Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade

0489498 - ÚFP 2019 RIV AT eng J - Journal Article
Krieger, K. - Balden, M. - Coenen, J.W. - Laggner, F. - Matthews, G. F. - Nille, D. - Rohde, V. - Sieglin, B. - Giannone, L. - Göths, B. - Herrmann, A. - de Marné, P. - Pitts, R.A. - Potzel, S. - Vondráček, Petr
Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade.
Nuclear Fusion. Roč. 58, č. 2 (2018), č. článku 026024. ISSN 0029-5515. E-ISSN 1741-4326
EU Projects: European Commission(XE) 633053 - EUROfusion
Institutional support: RVO:61389021
Keywords : fusion * melting * tungsten * plasma wall interaction * power exhaust
OECD category: Fluids and plasma physics (including surface physics)
Impact factor: 3.516, year: 2018
http://iopscience.iop.org/article/10.1088/1741-4326/aa9a05

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 cuixent 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 cuixent 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 cuixent 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.
Permanent Link: http://hdl.handle.net/11104/0283920