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@ARTICLE{Krieger:872608,
      author       = {Krieger, K. and Balden, M. and Coenen, J. W. and Laggner,
                      F. and Matthews, G. F. and Nille, D. and Rohde, V. and
                      Sieglin, B. and Giannone, L. and Göths, B. and Herrmann, A.
                      and de Marne, P. and Pitts, R. A. and Potzel, S. and
                      Vondracek, P.},
      title        = {{E}xperiments on transient melting of tungsten by {ELM}s in
                      {ASDEX} {U}pgrade},
      journal      = {Nuclear fusion},
      volume       = {58},
      number       = {2},
      issn         = {1741-4326},
      address      = {Vienna},
      publisher    = {IAEA},
      reportid     = {FZJ-2020-00101},
      pages        = {026024 -},
      year         = {2018},
      abstract     = {Repetitive melting of tungsten by power transients
                      originating from edge localized modes (ELMs) has been
                      studied in ASDEX Upgrade. Tungsten samples were exposed to
                      H-mode discharges at the outer divertor target plate using
                      the divertor manipulator II (DIM-II) system (Herrmann et al
                      2015 Fusion Eng. Des. 98–9 1496–9). Designed as near
                      replicas of the geometries used also in separate experiments
                      on the JET tokamak (Coenen et al 2015 J. Nucl. Mater. 463
                      78–84; Coenen et al 2015 Nucl. Fusion 55 023010; Matthews
                      et al 2016 Phys. Scr. T167 7), the samples featured a
                      misaligned leading edge and a sloped ridge respectively.
                      Both structures protrude above the default target plate
                      surface thus receiving an increased fraction of the parallel
                      power flux. Transient melting by ELMs was induced by moving
                      the outer strike point to the sample location. The temporal
                      evolution of the measured current flow from the samples to
                      vessel potential confirmed transient melting. Current
                      magnitude and dependency from surface temperature provided
                      strong evidence for thermionic electron emission as main
                      origin of the replacement current driving the melt motion.
                      The different melt patterns observed after exposures at the
                      two sample geometries support the thermionic electron
                      emission model used in the MEMOS melt motion code, which
                      assumes a strong decrease of the thermionic net current at
                      shallow magnetic field to surface angles (Pitts et al 2017
                      Nucl. Mater. Energy 12 60–74). Post exposure ex situ
                      analysis of the retrieved samples show recrystallization of
                      tungsten at the exposed surface areas to a depth of up to
                      several mm. The melt layer transport to less exposed surface
                      areas leads to ratcheting pile up of re-solidified debris
                      with zonal growth extending from the already enlarged grains
                      at the surface.},
      cin          = {IEK-4 / IEK-1},
      ddc          = {620},
      cid          = {I:(DE-Juel1)IEK-4-20101013 / I:(DE-Juel1)IEK-1-20101013},
      pnm          = {174 - Plasma-Wall-Interaction (POF3-174)},
      pid          = {G:(DE-HGF)POF3-174},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000419346000003},
      doi          = {10.1088/1741-4326/aa9a05},
      url          = {https://juser.fz-juelich.de/record/872608},
}