• Energy Research

In this work, the RAFM steels EUROFER, RUSFER and CLAM, along with a reference pure Fe sample, were all exposed to the same source of deuterium and analyzed using the same techniques, allowing a direct comparison of the experimental results. A 200 eV/D mass-selected deuterium ion beam at the SIESTA facility was used to bombard the samples to a fluence of 5 × 1024 D m−2 at 450 K. The surface morphology of the samples was investigated with Scanning Electron Microscopy. The sputter yield of the samples was determined by weight-loss measurements and confirmed by measurements of the eroded depth. The near-surface enrichment of W and Ta was investigated via Energy-dispersive X-ray spectroscopy, X-ray Photoelectron Spectroscopy and Rutherford Backscattering Spectrometry. Grain-orientation-dependent sputtering was studied with Confocal Laser Scanning Microscopy and Electron Backscatter Diffraction. Lastly, Nuclear Reaction Analysis and Thermal Desorption Spectrometry were employed to analyze deuterium retention in all the samples. The erosion behavior of all three steels under deuterium bombardment was confirmed to be similar. The measured sputter yield was comparable for all three steels, and significantly lower than that of pure Fe. Likewise, all steels develop a needle-like surface morphology under the given exposure conditions and a W- and Ta-enriched layer in the range of few nanometers, while the Fe sample remained smooth. Retained deuterium amounts were also comparable among the steel samples, and were overall larger than the retention measured for the pure Fe sample.

In this work, the RAFM steels EUROFER, RUSFER and CLAM, along with a reference pure Fe sample, were all exposed to the same source of deuterium and analyzed using the same techniques, allowing a direct comparison of the experimental results. A 200 eV/D mass-selected deuterium ion beam at the SIESTA facility was used to bombard the samples to a fluence of 5 × 1024 D m−2 at 450 K. The surface morphology of the samples was investigated with Scanning Electron Microscopy. The sputter yield of the samples was determined by weight-loss measurements and confirmed by measurements of the eroded depth. The near-surface enrichment of W and Ta was investigated via Energy-dispersive X-ray spectroscopy, X-ray Photoelectron Spectroscopy and Rutherford Backscattering Spectrometry. Grain-orientation-dependent sputtering was studied with Confocal Laser Scanning Microscopy and Electron Backscatter Diffraction. Lastly, Nuclear Reaction Analysis and Thermal Desorption Spectrometry were employed to analyze deuterium retention in all the samples. The erosion behavior of all three steels under deuterium bombardment was confirmed to be similar. The measured sputter yield was comparable for all three steels, and significantly lower than that of pure Fe. Likewise, all steels develop a needle-like surface morphology under the given exposure conditions and a W- and Ta-enriched layer in the range of few nanometers, while the Fe sample remained smooth. Retained deuterium amounts were also comparable among the steel samples, and were overall larger than the retention measured for the pure Fe sample.

We studied the influence of thin, electro-chemically grown tungsten (W) surface oxide films on hydrogen isotope release from W. As deuterium (D) reservoir underneath the oxide, we used a defect-rich, ion-irradiated W layer that was filled with D prior to oxidation. Several oxide films with thicknesses between 5 and 100 nm were studied and compared with tungsten with a natural oxide film. The release of D through the oxide film was analyzed with thermal desorption spectroscopy (TDS). The depth-resolved concentration profiles of D in the sample were measured with nuclear reaction analysis at all experimental steps. Changes of the morphology of the oxide film due to the release of D were investigated with scanning electron microscopy (SEM).In TDS studies, we found that the thin oxide films significantly influence the release behavior of D from W. The first D release peak (at 560 K) is shifted towards higher temperature (or later times) with increasing oxide thickness. This indicates that the oxide film acts as both a D reservoir and a transport barrier that delays D release at temperatures above 475 K. At this temperature, D also starts to interact chemically with the oxide film and is released not only as HD or D2 but also in the form of heavy water (HDO and D2O). Above 700 K, D is released only in form of heavy water as long as enough oxide is available. Accordingly, SEM images after TDS show a strong modification of the oxide film. For film thicknesses of 5–10 nm, all oxide is removed from the surface and smooth metallic W remains. For 15 nm, the surface is still partially covered by oxide islands with several micrometer of metallic W between them. From the fact that D is still only released as heavy water at high temperatures, we conclude that the mobility of D atoms at the surface is very high. Even D atoms that surface far from an oxide island apparently travel along the surface to form an O-D group at the W oxide before they recombine with another D atom to form water.Our results indicate that the oxide film becomes relevant for the D release during TDS if the ratio of O atoms on the surface to D atoms in the sample is larger than 5–10 %. Consequently, even the natural oxide film (1–2 nm) that forms on W upon contact with air may significantly influence the D release spectra from TDS for experiments with low D retention.

We studied the influence of thin, electro-chemically grown tungsten (W) surface oxide films on hydrogen isotope release from W. As deuterium (D) reservoir underneath the oxide, we used a defect-rich, ion-irradiated W layer that was filled with D prior to oxidation. Several oxide films with thicknesses between 5 and 100 nm were studied and compared with tungsten with a natural oxide film. The release of D through the oxide film was analyzed with thermal desorption spectroscopy (TDS). The depth-resolved concentration profiles of D in the sample were measured with nuclear reaction analysis at all experimental steps. Changes of the morphology of the oxide film due to the release of D were investigated with scanning electron microscopy (SEM).In TDS studies, we found that the thin oxide films significantly influence the release behavior of D from W. The first D release peak (at 560 K) is shifted towards higher temperature (or later times) with increasing oxide thickness. This indicates that the oxide film acts as both a D reservoir and a transport barrier that delays D release at temperatures above 475 K. At this temperature, D also starts to interact chemically with the oxide film and is released not only as HD or D2 but also in the form of heavy water (HDO and D2O). Above 700 K, D is released only in form of heavy water as long as enough oxide is available. Accordingly, SEM images after TDS show a strong modification of the oxide film. For film thicknesses of 5–10 nm, all oxide is removed from the surface and smooth metallic W remains. For 15 nm, the surface is still partially covered by oxide islands with several micrometer of metallic W between them. From the fact that D is still only released as heavy water at high temperatures, we conclude that the mobility of D atoms at the surface is very high. Even D atoms that surface far from an oxide island apparently travel along the surface to form an O-D group at the W oxide before they recombine with another D atom to form water.Our results indicate that the oxide film becomes relevant for the D release during TDS if the ratio of O atoms on the surface to D atoms in the sample is larger than 5–10 %. Consequently, even the natural oxide film (1–2 nm) that forms on W upon contact with air may significantly influence the D release spectra from TDS for experiments with low D retention.

Up to now, analyzing the production of dislocation-type defects in the subsurface region of plasma or ion-exposed tungsten samples has been hampered by the challenging production of suitable cross-section samples for transmission electron microscopy. We present two reliable methods based on precision electropolishing to prepare cross-sections of tungsten that allow direct imaging of dislocation-type defects by scanning as well as by transmission electron microscopy. Using these methods, we are able to demonstrate a clear enhancement of the dislocation density in the caps of blisters on tungsten exposed to H isotope plasma, i.e., of surface morphologies that are correlated to subsurface cavities. As a benchmark, we also show a cross-section of tungsten irradiated by 20 MeV W6+ ions.

Up to now, analyzing the production of dislocation-type defects in the subsurface region of plasma or ion-exposed tungsten samples has been hampered by the challenging production of suitable cross-section samples for transmission electron microscopy. We present two reliable methods based on precision electropolishing to prepare cross-sections of tungsten that allow direct imaging of dislocation-type defects by scanning as well as by transmission electron microscopy. Using these methods, we are able to demonstrate a clear enhancement of the dislocation density in the caps of blisters on tungsten exposed to H isotope plasma, i.e., of surface morphologies that are correlated to subsurface cavities. As a benchmark, we also show a cross-section of tungsten irradiated by 20 MeV W6+ ions.