Reactor relevant heat flux and displacement damage on ultra-fine grain tungsten*

Ultra-fine grain tungsten (UFG W) and ITER-grade W plasma-facing samples are exposed to reactor relevant conditions in the DIII-D divertor using DiMES. Effects of displacement damage on the deuterium (D) retention is examined using flat samples pre-damaged with 12 MeV Si ions up to 0.6 dpa. Damaged UFG W releases its trapped D inventory at temperatures < 500 C, but most of the inventory in damaged ITER W is still retained at 500 C. Tritium retention could therefore be mitigated in UFG W with routine low temperature tile baking. Surface damage effects are also examined using protruding angled samples exposed to heat fluxes up to 10 MW/m² during ELMs. The ITER W shows inter-granular cracks and develops < 1 μm diameter holes on the surface. UFG W develops cracks and vertically displaced grains along with surface features as large as 10s of μm consistent with local melting and droplet migration. These observations contradict e-beam experiments in which transient ELM-like heating causes less damage to the surface of UFG W materials than ITER W. Synergistic effects between heat and plasma particle fluxes may explain the differing results.