(551c) Helium Aggregation and Surface Morphology Near Surfaces and Grain Boundaries in Plasma-Facing Tungsten

Helium’s effective insolubility in metals has led to a host of problems with helium embrittlement and bubble formation. In fusion reactors, most bubbles form near the surface because of the high flux of ions there, yielding significant surface deformation. Efforts to understand the mechanisms relevant to helium-induced surface evolution are many, but simulations of these phenomena are often hampered by finite size effects and unrealistic flux or fluence values. We discuss here the effects of helium flux, surface orientation, and grain boundary orientation on helium retention and transport as well as tungsten surface evolution in situations intended to be representative of the ITER divertor under pure helium plasma. In particular, we focus on the effects of grain boundaries that intersect the surface, observing that helium collects at the grain boundaries because of reduced rates of transport of helium clusters on grain boundaries compared to the tungsten bulk. The orientation of the surface near the grain boundary is an important aspect of the predicted surface evolution, as it determines whether dislocations emitted by bubbles trapped at the interface will escape to the surface directly above the grain boundary or whether they will be emitted and form surface features farther away.