[en] Pulsed arc plasmas were diagnosed by means of emission spectroscopy. A capacitor was discharged through argon and hydrogen leading to a few cycles of damped current oscillation with ∼120 μs period and 5-12 kA maximum current. Spectroscopic measurements in the visible range were carried out in order to characterise the electron temperature and density in the arc channel as well as electron and gas temperatures in the afterglow plasmas. Spectra were integrated over 10 μs time windows and shifted in time from pulse to pulse. The plasmas also contained substantial fractions of electrode material (brass), namely copper and zinc. The electron density was measured in the conventional way from the broadening of H_β or from the Ar I Stark width. In the arc channel, it ranged from about 3.10²² to 2.10²³ m^-3. The broadening of Zn II lines could also be used. Ratios of Ar I to Ar II and of Zn I to Zn II line intensities were analysed for the electron temperature. Line pairs were found which lay conveniently close in one frame of the spectrometer allowing automatic on-line analysis without relying on reproducibility. Atomic physics models including opacity were developed for Ar II and Zn II in order to check the existence of a Boltzmann distribution of their excited states. These calculations showed that the observed levels were in fact close to thermodynamic equilibrium, in particular, if the resonance lines were optically thick. Electron temperature measurements yielded values between 14000 K and 21000 K. The gas temperature in the afterglow, where particles should have formed, was derived from the rotational and vibrational temperatures of C₂ molecular bands. Ratios between Cu I line intensities yielded the electron temperatures. Both were found to be a few 1000 K. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] Nitrogen is foreseen as seeding species in future magnetic confinement fusion reactors in order to reduce the power load from the plasma onto the divertor target tiles by radiative cooling. As a side effect it also gets implanted into the tungsten wall and forms tungsten nitrides (W_xN). The temperature-dependent W_xN formation was investigated in dedicated laboratory experiments. N ions of 300 eV kinetic energy were implanted into W samples under ultra-high vacuum conditions in the temperature range 300 K to 800 K. The N retention in W was monitored and quantitatively analysed by X-ray photoelectron spectroscopy (XPS). A method to calculate the statistical error of the measured data based on Bayesian statistics was developed. Argon sputter depth profiling was combined with XPS to measure N in W depth profiles which were compared with simulated N implantation profiles. Annealing of samples implanted with N at 300 K does not cause a loss of N up to 800 K. However, the retained N amount decreases linearly with increasing implantation temperature. It was found that this reduction is due to ion-irradiation-induced N release at elevated temperatures. Over the whole temperature range N diffusion into depth was not observed. N accumulation measurements showed no evidence for a phase transition in the W_xN layer. However, high resolution XPS measurements revealed that below 600 K a second photoelectron peak occurs in the N 1s signal which can be attributed to different local atomic arrangements of W_xN.

[en] Highlights: • Tungsten fibre (W_f) based CuCrZr and W compounds are proposed for high heat flux components. • W Fibre preforms have been manufactured with industrially viable textile techniques. • W_f/Cu tubes have been successfully produced in cooperation industrial partner. • Strongly increased fracture toughness of W_f/W confirmed in cyclic load tests. - Abstract: The European Fusion Roadmap foresees water cooled plasma facing components in a first DEMO design in order to provide enough margin for the cooling capacity and to only moderately extrapolate the technology which was developed and tested for ITER. In order to make best use of the water cooling concept copper (Cu) and copper-chromium-zirconium alloy (CuCrZr) are envisaged as heat sink whereas as armour tungsten (W) based materials will be used. Combining both materials in a high heat flux component asks for an increase of their operational range towards higher temperature in case of Cu/CuCrZr and lower temperatures for W. A remedy for both issues- brittleness of W and degrading strength of CuCrZr- could be the use of W fibres (W_f) in W and Cu based composites. Fibre preforms could be manufactured with industrially viable textile techniques. Flat textiles with a combination of 150/70 µm W wires have been chosen for layered deposition of tungsten-fibre reinforced tungsten (W_f/W) samples and tubular multi-layered braidings with W wire thickness of 50 µm were produced as a preform for tungsten-fibre reinforced copper (W_f /Cu) tubes. Cu melt infiltration was performed together with an industrial partner resulting in sample tubes without any blowholes. Property estimation by mean field homogenisation predicts strongly enhanced strength of the W_f/CuCrZr composite compared to its pure CuCrZr counterpart. W_f /W composites show very high toughness and damage tolerance even at room temperature. Cyclic load tests reveal that the extrinsic toughening mechanisms counteracting the crack growth are active and stable. FEM simulations of the W_f/W composite suggest that the influence of fibre debonding, which is an integral part of the toughening mechanisms, and reduced thermal conductivity of the fibre due to the necessary interlayers do not strongly influence the thermal properties of future components.

[en] Highlights: • EUROFER has been eroded by deuterium ions to fluences of several 10²⁴ atoms/m². • Resulting sputter yields deviate considerably from SDTrimSP simulations. • Tungsten surface enrichment is observed e.g. in Rutherford backscattering. • Resulting surface morphologies have strong grain dependencies. • Morphology evolution is a key parameter affecting the sputter yield. - Abstract: The erosion behaviour of EUROFER steel due to mono-energetic deuterium (D) ion bombardment in the energy range of 100–1000 eV/D was investigated. At low fluences, the sputtering yield is comparable to that of pure iron (Fe). It then decreases with increasing fluence and tends toward a steady state at larger fluences. The largest experimentally investigated fluences are of the order of several 10²⁴ D/m². The yield reduction is more pronounced for lower D impinging energies. A simple model is presented within which the evolution of the yield can be described by an exponential decay, using empirical values for the fitting parameters. In this model, the yield reduction is caused by preferential sputtering of Fe and the consequent development of a tungsten- (W-) enriched surface layer. SDTrimSP simulations also confirm the appearance of a W-enriched surface layer. However, the experimentally observed fluence dependence of the sputtering yield could not satisfactorily be reproduced in these simulations. The resulting enrichment layer thicknesses below 1 Å stretch the physics model implemented in SDTrimSP beyond its validity range. Experimentally, surface enrichment of W was qualitatively confirmed in Rutherford backscattering and sputter X-ray photoelectron spectroscopy measurements. However, the measured depth profiles are very likely influenced by the observed surface roughening caused by the D irradiation. Electron microscopy revealed the inhomogeneous W distribution on the un-irradiated sample as well as a grain-dependence of erosion and grain-dependent topography development. The inclusion of surface topography in the description of the erosion could be the key to improve the agreement between model predictions and the experiment.

[en] A comparison of high-heat-flux tested carbon-fiber reinforced carbon (CFC)/Cu materials of Tore Supra and Wendelstein 7-X plasma-facing components is made in order to understand the different fatigue behavior of the bonding interfaces, in particular for the Tore Supra materials. The elemental distribution around the bonding layer and the chemical composition of the active element titanium are characterized by secondary ion mass spectrometry and x-ray photoelectron spectroscopy. The results show that the improved bonding of the Wendelstein 7-X target elements compared to the Tore Supra pump limiter elements is due to a modified silicon and titanium distribution at the bonding interface. However, the difference in fatigue behavior between the two Tore Supra components cannot be attributed to the bonding interface, since the elemental distribution and chemistry of these components are identical and no degradation is observed after an extended heat flux exposure.

[en] To avoid failures caused by the inherent brittleness of pure tungsten below its ductile to brittle transition temperature, tungsten heavy alloy was proposed as an alternative solution of the plasma facing material. Plasma facing components manufactured of tungsten heavy alloy have been installed partly in the tokamak ASDEX Upgrade instead of pure tungsten targets. Opposite to the pure tungsten targets, no deep cracks have been identified in these targets after one experimental campaign. In order to understand and improve the material behavior of tungsten heavy alloy as plasmam facing material in ASDEX Upgrade, the impact of heat treatment on tensile properties of 97W2Ni1Fe heavy alloy has been studied in this work. The heat treatments have been conducted in the vacuum for the tensile specimen cut from an ASDEX Upgrade target at different temperatures (600 °C, 1100 °C and 1350 °C) with various durations (15 min, 60 min and 120 min). After the heat treatment, the tensile properties were remarkably improved. The increase of the interfacial strength between tungsten grains and matrix phase as well as growth of grains are considered as the main reasons for the increase of elongation after heat treatment. The experimental results reveal that electrical discharge machining reduces the total elongation of the specimens. Furthermore, no big impact on the tensile properties is found after low energy deuterium plasma implantation.

[en] Due to the unique combination of excellent thermal properties, low sputter yield, hydrogen retention and activation, tungsten is the main candidate for the first wall material in future fusion devices. However, its intrinsic brittleness and its susceptibility to operational embrittlement is a major concern. To overcome this drawback, tungsten fiber reinforced tungsten composites featuring pseudo ductility have been developed. Bulk material can be successfully produced utilizing chemical vapor deposition of tungsten fabrics. However, a fully dense composite with a high fiber volume fraction is still a huge challenge. Therefore, a model is currently developed in Comsol including the complex coupling of transport phenomena and chemical reaction kinetics. To validate the model with experimental data, fibers were deposited in heated tubes under controlled parameter variation. The temperature and tungsten growth rate were measured along the fibers and inner tube surfaces for different heater temperatures, partial pressures and gas flows. With the experimental results the prediction of the model has been improved. As next step the model will be applied to design infiltration experiments to fabricate fully dense Wf/W composites with a high fiber volume fraction.

[en] Highlights: • Clear evidence of microscopic damage and crack formation at the notch root in the early stage of the fatigue loading (50–100 load cycles). • Propagation of fatigue crack at the notch root in the course of subsequent cyclic heat-flux loading followed by saturation after roughly 600 load cycles. • No sign of damage on the notch-free surface up to 800 load cycles. • No obvious effect of the pulse time duration on the crack extension. • Slight change in the grain microstructure due to the formation of sub-grain boundaries by plastic deformation. - Abstract: Recently, the idea of bare steel first wall (FW) is drawing attention, where the surface of the steel is to be directly exposed to high heat flux loads. Hence, the thermo-mechanical impacts on the bare steel FW will be different from those of the tungsten-coated one. There are several previous works on the thermal fatigue tests of bare steel FW made of austenitic steel with regard to the ITER application. In the case of reduced-activation steel Eurofer97, a candidate structural material for the DEMO FW, there is no report on high heat flux tests yet. The aim of the present study is to investigate the thermal fatigue behavior of the Eurofer-based bare steel FW under cyclic heat flux loads relevant to DEMO operation. To this end, we conducted a series of electron beam irradiation tests with heat flux load of 3.5 MW/m² on water-cooled mock-ups with an engraved thin notch on the surface. It was found that the notch root region exhibited a marked development of damage and fatigue cracks whereas the notch-free surface manifested no sign of crack formation up to 800 load cycles. Results of extensive microscopic investigation are reported

[en] Understanding the influence of the microstructure of tungsten on hydrogen transport is crucial for the use of tungsten as first-wall material in fusion reactors. Here, we report the results of molecular dynamics and transition state studies on the influence of grain boundaries in tungsten on the transport of hydrogen. An exhaustive mapping of possible minimum activation energy migration trajectories for hydrogen as the trace impurity reveals a strongly modified activation energy distribution in the neighborhood of grain boundaries together with an altered connectivity matrix. The results indicate that grain boundaries in polycrystalline tungsten may provide an important transport channel, especially for neutron-damaged tungsten.

[en] In tungsten fibre-reinforced tungsten composites (W_f/W) the brittleness problem of tungsten is solved by utilizing extrinsic toughening mechanisms. The properties of the composite are very much related to the properties of the drawn tungsten wire used as fibre reinforcements. Its high strength and capability of ductile deformation are ideal properties facilitating toughening of W_f/W. Tensile tests have been used for determining mechanical properties and study the deformation and the fracture behaviour of the wire. Tests of as-fabricated and straightened drawn wires with a diameter between 16 and 150 μ m as well as wire electrochemically thinned to a diameter of 5 μ m have been performed. Engineering stress–strain curves and a microscopic analysis are presented with the focus on the ultimate strength. All fibres show a comparable stress–strain behaviour comprising necking followed by a ductile fracture. A reduction of the diameter by drawing leads to an increase of strength up to 4500 MPa as a consequence of a grain boundary hardening mechanism. Heat treatment during straightening decreases the strength whereas electrochemical thinning has no significant impact on the mechanical behaviour. (paper)