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K. Krieger, M. Balden, J. Coenen, F. Laggner, G. Matthews, D. Nille, V. Rohde, B. Sieglin, L. Giannone, B. Göths, A. Herrmann, P. de Marné, R. Pitts, S. Potzel, V. Vondracek, . ASDEX Upgrade Team, -. EUROfusion MST1 Team:
"Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade";
Nuclear Fusion, 58 (2018), 2; 02602401 - 02602419.

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.

fusion, melting, tungsten, plasma wall interaction, power exhaust