Wall Forum 2017

Recent laboratory experiments revealed that He admixture to deuterium (D) plasmas reduces D retention at elevated temperature for tungsten [1]. This change in retention is accompanied by reduced blistering and the growth of He nano bubbles below the surface. While there are several speculative attempts to explain these observations, the actual cause for the reduced retention remains unclear. One possibility is that He might act as diffusion barrier for D. Likewise nano-sized bubbles might open up additional pathways for D to reach the surface thereby decreasing its transport into the bulk. Contrary to these experimental findings density functional theory (DFT) calculations show strong attraction between He and hydrogen [2], indicating increased trapping and hence retention of D around He clusters should be expected. In order to unravel this mystery we took a novel experimental approach. We separated the surface effect from the He effect by moving the He interaction zone into depth. Moreover, He implantation and D transport was decoupled. With this we show for the first time unambiguously that the presence of He does locally increase D trapping. He does not act as diffusion barrier. Rate equation modelling is presented which can explain the observed effects without the need for free fitting parameters. ([1] M. J. Baldwin et al. Nucl. Fusion 51 (2011) 103021 [2] H-B. Zhou, Nucl. Fusion 50 (2010) 115010) [mehr]

The retention of hydrogen isotopes (H, D and T) in the first, plasma exposed wall isone of the key concerns for the operation of future long pulse fusion devices. It affects theparticle-, momentum and energy balance in the scrape-off layer as well as the retention ofhydrogen isotopes (HIs) and their permeation into the coolant. The currently acceptedpicture that is used for interpreting current laboratory and tokamak experiments is thatof diffusion hindered by trapping at lattice sites. This paper summarises recent resultsthat show that this current picture of how HIs are transported and retained in W needsto be extended: The modification of the surface (e.g. blistering) can lead to the formationof fast loss channels for near surface H. Trapping at single occupancy traps with fixed de-trapping energy fails to explain isotope exchange experiments, instead a trapping modelwith multi occupancy traps and fill level dependent de-trapping energies is required. Thepresence of interstitial impurities like N or C affects the solute HI diffusion coefficient. Thepresence of HIs during damage creation by e.g. neutrons stabilises defects and reducesdefect annealing at elevated temperatures. [mehr]

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Hydrogen (H) in the near-surface region of solids is of great relevance in science and technology. Hydrogen diffusion across the gas/solid interface of H-absorbing metals is exploited routinely in metal hydride H storage and H₂ permeation membranes to produce purified H₂ for fuel cell utilization. While palladium (Pd) absorbed hydrogen enables olefin hydrogenation catalysis [1], H in the interior of metals may also cause embrittlement and pose other problems, such as the retention of hydrogen isotopes in plasma-facing walls of fusion reactor devices. This talk introduces our unique combination of ¹H-specific depth profiling via ¹H(¹⁵N,aγ)¹²C nuclear reaction analysis (NRA) [2] with thermal desorption spectroscopy (TDS) as a powerful technique to identify and evaluate the thermal stability of H states in the near-surface region of solids in a highly depth-resolved fashion. I describe our recent research applying this method and isotope-labeling (H, D) of surface hydrogen to structurally well-defined Pd single crystal surfaces, which clarifies the atomic-level H transportation mechanisms at H₂-exposed Pd surfaces during H₂ absorption and thermal desorption [3]. I will further demonstrate that the insight thus obtained into the surface structure-sensitive H ingress can be used to achieve control over the thermal desorption of Pd-dissolved H in a wide range of temperatures [4] and discuss implications for industrial olefin hydrogenation over Pd catalysts [1, 5]. The NRA(/TDS) technique is equally applicable to nanostructured metals [6], ultrathin oxide films, and semiconductor/dielectric material stacks relevant to the microelectronics industry [7]. (References: [1] M. Wilde, K. Fukutani, W. Ludwig, B. Brandt, J.H. Fischer, S. Schauermann, H.J. Freund, Angew. Chem. Int. Ed., 47 (2008) 9289-9293. [2] M. Wilde, K. Fukutani, Surf. Sci. Rep., 69 (2014) 196-295. [3] S. Ohno, M. Wilde, K. Fukutani, J. Chem. Physics, 140 (2014) 134705. [4] S. Ohno, M. Wilde, K. Fukutani, J. Phys. Chem. C, 119 (2015) 11732-11738. [5] S. Ohno, M. Wilde, K. Mukai, J. Yoshinobu, K. Fukutani, J. Phys. Chem. C, 120 (2016) 11481-11489. [6] M. Wilde, K. Fukutani, M. Naschitzki, H.J. Freund, Phys. Rev. B, 77 (2008) 113412. [7] Z. Liu, S. Fujieda, H. Ishigaki, M. Wilde, K. Fukutani, ECS Transactions, 35 (2011) 55-72.) [mehr]

With the well-succeeded commissioning of the ToF-RBS, we performed a series of experiments aiming new insights on the variation of the surface composition of Tungsten-containing steel alloys due to the preferential sputtering induced by low energy Deuterium irradiation. We exposed W-Fe models systems with three different Tungsten concentrations and EUROFER to a Deuterium beam with 200 eV energy at fluences of 10^23 D/m^2. The ToF-RBS with a 2.0 MeV Silicon ion beam achieved enough depth-resolution to reveal the resulting profiles after irradiation. The combination of the ToF-RBS data with ToF-ERD data, together with surface morphology information obtained by AFM, provided an improved computational modeling of the samples using the MultiSIMNRA software. The profiles revealed that the light elements present in the near-surface regions, i.e. Oxygen and Carbon, may play some hole of importance either in the dynamics of the process (considering the presence of these elements prior the irradiation) or in the interpretation of the results (considering the incorporation of these elements after the exposure). The obtained profiles are in good agreement with the ones obtained by XPS, with the advantage of a direct assessment of the W-profiles with no need for corrections. The analysis of samples irradiated at a high temperature indicates a different behavior for the EUROFER when compared to the model systems, possibly due to Tungsten segregation that occurs in the latter. SEM-EDX measurements confirm segregation on model systems and not on EUROFER. [mehr]

During the course of this masters thesis the existing temperature programmed desorption (TPD) setup TESS was upgraded with an electron beam heater in order to achieve high temperatures (> 2000 K) needed for He TPD from W. First results were obtained and cross-checked by elastic recoil detection analysis with O ions. [mehr]

Rehearsal for an invited talk at the ICFRM conference (Aomori, Japan) 2017. [mehr]

During the course of this master's thesis, the interdiffusion coefficient between tungsten and iron was determined. From data obtained by means of Rutherford backscattering spectrometry, an effective diffusion coefficient was determined via finite element simulations and the Boltzman-Matano method. SEM images on FIB-cuts into a layered tungsten-iron systems reveal the formation of an intermediate phase. From its growth with respect to time, a diffusion coefficient could be determined. A simulation code, describing the formation and growth of such intermediate phases, was developed. [mehr]