Wall Forum 2022

Tungsten (W) shows a pronounced transition from ductile deformation behaviour at high temperatures to brittle behaviour at low temperatures (ductile-to-brittle transition). Undeformed, coarse-grained W exhibits transition temperatures around 200 °C to 300 °C, rendering it unsuitable for structural applications at room temperature. The transition is shifted to lower temperatures by cold-working W in wire drawing, rolling or a severe plastic deformation process. Sufficiently cold-worked W materials deform plastically at room temperature and below. The reasons for the shift of the transition temperature are connected to the deformation-induced microstructural changes. We performed the first systematic study of the relationship between microstructure and deformation mechanisms on a series of sequentially drawn potassium-doped W wires. The deformation behaviour and the strength of the wires were investigated using tensile tests and transient mechanical tests at room temperature; the microstructure was characterised using electron backscatter diffraction (EBSD). Quantitative results on the deformation-induced microstructural evolution and its effect on the mechanical properties will be presented. Furthermore, possible theories on the reasons for the shift of the ductile-to-brittle transition upon cold-working are discussed. [mehr]

Helium transport in tungsten during fuzz growth was investigated to provide insight into the helium-induced nanostructuring process which adversely affects the near-surface properties of plasma-facing tungsten components. Fuzz layers with thicknesses of 1.0 or 1.7 µm were initially formed with pure ⁴He plasma at 1100 K, then sequentially exposed to a mixed ³He-⁴He plasma at a range of sample temperatures (i.e., 316-1053 K). ³He retained in the fuzzy layer and in the bulk substrate after plasma exposure were measured by nuclear reaction analysis. An increase in ³He retention was observed at 1053 K in the bulk, but no comparable decrease was observed the fuzz layer, suggesting more efficient trapping of helium in the bulk at this temperature, and no diffusion of helium from the fuzz layer to the bulk. An analytical model of helium transport in vacuum within the fuzz layer was developed for comparison. Model results were found to compare well with experimental results of retention in the bulk when considering helium reflection from the fibers sides and when considering that only ~10^-3% of helium incident on bulk tungsten is retained. Using model results for helium implantation in the bulk, the helium fluence-fuzz thickness relation was derived, which matched well with experimental data. Hence it was determined that fuzz growth is governed by helium content in the bulk, which occurs by implantation and not diffusion. [mehr]

The WallDYN code has been used in the past to make prediction about Be erosion and deposition in ITER and the resulting fuel retention by co-deposition. These calculations were done in 2D assuming a flat toroidally symmetric frist wall. Recently the calculations were re-done in 3D taking the geometric details of the shaped Be first wall of ITER into account. The presentation will show how the 3D plasma was "obtained" from a 2D SolPS/OSM solution and how the shaped first wall affects Be erosion/deposition and the resulting fuel retention. The new 3D results are also compared to the old 2D calculation and to ERO2.0 predictions. [mehr]

The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) operates several user facilities for materials research and defect characterization employing positron annihilation spectroscopy. In this talk, you will obtain an overview of positron basics, the Monoenergetic Positron Source (MePS) as well as the other source-based positron beams and setups at the HZDR. [mehr]

The transmission electron microscope (TEM) is the perfect tool for the structural and chemical characterization of metallic materials at the nanoscale. It is one of the few techniques capable of resolving nanosized precipitates and determining their composition and structure, as well as visualizing structural defects such as dislocations. The imaging using high angular dark field (HAADF) technique as well as analysis using electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS), has provided the comprehensive characterization of the materials microstructure. The application of advanced TEM methods to fusion-relevant materials at IAM-AWP includes the microstructural examination of neutron irradiated alloys of EUROFER type, beryllium and tungsten.The effect of neutron irradiation up to 16 dpa damage dose and on the microstructure of EUROFER-ODS (0.5% Y2O3) was compared to that of pure EUROFER97 irradiated under the same conditions. The study allows to determine the influence of ODS particles and their morphology on radiation resistance. Analysis of neutron irradiated beryllium detected and analyzed the presence of transmutation induced helium and tritium inside the alloy. Application of 2D elemental analysis of neutron-irradiated W supports the conclusion that transmutation-induced Re and Os segregate differently on the structural defects. All these results contribute significantly to the understanding of structural damage, provide valuable approaches for theoretical modelling, and allow conclusions concerning applicability of analysed materials in fusion reactor. [mehr]

Helium diffusion, clustering and bubble nucleation and growth is modelled using the finite element method. The existing model from Faney et al. (Model Simul Mater Sci Eng 22:065010, 2018; Nucl Fusion 55:013014, 2015) is implemented with FEniCS and simplified in order to greatly reduce the number of equations. A parametric study is performed to investigate the influence of exposure conditions on helium inventory, bubbles density and size. Temperature is varied from 120 K to 1200 K and the implanted flux of 100 eV He is varied from 1017 m−2 s−1 to 5 × 1021 m−2 s−1. Bubble mean size increases as a power law of time whereas the bubble density reaches a maximum. The maximum He content in bubbles was approximately 4 × 108 He at 5 × 1021 m−2 s−1. After 1 h of exposure, the helium inventory varies from 5 × 1016 m−2 at low flux and high temperature to 1025 m−2 at high flux and low temperature. The bubbles inventory varies from 5 × 1012 bubbles m−2 to 2 × 1019 bubbles m−2. Comparison with experimental measurements is performed. The bubble density simulated by the model is in quantitative agreement with experiments. [mehr]

Defects like vacancies, dislocations and grain boundaries are discontinuities of the crystal lattice and may attract dissolved hydrogen atoms leading to a distribution of site energies and a higher solubility of hydrogen at a given partial pressure of hydrogen gas. On the other hand hydrogen atoms at discontinuities may either enhance or retard defect motion. Hydrogen also affects the generation of defects reducing their formation energies to zero or even negative values, which is the main reason for hydrogen embrittlement of metals. The basic physico-chemical laws describing this behavior of hydrogen are presented together with examples of experiments, where these laws play a major role. [mehr]

The assessment of transport phenomena as well as fuel retention plays a major role in the operation of a fusion device. That is why the instruments and methods designed for in situ measurements of erosion-deposition phenomena or fuel retention will play a leading role. LIBS (Laser-Induced Breakdown Spectroscopy) measurements is one of the investigation technique with a large perspectives in this respect. In order to assure a correct assessment of the investigated regions, these investigation instruments should be calibrated using reference samples with composition as close as possible with the real tiles exposed to fusion plasma.In this talk we report the results obtained in production of W based coatings with gas inclusions (D, He etc). The production of the coatings was performed by using an equipment provided with 2 magnetrons. The magnetrons can be energized independently by using DC or HiPIMS (High Power Impulse Magnetron Sputtering) power supplies. This fact allows a higher control of the deposition rates, and consequently of the stoichiometry of the deposited compound when 2 different target materials are used. The magnetrons are mounted in the deposition chamber in such a way that the substrate holder is situated in confocal position.The chemical composition (elemental depth profile) of the coatings was performed by using Glow Discharge Optical Emission Spectrometry (GDOES). Thermal Desorption Spectrometry (TDS) measurements were also performed in order to evaluate the gas content (D, He, Ar) released from the coatings. [mehr]

The production of a W-Cu divertor monoblock assembly shall be done using the HRP (Hot Radial Pressing) method. This process uses high temperatures and pressures to join the different components that compose the monoblock. Due to the large temperature transient that takes place during HRP and the different thermo-mechanical properties of tungsten and copper, stresses build up in the component. Previous studies have analyzed the stress build-up during HRP computationally and experimentally. However, no systematic analysis was done that considered viscoplastic effects in the relevant materials. In this master thesis, the impact of viscous stress relaxation during HRP is further analyzed. The results will be used to support the interpretation of the experimental data already obtained and of future analysis. The thesis also presents an innovative method to simulate cutting processes, which are needed to compare simulated results with specific experimental analysis. In addition, the implication of the findings is discussed in terms of the component design, material testing and experimental stress measurements. [mehr]

The quality of data obtained via nuclear reaction analysis is directly dependent on the quality of cross-section data used. Unfortunately, the data available for the cross-sections of helium-3 on carbon-12 and carbon-13 either significantly disagree or there are no data available at all. For this master thesis systematic measurement of the ¹²C(³He,p_x)¹⁴N and ¹³C(³He,p_x)¹⁵N cross-sections has been performed for peaks with x=0..6 and x=0..11 for ¹²C and ¹³C, respectively, energies from 1.5 MeV to 6 MeV and angles of 135° and 175° using thin a-C:H layers. The ¹²C cross-section results have been benchmarked using thick pyrolytic graphite targets and SimNRA simulations and were found to be very accurate. As part of the measurements the accelerator beam energy was calibrated with a novel method combining simulations and measurements. [mehr]

An overview of the Hydrogen-3 Advanced Technology (H3AT) facility, currently nearing completion at Culham. This will be the worlds largest civilian tritium research facility. The UKAEA’s H3AT department has been developing a tritium R&D programme to utilise the new facility to enable us to provide world class tritium solutions that will enable the delivery of fusion devices. The research themes and current experimental efforts will be presented, with a focus on the new tritium compatible ion exposure system. [mehr]

Solid tungsten tiles exposed at the outer strike point module of the ASDEX Upgrade divertor show cracks parallel to surface in the bulk and humps at the surface. At ASDEX Upgrade horizontal cracks were observed for the first time. About 10 % of all tiles exhibit protrusions originated by horizontal cracks about 5 mm below the surface. The position of this damage coincidence with the location of highest deposited energy, which varies for the different tiles due to the mounting. The tungsten at the locations of the horizontal cracks exhibits recrystallization and grain growth, presumably due to the worse thermal contact caused by the horizontal cracks. Although, the operation of AUG was not hampered by these damages, these tiles had to be exchanged preventing progression of these damages. High thermal stress due to temperature gradients is discussed to produce these cracks. Additionally, humps are found at the surface at the position of high heat flux. Their occurrence seems to be related to the increased number of high power discharges. Focused ion beam prepared cross sections indicate pores and cavities in and below these humps. This finding is discussed in the view of He bubble formation in tungsten. [mehr]

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Surface roughness is known for many years to affect the sputtering behaviour of materials under ion bombardment [1]. While theoretical models and simulation codes are often approximating surfaces to be perfectly flat, applications in technology, especially for first wall materials in nuclear fusion devices, may require a more detailed consideration of the roughness.In recent years, the question arose whether an accessible surface roughness parameter exists which allows to describe (static) effects on sputtering yields for conventionally rough surfaces. Well-known parameters like the root-mean-square roughness failed to describe the trends, but it was observed that the mean value of the surface inclination angle distribution enables a scale-independent characterisation, even allowing to predict effective sputtering yields without need for detailed simulations or experiments [2]. Understanding these geometric effects from surface topography on the sputtering yield of conventionally rough surfaces, it was possible to optimise tungsten surfaces by implementing nanocolumnar structures, thereby achieving very low sputtering yields, e.g., as desired for first wall applications in nuclear fusion devices [3]. In this talk, an overview on the experimental methods (quartz crystal microbalance), numerical simulation techniques (SPRAY) and analytical models (roughness parameters for sputter yield prediction), which were developed in Vienna to investigate structured surfaces under ion bombardment, is given. Furthermore, a comparison of the characteristic sputtering behaviour of flat, conventionally rough [2] and oriented nanocolumnar tungsten surfaces [3] will be presented during this talk, also indicating the relevance and potential for application in nuclear fusion devices.[1] M. Küstner et al., Nucl. Instrum. Methods Phys. Res. B, 145 (1998), 320-331[2] C. Cupak, et al., Appl. Surf. Sci. 570 (2021) 151204[3] A. Lopez-Cazalilla, C. Cupak, et al., Phys. Rev. Mat. 6 (2022) 075402 [mehr]

The influence of pre-existing displacement damage on the early stages of helium (He) interaction with tungsten (W) and the resulting defect creation was investigated experimentally. Samples were irradiated with 20.3 MeV W ions to different displacement-damage levels of 0.005, 0.01 and 0.1 displacements per atom (dpa). Displacement-damaged samples were exposed to a He plasma at room temperature at a He flux in the range of 10¹⁸ He/m²s to fluences of up to 10²² He/m² together with pristine W samples. He ion energy of 100 eV was used to remain well below the threshold for displacement damage creation in the bulk. Elastic recoil detection analysis (ERDA) shows that the He retention in the damaged samples is one order of magnitude larger than in the undamaged sample. Detailed He depth distributions were derived by stepwise removal of near-surface layers (via anodic oxidation and dissolution of the oxide) and subsequent ERDA measurements of the remaining He content. Pre-damaged samples show a significantly faster decrease in He concentration with depth than the undamaged sample, indicating that He is efficiently stopped from diffusing into deeper regions beyond 30 nm by pre-existing defects. The undamaged sample exhibits a lower He concentration in the near surface region and a flatter distribution of He up to a depth of 100 nm. From the clear influence of the initial displacement damage level on the He uptake and retention we conclude that He self-trapping mechanisms do not yet have a strong effect on the He diffusion depth in W at low fluxes used in this study. [mehr]