Abstract
Helium plasma irradiation to tungsten leads to the formation of nanostructures on the surface. On the other hand, when the surface temperature is raised without helium irradiation, the nanostructure is annealed out. In this study, the boundary between the formation and annealing of tungsten nanostructures are investigated in the divertor simulator NAGDIS-II. When the helium ion flux to the surface was gradually decreased at the surface temperature of 1300–1600 K, the nanostructure growth was terminated at some point, and the nanostructures started to be annealed out. The helium ion flux threshold that determines the boundary between the growth and the annealing was varied by the irradiation history, and the flux threshold was in the range of 10 21 –10 22 m −2 .
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