Helium effects on tungsten under fusion-relevant plasma loading conditions

Experiments were performed in the Pilot-PSI linear plasma device to study the formation of helium-induced nanostructures under high heat fluxes (> 10MWm 2), the erosion of a tungsten surface at the elevated temperatures where those structures typically form and most importantly the behavior of a helium-induced fibreform structure during ELM-like pulses. A strong correlation between the characteristic size of the helium- induced morphology and the size of the voids observed in the near- surface region has been found, and both increase in size with increasing temperatures. Temperature-dependent erosion of tungsten surfaces was observed despite the ion energy being below the sputtering threshold, with an exponential increase of the erosion for temperatures higher than 2400 degrees C. The same effect was observed during ELM- like plasma pulses. Finally, a complete disappearance of the tungsten fibreform structure was observed after only one plasma pulse for energy densities higher than 0.5 MJ m (2). (C) 2013 Elsevier B. V. All rights reserved.