Self-Consistent Simulations of NSTX-U with Lithium Plasma-Facing Components Using UEDGE and Wall-Li

The use of lithium as a plasma-facing component (PFC) in NSTX-U will be assessed for its potential benefits in tokamak divertor power exhaust and core plasma management. This study explores how the lithium surface response and impurity dynamics evolve with increasing exhaust power from the core plasma. Results are presented from self-consistent simulations of the NSTX-U boundary plasma using UEDGE with lithium PFCs, incorporating full magnetic and E×B drifts physics, and coupled to the dynamic lithium surface model Wall-Li. In the model, the divertor target plates are assumed to consist of a carbon substrate covered with a thin layer of lithium. A scan up to 10~MW core input power assesses the impact on vapor shielding and surface temperature (T_surf), which regulates lithium sourcing. The simulations show that increasing the exhaust power leads to saturation of the lithium T_surf below 600°C, due to lithium vapor shielding that limits further temperature rise. The inclusion of magnetic and E×B drifts reduces the upstream lithium density, as lithium impurity ions are redirected toward the inner divertor through the private flux region, thereby extending the operating surface temperature window. Drifts enhance convective transport significantly and broaden λ_q by spreading heat over a wider area, which reduces peak PFC heat loads. Consequently, the anomalous heat diffusivity, Χ, needed to match experimental λ_q is significantly reduced.