Analysis of structural changes and high-heat-flux tests on pre-damaged tungsten from tokamak melt experiments

The characteristics of W plasma-facing components under high heat fluxes were investigated in view of the material choice for the divertor in future fusion devices. Thin W sheets were exposed in the TEXTOR tokamak to multiple steady-state loads (20 MW at 36 degrees), resulting in substantial W melting. One sample was subjected to post-exposure edge localized mode (ELM) simulation in the electron beam facility JUDITH 1 to determine the modified behavior of the pre-damaged components. The material structure after exposure and the cracking thresholds and patterns under repetitive transient thermal loads were investigated. Grain sizes from the original mu m up to the mm range were observed. Void-like structures from the low mu m up to the mm range were found at different regions of the resolidified sample. Exposure of the molten and recrystallized specimens to ELM-like thermal loads revealed a shift of the cracking threshold towards 300 degrees C, indicating an unfavorable increase in the ductile-to-brittle transition temperature.