Abstract The evaluation of the impact of plasma-facing units (PFUs) damage on subsequent plasma operation is an important issue for ITER. This is one of the topic addressed by the operation of ...dedicated PFUs in WEST. From the start of WEST in 2017, few ITER grade divertor plasma-facing units (PFUs) were tested in the lower divertor for short plasma exposure (experimental campaigns C1-C5). A PFU is an assembly of tungsten blocks on CuCrZr tube via pure copper interlayer. Since 2022, WEST operates with a lower divertor fully equipped with ITER grade PFUs (experimental campaign C6-C7) with long duration plasma exposure capabilities. To test the ageing of the ITER-grade PFUs on an accelerated basis, well controlled damage was implemented on purpose with electron beam loading on the plasma facing surface of two different PFUs (pre-damage#1 and #2 featuring different level of damage). Damage tackled in this study is induced by the loading with numerous transient heat loading events to mimic the ELM impact on the ITER grade PFU. This paper presents the damage before and after WEST plasma exposure and shows the tungsten sources and temperature of the block measured during the operation. On damaged area, surface roughening is heterogeneous. With WEST campaigns (C3, C4 and C7) respectively ended in (2018, 2019 and 2023) a global smoothing of the pre-damaged surfaces, in particular those located in the high loaded erosion area, is observed. No evolution of the crack is observed in the electron beam loaded area, while new cracking are observed on non pre-damaged area on the tungsten top surfaces in highly loaded areas (outer strike point) after significant plasma duration and cumulated energy (first observation in C4 and tends to be confirmed with preliminary observations after the C7 campaign). Smoothing is less pronounced after the 2018 campaign, which may be due to the related short plasma duration. The surface smoothing of blocks also depends on their poloidal position. For PFU which has no shaping, toroidal positioning within a block has no influence on the surface smoothing. After 2019 and 2023 campaigns, equivalent relative smoothing is obtained (between 19 and 32%). For the explored plasma conditions, damage has no influence on the maximum brightness of W.
Stainless steels are widely used as structural materials due to their excellent corrosion resistance and fabricability. In some applications, in nuclear fusion reactors for instance, these materials ...may be exposed to harsh conditions, including high heat flux and ion bombardment in a vacuum environment, which can significantly affect their surface properties and composition. In this work, we investigate the temperature-dependent evolution of the surface composition and optical properties (reflectivity, emissivity) of stainless steel 316L. X-ray photoelectron spectroscopy analyses, temperature programmed desorption and reflectivity measurements in the visible/near-infrared range were performed on polished samples from ambient temperature to 1000 K. It was found that around 750 K, the initial Fe/Cr surface abundance of 90/10 changes to 35/55 following the onset of Cr and CrO desorption at 575 K, with Cr scavenging oxygen initially bound to Fe. In addition, Mn segregated at the surface and oxidized, while small amounts of metallic Ni segregated. Despite these modifications in surface composition, no significant change in reflectivity up to 900 K was observed. These results provide insight into the behavior of stainless steel under extreme conditions which is relevant for understanding its performance and durability in high-temperature applications.
Raman microscopy is one of the methods that could be used for future post-mortem analyses of samples extracted from ITER plasma facing. This study shows that this technique is useful for studying ...tungsten-based materials containing impurities including oxides and nitrides. Here, we apply pulsed laser deposition and DC argon glow discharges to produce tungsten-containing synthetic films (compact, porous) and nanoparticles and investigate the influence of their morphology on the measured Raman spectra. The amounts of oxygen and/or nitrogen in the films are also investigated. Comparative data are obtained by X-ray Photoelectrons Spectroscopy, Atomic Force Microscopy, Electron Microscopies (Scanning and Transmission), Energy Dispersive X-ray spectroscopy, Time-of-Flight Elastic Recoil Detection Analysis. The power density of the laser beam used to perform Raman microscopy is varied by up to 4 orders of magnitude (0.01-20 mW/\(\mu\)m 2) to investigate thermal stability of films and nanoparticles. As a first result, we give evidence that Raman microscopy is sensitive enough to detect surface native oxides. Secondly, more tungsten oxides are detected in porous materials and nanoparticles than in compact films, and the intensities of the Raman band correlate to their oxygen content. Thirdly, thermal stability of these films (i.e. structural and chemical modification under laser heating) is poor when compact films contain a sufficiently large amount of nitrogen. This finding suggests that nitrogen can be substituted by oxygen during Raman laser induced heating occurring in ambient air. Finally, our methodology can be used to rapidly characterize morphology and chemistry of the samples analyzed, and also to create oxides at the micrometer scale. keywords: PLD, Raman spectroscopy, tungsten oxide, tungsten nitride, plasma wall interaction, laser heating, post-mortem analysis 2
Despite advances in the differentiation of insulin-producing cells from human embryonic stem cells, the generation of mature functional β cells in vitro has remained elusive. To accomplish this goal, ...we have developed cell culture conditions to closely mimic events occurring during pancreatic islet organogenesis and β cell maturation. In particular, we have focused on recapitulating endocrine cell clustering by isolating and reaggregating immature β-like cells to form islet-sized enriched β-clusters (eBCs). eBCs display physiological properties analogous to primary human β cells, including robust dynamic insulin secretion, increased calcium signalling in response to secretagogues, and improved mitochondrial energization. Notably, endocrine cell clustering induces metabolic maturation by driving mitochondrial oxidative respiration, a process central to stimulus-secretion coupling in mature β cells. eBCs display glucose-stimulated insulin secretion as early as three days after transplantation in mice. In summary, replicating aspects of endocrine cell clustering permits the generation of stem-cell-derived β cells that resemble their endogenous counterparts.
Lasing in organic 2D photonic crystal microcavity is investigated under optical pumping of Alq3:DCJTB as a gain medium. The resonant mode of the microcavity was measured at 662 nm with a threshold of ...0.6 nJ/pulse.
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