The role of lithium thin-film coatings on W surface morphology evolution under high-fluence and high temperature He irradiation*

Tungsten is the candidate plasma-facing component material for the ITER divertor due to its high sputter threshold, high melting temperature, and excellent thermal conductivity. However, when exposed to He ions with E = 0.01-1.0 keV and high fluences $>$10$^{26}$ m$^{-2}$ [1], as those expected in a burning plasma fusion tokamak divertor, the damage to the surface can include the creation of bubbles, holes and tendril-like fuzz morphology. Recent studies show that adding low-Z impurities (C and Be) to a He plasma can inhibit the growth of fuzz. In other applications, lithium (Li) as a PFC coating in multiple tokamaks has improved plasma performance, yet its interaction with high-Z materials (i.e. W) and its role inhibiting fuzz formation is not well understood. We investigated the effect of a thin $\sim$1000 nm Li coating on formation of W surface defect morphology under high fluence and temperature conditions. Samples were exposed with fluxes of $\sim$10$^{24}$ m$^{-2}$s$^{-1}$ and T$_{surf}$ $\sim$1100 $^{\circ}$C. After irradiation, the surfaces of the samples were characterized with SEM. These results are presented along with XPS and SIMS results elucidating the persistence of Li coatings under these conditions. $^{1}$O. El-Atwani, et al., Nucl. Fusion 54 (2014) 083013.