Search

EUROFER is a Reduced Activation Ferritic Martensitic (RAFM) steel developed as structural material for future fusion power plants and is considered as first-wall material in recessed areas of DEMO. Recent work has shown a fluence-dependent decrease of the sputter yield for bombardment with deuterium ions in the energy range of 100 to about 500 eV. This decrease was previously attributed to preferential sputtering of the lower mass constituents in EUROFER, such as Fe and Cr, compared to the higher mass alloying elements. This leads to an increase of the surface tungsten concentration. However, it was also observed that, after sputtering, the samples had developed a very rough surface morphology. In this work, the combined influence of surface roughness and W surface enrichment on the sputter yield of EUROFER under bombardment with 200 eV deuterium ions was studied. The influence of surface roughness was determined with the aid of Scanning Electron Microscopy (SEM) and SDTrimSP-3D simulations. W surface enrichment was investigated applying sputter X-Ray Photoelectron Spectroscopy (XPS) depth-profiling and Rutherford backscattering spectrometry (RBS). After bombardment to a fluence of 1024 D m−2 (at 200 eV per deuterium) a reduction of the sputter yield to 29% ± 5% of the initial value was measured by weight-loss measurements. This reduction is in good agreement with published values. Two distinct surface morphologies, consisting of smooth and spiked surfaces, were observed on the EUROFER sample after sputtering. Based on the experimental results, the combined effect of the two factors, surface roughness and W surface enrichment, is estimated to be responsible for a reduction in the sputter yield to 27% ± 4% of the initial value, which is in excellent agreement with the measured value. Our assessment shows that both surface morphology and W surface enrichment contribute significantly to the reduction of the sputter yield of EUROFER under the given experimental conditions, and are sufficient to fully explain the experimentally observed reduction in the sputter yield.

In present tokamaks nitrogen seeding is used to reduce the power load onto the divertor tiles. Some fraction of the seeded nitrogen reacts with hydrogen to form ammonia. The behaviour of ammonia in ASDEX Upgrade is studied by mass spectrometry. Injection without plasma shows strong absorption at the inner walls of the vessel and isotope exchange reactions. During nitrogen seeding in H-mode discharges the onset of a saturation of the nitrogen retention is observed. The residual gas consists of strongly deuterated methane and ammonia with almost equal amounts of deuterium and protium. This confirms the role of surface reactions in the ammonia formation. The results are consistent with findings in previous investigations. A numerical decomposition of mass spectra is under development and will be needed for quantitative evaluation of the results obtained.

The desorption of D implanted into Be with a superficial oxide layer is studied. The different oxide thicknesses and implantation at different energies resulted in a strong variation of the fraction stopped within the oxide layer. Thermal desorption of D was subsequently performed, intermitted by nuclear reaction analysis for assessment of the D depth distributions and total retained amounts. For the conditions, where part of the D was deposited in the Be substrate, a sharp decrease of the retained amount of D occurs around 200 °C. This is attributed to the release from metallic Be. Correspondingly, the D and O depth profiles show that above 200 °C the remaining D is only retained in the BeO layer. Apparently, the superficial BeO layer does not act as a diffusion barrier for D that is released from the metallic substrate. The retained amount of D deposited within the BeO layer decreases steadily and is not completely released at 350 °C, the foreseen bake-out temperature in ITER.

In this paper well defined test samples are used to show how the analysis depth and the matrix material influence the depth resolution of nuclear reaction analysis. The reaction D( 3 He, p)α is used to detect. The influence of the atomic number and the overlayer thickness on the depth resolution is studied by covering 10 nm thin deuterated amorphous carbon (a–C:D) films on silicon with tungsten (ZW=74) and titanium (ZTi=22) of various thicknesses between 500 nm and 8 μm. The most probable depth profiles are calculated from the experimental data with the program NRADC, which implements Bayesian statistics. The resulting apparent layer width of the deuterium containing layer broadens with increasing thickness of the coating and this broadening is more pronounced for coatings with higher Z. These apparent layer widths are a measure for the achievable depth resolution. They are in good agreement with depth resolutions calculated with RESOLNRA. To investigate the

The fraction of deuterium (D) that is retained upon irradiation of beryllium (Be) as well as the temperatures at which implanted D is released are of importance for the international fusion experiment ITER, where Be will be used as an armor material. The influence of single parameters on retention and release is investigated in laboratory experiments performed under well defined conditions with the aim to identify dominant underlying mechanisms and thus be able to predict the behavior of the Be wall in ITER. Recent progress in the quantification of retained fractions and release temperatures as well as in the understanding of the governing mechanisms is presented. The retained fraction upon implantation of D at 1 keV into Be ( 1 1 2 ¯ 0 ) to fluences far below the saturation threshold of 1021 m−2 is almost 95%, the remaining 5% being attributed to reflection at the surface. At these low fluences, no dependence of the retained fractions on implantation energy is observed. At fluences of the order of 1021 m−2 and higher, saturation of the irradiated material affects the retention, leading to lower retained fractions. Furthermore, at these fluences the retained fractions decrease with decreasing implantation energies. Differences in the retained fractions from implanted Be ( 1 1 2 ¯ 0 ) and polycrystalline Be are explained by anisotropic diffusion of interstitials during implantation, leading to an amount of surviving D–trap complexes that depends on surface-orientation. Temperature-programmed desorption (TPD) spectra are recorded after implantation of fluences of the order of 1019 m−2 at various energies and simulated by means of a newly developed code based on coupled reaction–diffusion systems (CRDS). The asymmetric shape of the TPD peaks is reproduced by introducing a local D accumulation process into the model.

The temperature-driven release of deuterium implanted as keV ions into metallic beryllium is measured by temperature-programmed desorption (TPD). TPD spectra from single and polycrystalline Be implanted with 1 keV ions are compared. The high-temperature desorption stage (T > 700 K) is attributed to the release of deuterium trapped at several types of energetically different ion-induced defects. A release peak around 850 K is recorded in the single crystal, while in the polycrystal all deuterium desorbs below this temperature. An increase in the maximum release temperature is observed after implantation of the polycrystal with higher ion energies (2 and 3 keV). We propose an interpretation of the experimental results based on two types of traps, with depth distributions adapted to the implantation energy. Preliminary TMAP7 calculations qualitatively reproduce the shifts in the maximum desorption temperature, observed in the polycrystal at different implantation energies. The difference between the single and the polycrystal is explained by a higher density of surviving defects in the single crystal. Diffusion of mobile defects to grain boundaries and subsequent annihilation is proposed as the dominant mechanism for differences in deuterium desorption from Be(1 1 2 ¯ 0) and Be(poly).