%0 Journal Article
%A Brezinsek, S.
%A Hakola, A.
%A Greuner, H.
%A Balden, M.
%A Kallenbach, A.
%A Oberkofler, M.
%A De Temmerman, G.
%A Douai, D.
%A Lahtinen, A.
%A Böswirth, B.
%A Brida, D.
%A Caniello, R.
%A Carralero, D.
%A Elgeti, S.
%A Krieger, K.
%A Mayer, H.
%A Meisl, G.
%A Potzel, S.
%A Rohde, V.
%A Sieglin, B.
%A Terra, A.
%A Neu, R.
%A Linsmeier, Ch.
%T Surface modification of He pre-exposed tungsten samples by He plasma impact in the divertor manipulator of ASDEX Upgrade
%J Nuclear materials and energy
%V 12
%@ 2352-1791
%C Amsterdam [u.a.]
%I Elsevier
%M FZJ-2017-00329
%P 575-581
%D 2017
%X 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.
%F PUB:(DE-HGF)16
%9 Journal Article
%U <Go to ISI:>//WOS:000417293300091
%R 10.1016/j.nme.2016.11.002
%U https://juser.fz-juelich.de/record/826069