Surface modification of He pre-exposed tungsten samples by He plasma impact in the divertor manipulator of ASDEX Upgrade

Brezinsek, S.; Linsmeier, Ch.; Mayer, H.; Caniello, R.; De Temmerman, G.; Elgeti, S.; Böswirth, B.; Terra, A.; Brida, D.; Oberkofler, M.; Balden, M.; Lahtinen, A.; Rohde, V.; Krieger, K.; Sieglin, B.; Neu, R.; Meisl, G.; Kallenbach, A.; Potzel, S.; Greuner, H.; Hakola, A.; Carralero, D.; Douai, D.

doi:10.1016/j.nme.2016.11.002

Journal Article

FZJ-2017-00329

Surface modification of He pre-exposed tungsten samples by He plasma impact in the divertor manipulator of ASDEX Upgrade

Brezinsek, S. (Corresponding author)FZJ* ; Hakola, A. ; Greuner, H. ; Balden, M. ; Kallenbach, A. ; Oberkofler, M. ; De Temmerman, G. ; Douai, D. ; Lahtinen, A. ; Böswirth, B. ; Brida, D. ; Caniello, R. ; Carralero, D. ; Elgeti, S. ; Krieger, K. ; Mayer, H. ; Meisl, G. ; Potzel, S. ; Rohde, V. ; Sieglin, B. ; Terra, A.FZJ* ; Neu, R. ; Linsmeier, C.FZJ*

2017
Elsevier Amsterdam [u.a.]

Nuclear materials and energy 12, 575-581 (2017) [10.1016/j.nme.2016.11.002]

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Please use a persistent id in citations: http://hdl.handle.net/2128/15701 doi:10.1016/j.nme.2016.11.002

Abstract: Tungsten (W) will be used as material for plasma-facing components (PFCs) in the divertor of ITER and interact with Helium (He) ions either from initial He plasma operation or from Deuterium-Tritium (DT) fusion reactions in the active operation phase. Laboratory experiments reported that in a specific operational window of impact energy, ion fluence, and surface temperature (Ein ≥ 20 eV, ϕ ≥ 1 × 1024 Hem Tsurf ≥ 1000 K) a modification of W surfaces occurs resulting in the formation of He-induced W nanostructures. Experiments in ASDEX Upgrade H-mode plasmas ( T, MA, Paux ≃ 8.0 MW) in He have been carried out to investigate in detail (a) the potential growth of W nanostructures on pre-damaged W samples incorporating He nanobubbles, and (b) the potential ELM-induced erosion of W nanostructure. Both W surface modifications were generated artificially in the GLADIS facility by He bombardment of W samples at keV (a) to ϕ ≃ 0.75 × 1024 He0m at Tsurf ≃ 1800 K and (b) ϕ ≃ 1 × 1024 He0m at Tsurf ≃ 2300 K prior to exposure in the divertor manipulator of ASDEX Upgrade. Though in part (a) conditions of W nanostructure growth with a total He ion fluence of ϕ ≃ 1.6 × 1024 Hem and peak He ion impact energies above 150 eV were met, no growth could be detected. In part (b) lower density plasmas with more pronounced type I ELMs, carrying energetic He ions in the keV range, were executed with the strike-line positioned on 2 µm thick W nanostructure accumulating a fluence of ϕ ≃ 0.8 × 1024 Hem. Post-mortem analysis revealed that co-deposition by predominantly W, and Boron (B), eroded at the main chamber wall and transported into the divertor, took place on all W samples. Erosion of W nanostructure or its formation was hindered by the fact that the outer divertor at the location of the samples was turned under these He plasma conditions into a net deposition zone by W, B and Carbon (C) ions. The surface morphology with large roughness and effective surface area act as a catcher for the impinging impurities. Thus, apart from operation in the existence diagram of W nanostructure with respect to Tsurf, ϕ, and Ein, also the impinging impurity flux contribution needs to be considered in predictions concerning the formation of W nanostructures.

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