Ch. Linsmeier

Enhanced toughness and stable crack propagation in a novel tungsten fibre-reinforced tungsten composite produced by chemical vapour infiltration

Tungsten is a promising candidate for the plasma-facing components of a future fusion reactor, but its use is strongly restricted by its inherent brittleness. An innovative concept to overcome this problem is tungsten fibre-reinforced tungsten composite. In this paper we present the first mechanical test of such a composite material using a sample containing multiple fibres. The in situ fracture experiment was performed in a scanning electron microscope for close observation of the propagating crack. Stable crack propagation accompanied with rising load bearing capacity is observed. The fracture toughness is estimated using the test results and the surface observation.

Materials for DEMO and reactor applications-boundary conditions and new concepts

DEMO is the name for the first stage prototype fusion reactor considered to be the next step after ITER towards realizing fusion. For the realization of fusion energy especially, materials questions pose a significant challenge already today. Heat, particle and neutron loads are a significant problem to material lifetime when extrapolating to DEMO. For many of the issues faced, advanced materials solutions are under discussion or already under development. In particular, components such as the first wall and the divertor of the reactor can benefit from introducing new approaches such as composites or new alloys into the discussion. Cracking, oxidation as well as fuel management are driving issues when deciding for new materials. Here composites as well as strengthened CuCrZr components together with oxidation resilient tungsten alloys allow the step towards a fusion reactor. In addition, neutron induced effects such as transmutation, embrittlement and after-heat and activation are essential. Therefore, when designing a component an approach taking into account all aspects is required.

Properties of drawn W wire used as high performance fibre in tungsten fibre-reinforced tungsten composite

High strength and creep resistance also at high temperature, combined with a high thermal conductivity and high melting point make tungsten (W) an ideal material for highly loaded areas in future fusion reactors. However, as a typical bcc metal tungsten features an intrinsic brittleness up to very high temperature and is prone to operational embrittlement. Tungsten fibre-reinforced tungsten composite (Wf/W) utilizes extrinsic toughening mechanisms similar to ceramic fibre-reinforced ceramics and therefore overcomes the brittleness problem. The properties of the composite are to a large extend determined by the properties of the drawn tungsten wire used as reinforcement fibres. W wire exhibits a superior strength and shows ductile behaviour with exceptional local plasticity. Beside the typical mechanisms observed for ceramic composites the ductile deformation of the fibres is therefore an additional very effective toughening mechanism. Tension tests were used to investigate this phenomenon in more detail. Results show that there is a region of enhanced localized plastic deformation. The specific energy consumption in this region was estimated and used to suggest optimisation options for Wf/W composites.

Morphology and composition of Fe–W coatings after deuterium plasma exposure as a model system for RAFM steels

A model system representing the RAFM steel EUROFER-97 is produced by magnetron sputter deposition of iron and 1.5 at% tungsten and investigated in order to study the consequences of plasma exposures. The alloy is deposited as coatings with a thickness of 400 nm on polycrystalline, high purity iron substrates. To understand the erosion mechanisms and morphology changes the coatings were exposed to a linear plasma device with an ion flux of 3×1021 D+ m−2 s−1 and an electron temperature of 13 eV. Samples were exposed at sample temperatures of about 420 and 770 K at incident ion energy of 30 eV (floating potential), 70 and 190 eV. Additionally, the effect of ion fluence was investigated. The coatings before and after plasma exposure were investigated by electron microscopy and glow discharge optical emission spectroscopy (GD-OES). Microstructure observation revealed a complex morphology with distinct sharp spikes formed under the plasma exposure at incident ion energies of 70 and 190 eV. The tungsten enrichment by a factor of 3 in the spikes was visualized by backscatter electron observation and confirmed by both energy-dispersive x-ray spectroscopy and GD-OES. No visible erosion and, by that, tungsten enrichment was observed after the plasma exposure at an incident ion energy of 30 eV, as expected since it is below the threshold energy for sputtering of iron.

Impact on the deuterium retention of simultaneous exposure of tungsten to a steady state plasma and transient heat cycling loads

The impact on the deuterium retention of simultaneous exposure of tungsten to a steady-state plasma and transient cyclic heat loads has been studied in the linear PSI-2 facility with the main objective of qualifying tungsten (W) as plasma-facing material. The transient heat loads were applied by a high-energy laser, a Nd:YAG laser (λ = 1064 nm) with an energy per pulse of up to 32 J and a duration of 1 ms. A pronounced increase in the D retention by a factor of 13 has been observed during the simultaneous transient heat loads and plasma exposure. These data indicate that the hydrogen clustering is enhanced by the thermal shock exposures, as seen on the increased blister size due to mobilization and thermal production of defects during transients. In addition, the significant increase of the D retention during the simultaneous loads could be explained by an increased diffusion of D atoms into the W material due to strong temperature gradients during the laser pulse exposure and to an increased mobility of D atoms along the shock-induced cracks. Only 24% of the retained deuterium is located inside the near-surface layer (d<4 μm). Enhanced blister formation has been observed under combined loading conditions at power densities close to the threshold for damaging. Blisters are not mainly responsible for the pronounced increase of the D retention.

Time resolved imaging of laser induced ablation spectroscopy (LIAS) in TEXTOR and comparison with modeling

Laser based methods are investigated as in situ diagnostic for plasma facing materials (PFMs) in magnetic fusion research to study PFM composition and retention. In laser induced ablation spectroscopy (LIAS) the wall material is ablated by a laser beam. The released material enters the edge plasma region of a fusion experiment and the resulting optical emission is observed. To conclude from the observed photons to the number of ablated atoms, a detailed knowledge of the velocity distribution of the ablated material is required. In this work the LIAS emission in discharges at TEXTOR was studied using an Ametek Phantom v711 camera. In this paper a method is developed to conclude from the observed emission the velocity distribution of the ablated species. The obtained velocity distribution is used for our numerical LIAS model, demonstrating good agreement with our experimental observations. Implications are discussed.

Hydrogen retention in beryllium: concentration effect and nanocrystalline growth

We herein report on the formation of BeD2 nanocrystalline domes on the surface of a beryllium sample exposed to energetic deuterium ions. A polycrystalline beryllium sample was exposed to D ions at 2 keV/atom leading to laterally averaged deuterium areal densities up to 3.5 10(17) D cm(-2), and studied using nuclear reaction analysis, Raman microscopy, atomic force microscopy, optical microscopy and quantum calculations. Incorporating D in beryllium generates a tensile stress that reaches a plateau atu2009u2009≈1.5 10(17) D cm(-2). For values higher than 2.0 10(17) cm(-2), we observed the growth ofu2009u2009≈90u2009nm high dendrites, covering up to 10% of the surface in some zones of the sample when the deuterium concentration was 3u2009u2009×u2009u200910(17) D cm(-2). These dendrites are composed of crystalline BeD2, as evidenced by Raman microscopy and quantum calculations. They are candidates to explain low temperature thermal desorption spectroscopy peaks observed when bombarding Be samples with D ions with fluencies higher than 1.2 10(17) D cm(-2).

Tungsten erosion under combined hydrogen/helium high heat flux loading

We investigated the erosion behaviour of tungsten under irradiation with a fusion reactor-relevant H/He mixture of 94%/6% at a power density of 10 MW m−2. The investigated surface temperatures range up to 2000 °C and the range of the applied fluence was up to 7 × 1025 m−2. The erosion yield we observe exceeds the value expected from physical sputtering data by a factor of 2. In addition we observe an Arrhenius-like increase of the erosion yield with temperature with an activation energy of 0.04 eV.

Modeling of crack formation after pulse heat load in ITER-grade tungsten

The one-dimensional numerical simulations of the deformation during and after the pulsed heat load are carried out for tungsten manufactured according to ITER specifications. The calculations for stress-relieved tungsten demonstrated the cracking at temperatures above ductile-to-brittle transition temperature. The features of the stresses caused by thermal expansion are discussed.

Studies of protection and recovery techniques of diagnostic mirrors for ITER

In optical diagnostic systems of ITER, mirrors will be used to guide the light from plasma towards detectors and cameras. The mirrors will be subjected to erosion due to fast particles and to deposition of impurities from the plasma which will affect adversely the mirror reflectivity and therefore must be suppressed or mitigated at the maximum possible extent. Predictive modeling envisages the successful suppression of deposition in the diagnostic ducts with fins trapping the impurities on their way towards mirrors located in the end of these ducts. To validate modeling predictions, cylindrical and cone-shaped diagnostic ducts were exposed in TEXTOR for 3960 s of plasma operation. After exposure, no drastic suppression of deposition was observed in the cylindrical ducts with fins. At the same time, no detectable deposition was found on the mirrors located at the end of cone-shaped ducts outlining the advantages of the cone geometry. Analyses of exposure provide evidence that the contamination of exposed mirrors was due to wall conditioning discharges and not due to working plasma exposure. Cleaning by plasma sputtering was performed on molybdenum mirrors pre-coated with a 100 nm thick aluminum film. Aluminum was used as a proxy of beryllium. During exposure in electron cyclotron resonance-generated helium plasma, the entire coating was sputtered within nine hours, leaving no trace of aluminum and leading to the full recovery of the specular reflectivity without detrimental effects on the mirror surface.