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| Home > Publications database > Microscopy study of the growth and erosion of fuzz on tungsten by helium plasma exposure on ASDEX Upgrade |
| Abstract | FZJ-2021-01647 |
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2021
Abstract: Microscopy study of the growth and erosion of fuzz on tungsten by heliumplasma exposure on ASDEX UpgradeM. Balden1, S. Brezinsek2, A. Hakola3, K. Krieger1, R. Neu1, M. Rasinski2,the ASDEX Upgrade Team4, and the EUROfusion MST1 Team51Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany2Forschungszentrum Jülich, Institut für Energie und Klimaforschung - Plasmaphysik, 52425 Jülich, Germany3VTT, P.O. Box 1000, 02044 VTT, Finland4see author list of H. Meyer et al. 2019 Nucl. Fusion 591120145see author list of B. Labit et al. 2019 Nucl. Fusion59 086020Martin.Balden@ipp.mpg.deTungsten (W) will be used as plasma-facing material in the ITER divertor due to its superiorproperties leading to good power handling, low fuel retention and low sputtering. Its advantageshave previously been demonstrated for hydrogen plasma operation. However, a campaign withhelium (He) plasmas is considered in the start-up phase of ITER, which might lead tomicroscopic changes as observed for various He exposure conditions in laboratory studies.Helium can form bubbles in W, which may in turn lead to formation of a nano-fibrous surfacelayer, referred to as fuzz. In order to assess whether and to which extent this happens in atokamak environment and how preformed fuzz survives under high power loads, a dedicatedexperiment was performed during the He plasma campaign of ASDEX Upgrade in 2019.The effect of He exposure on a W-fuzz surface was studied by exposing a set of 12pre-characterized and partially He pre-exposed samples to a series of 10 H-mode and 6 L-modeHe plasma discharges. To separate the effects of the respective plasma exposure conditions,both scenarios were run with well-separated strike line positions. The samples were arranged intwo poloidal stripes embedded in a dedicated Mo-coated W divertor tile. He pre-exposedsamples (with fuzz or only roughened) were prepared in the high heat-flux neutral beam teststand GLADIS (Garching) as well as in the linear plasma device PSI-2 (Jülich). The pre- andpost-exposure surface morphology and composition was analyzed in detail by scanning electronmicroscopy combined with energy-dispersive X-ray spectroscopy and focused ion beam cutting.The analyses revealed that:(i) Above the H-mode strike line, where the target got rather hot, new fuzz has been formed witha thickness of a few hundred nanometers. Its microstructure shows no significant lateralvariation and is independent of the initial surface structure before AUG exposure.(ii) Directly at the H-mode strike line, ~200 nm of the initial W-fuzz structure on Hepre-exposed samples has been eroded.(iii) At and below the H-mode strike line, strong arcing occurred on the sample with a thickpre-established W-fuzz layer. The arc traces reduced, however, the fuzz thickness only slightly.(iv) At the strike line of the L-mode sub-series (~7 cm below that of the H-mode sub-series),only marginal erosion and slight deposition is observed on He pre-exposed samples.(v) Thickness and composition of deposited material varies with poloidal position, with amaximal thickness of 1 μm. Tungsten is the dominant species in the deposited material, whileMo and Ni+Fe (from heavy alloy divertor tiles) are found at least as traces.
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