Beryllium Surface Reconstructions and Intermixing on Tungsten*

Tungsten and Beryllium are plasma facing materials (PFMs) chosen as the diverter and first wall material, respectively, at ITER’s tokamak fusion reactor. The Be walls have been shown to erode under the intense conditions of the reactor and Be can deposit on the W diverter. A significant buildup of Be can lead to intermixing and the formation of Be-W alloys. These alloys have significantly lower melting temperatures and different thermal conductivity compared to pure tungsten which could lead to reduced diverter lifetimes and impact the safety of the reactor. The first step in understanding Be-W alloy formation is the assessment of Be intermixing into W. 

Density functional theory is used to discover optimal Be structures as a function of coverage on W(110) and W(211) surfaces. On W(110), a densely packed hexagonal surface reconstruction, which constitutes a full single layer, has been found. Below this density, a superposition of this high and a low-density (1x1) coverage is seen. The W(211) surface has deep troughs which fill with Be and, at the highest single layer coverage, Be atoms are stacked in twisted columns close to the [011] direction. The properties and relative stability of these structures are analyzed using a work function calculation, surface energies and comparison to experimental images. Intermixing is shown to be energetically unfavorable with less than or equal to a full single layer coverage which qualitatively agrees with experimental findings that a critical amount of Be build up is required for significant intermixing and alloying