Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade

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 [1]. Designed as near replicas of the geometries used also in separate experiments on the JET tokamak [2-4], 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 [5]. 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.

Simulations of thermionic suppression during tungsten transient melting experiments

“…As a consequence, a concentrated experimental and modelling effort has been put forward in the past years in order to improve the understanding of the effect of molten PFCs on plasma operation and wall degradation. In dedicated JET [1][2][3] and AUG [4,5] experiments, specially designed tungsten lamellae (referred as special lamellae further) were exposed in the divertor during high power Type I ELMy H-modes, where transient melting was achieved following repetitive ELM heat load impacts. In both tokamaks, the special lamellae had identical geometry, featuring either a protruding leading edge oriented nearly perpendicular to the magnetic field or a slope at α = 17.5 • with respect to the magnetic field.…”

Investigation of transient melting of tungsten by ELMs in ASDEX Upgrade

“…The main tool for modelling of melt motion at metallic plasma facing components is the 3D MEMOS (Melt Motion at Surfaces) code described in [5,6]. To validate the MEMOS code model for tokamak conditions, simulations of tungsten melting and resulting melt motion were compared to experiments in TEXTOR [7,8] on sustained melting and experiments in JET and ASDEX Upgrade on transient melting at special target tiles equipped with a leading edge [9][10][11][12][13]. In all cases the main driving force for melt motion was identified as the j´B force originating from thermionic electron emission at the hot surface.…”

“…This leaves significant uncertainties resulting from the range of published data for the tungsten work function and from assumptions on the possible attenuation of the emission current by space charge effects and locally returning electrons [1,15]. To overcome this limitation, a corresponding experiment was carried out in ASDEX Upgrade with samples designed to allow direct measurement of the thermionic emission current [12]. The replacement current flowing through the samples as result of thermionic emission and other effects, such as secondary electron emission and local gyro-orbit effects, could be measured during transient melting.…”

Dynamics of a high-density plasma in a magnetic field

“…Plasma of a large density in the magnetic field arises widely at powerful plasma sources and especially at fusion devices [1][2][3]. Plasma-surface interactions with the first wall of fusion devices can be accompanied by ignition of a self-sustained unipolar arcing producing dense plasma splashes, which is attributed to so-called vacuum electrical discharge [4][5][6][7].…”