is necessary over the entire divertor PFU operational temperature window (up to 2000 °C). Some data exist for temperature loading lower than 1350 °C. For higher temperatures, some constraints due to the heating system and its impact on recrystallization process limit the possibility of high temperature studies. For this reason, in this study, a laser heating device and related diagnostics are used for the determination of the recrystallization kinetics. The Johnson Mehl-Avrami Kolmogorov (JMAK) equation is used to model the recrystallization process. Recrystallization kinetics for annealing temperatures higher than 1400 °C are provided for two tungsten materials produced according to the ITER specifications. At 1600 °C (resp. 1800 °C), recrystallization fraction of 50% is obtained after 200 s (resp. 10 s) thermal loading for one batch and after 500 s (resp. 50 s) for the other one. more...

We report a systematic study that quantifies nitrogen retention and ammonia production on tungsten and that sheds light on the mechanism for ammonia formation on ITER’s divertor material. Saturation of the nitrogen-implanted layer in polycrystalline tungsten is observed at room temperature for a nitrogen ion fluence in the low 1021 N+ m−2 range. Nitrogen desorption from this N-implanted layer occurs in the 800–1100 K temperature range and exhibits a zero-order kinetics with an activation energy of 1.45 eV and a prefactor of 5 × 1024 m−2 s−1. Following nitrogen and deuterium co-implantation, deuterated ammonia production is observed during temperature programmed desorption between 350 K and 650 K in conjunction with deuterium desorption. In contrast, nitrogen desorption still occurs above 800 K. Significant production of ammonia is obtained only when the nitrogen layer created by ion implantation is approaching saturation and the amount of nitrogen lost to ammonia production is only in the percent range. This result is understood by repeating cycles of deuterium implantation and thermo-desorption below the desorption temperature of the nitrogen layer. The exponential decay of the amount of produced ammonia with cycle number demonstrates that nitrogen diffusion to the surface is negligible in the ammonia production temperature range and that ammonia formation occurs at the outermost surface layer. The maximum quantity of ammonia produced from the present N implanted layer is below 2 × 1018 ND3 m−2, which is limited by the nitrogen atom surface density. Surface vibrational spectroscopy demonstrates the presence of ammonia precursors on the nitrogen-implanted tungsten surface upon deuterium implantation. These ammonia precursors can be created also at room temperature through the dissociative chemisorption of thermal D2 catalysed by nitrogen present at the tungsten surface and, more efficiently, by adsorption of deuterium atoms. more...

International audience; The time evolution of fuel retention in materials relevant for future fusion reactors is compared for two different tungsten microstructures: single-crystal versus recrystallized poly-crystals. The initial retention of both types of sample is similar. It decays exponentially with a time constant of ~18 hours at 300 K (the so-called short-term retention). After 48 hours at room temperature, a constant deuterium retention is measured (long-term retention) with the single-crystal containing systematically less deuterium than poly-crystals. Macroscopic rate equations models are built with density functional theory inputs to reproduce deuterium desorption observables with the MHIMS-R code. We found that the native oxide layer could explain the desorption peak located at ~450 K as well as most of the short-term retention in the single-crystal. The native oxide together with, dislocations for single-crystal and grain boundaries for poly-crystals, are responsible for the long-term retention. Dislocations should explain the desorption peak located at ~815 K for mechanically polished samples. The dual role of most of tungsten defects is related to their multi-trapping properties with filling-level-dependent detrapping energies. Finally, the use of an effective diffusivity of deuterium through the native oxide layer, i.e. its diffusion barrier character, is evaluated. more...

Raman microscopy is one of the methods that could be used for future post-mortem analyses of samples extracted from ITER plasma facing. This study shows that this technique is useful for studying tungsten-based materials containing impurities including oxides and nitrides. Here, we apply pulsed laser deposition and DC argon glow discharges to produce tungsten-containing synthetic films (compact, porous) and nanoparticles and investigate the influence of their morphology on the measured Raman spectra. The amounts of oxygen and/or nitrogen in the films are also investigated. Comparative data are obtained by X-ray Photoelectrons Spectroscopy, Atomic Force Microscopy, Electron Microscopies (Scanning and Transmission), Energy Dispersive X-ray spectroscopy, Time-of-Flight Elastic Recoil Detection Analysis. The power density of the laser beam used to perform Raman microscopy is varied by up to 4 orders of magnitude (0.01-20 mW/$\mu$m 2) to investigate thermal stability of films and nanoparticles. As a first result, we give evidence that Raman microscopy is sensitive enough to detect surface native oxides. Secondly, more tungsten oxides are detected in porous materials and nanoparticles than in compact films, and the intensities of the Raman band correlate to their oxygen content. Thirdly, thermal stability of these films (i.e. structural and chemical modification under laser heating) is poor when compact films contain a sufficiently large amount of nitrogen. This finding suggests that nitrogen can be substituted by oxygen during Raman laser induced heating occurring in ambient air. Finally, our methodology can be used to rapidly characterize morphology and chemistry of the samples analyzed, and also to create oxides at the micrometer scale. keywords: PLD, Raman spectroscopy, tungsten oxide, tungsten nitride, plasma wall interaction, laser heating, post-mortem analysis 2, Comment: Nuclear Fusion, IOP Publishing, 2020 more...

International audience; The knowledge of optical properties of tungsten at high temperatures is of crucial importance in fields such as nuclear fusion or aerospace applications. The optical properties of tungsten are well known at room temperature, but little has been done at temperatures comprised between 300 K and 1000 K in the visible and near-infrared domains. Here we investigate the temperature dependence of tungsten reflectivity from the ambient to high temperatures ( more...

International audience; 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 of a thin defective oxide layer following the experimental observation of the presence of an oxygen layer on the surface even after annealing to 1300 K. more...