Investigation of 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 the tokamak experiment ASDEX Upgrade. Tungsten samples were exposed to H-mode discharges at the outer divertor target plate using the Divertor Manipulator II (DIM-II) system. The exposed sample was designed with an elevated sloped surface inclined against the incident magnetic field to increase the projected parallel power flux to a level were transient melting by ELMs would occur. Sample exposure was controlled by moving the outer strike point (OSP) to the sample location. As extension to previous melt studies in the new experiment both the current flow from the sample to vessel potential and the local surface temperature were measured with sufficient time resolution to resolve individual ELMs. The experiment provided for the first time a direct link of current flow and surface temperature during transient ELM events. This allows to further constrain the MEMOS melt motion code predictions and to improve the validation of its underlying model assumptions. Post exposure ex-situ analysis of the retrieved samples confirms the decreased melt motion observed at shallower magnetic field line to surface angles compared to that at leading edges exposed to the parallel power flux.

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.…”

Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade

“…Poloidal cross-section of the ASDEX Upgrade vessel with divertor manipulator II structure[21] and flux surfaces of reference H-mode discharge #33499 in the flat top phase at t = 3 s with the outer strike point at the position preceding sample exposure.…”

“…CAD view of the two instrumented tiles mounted on the DIM-II probe head[21] with installed samples, ceramic insulation, electrical contact points and mantle thermocouples.…”

A brief summary of tungsten technology development in Korea

“…They are heavily bombarded by energetic particles during plasma shots. Consequently, the temperature of the leading edge could reach the melting point of tungsten, resulting in the movement of molten tungsten in the direction of j × B [17][18][19]. By tilting and shaping of the castellated structure, and by changing its angle with respect to the field line, leading edges can be hidden from the plasma particles.…”

“…It was also found that the molten droplets of W were directed towards the high field side due to j × B force as reported in Refs. [17][18][19].…”