Investigation of Drift Effects in <scp>UEDGE</scp> Simulations of <scp>NSTX</scp> ‐U Edge Plasma With Lithium Divertors

ABSTRACT Lithium is a low‐Z material, and lithium‐based plasma‐facing components (PFCs) are planned for the National Spherical Torus Experiment Upgrade (NSTX‐U) to explore potential benefits for divertor power exhaust and core plasma management. NSTX‐U is a medium‐sized spherical tokamak with up to 12 MW of auxiliary heating, capable of generating reactor‐relevant plasma conditions. This work presents boundary plasma simulations for NSTX‐U with lithium PFCs using the UEDGE code, incorporating full magnetic and drift physics. The simulations show that drifts strongly influence heat and particle transport: they enhance convective transport, broaden the scrape‐off layer heat‐flux width , and reduce the anomalous heat diffusivity required to reproduce predicted SOL heat‐flux width. drifts provide poloidal transport, while (which includes both gradB and curvature) drifts provide radial heat and particle transport. Lithium transport is also affected by drifts, with lithium ions migrating from the outer divertor to the inner divertor through the private flux region (PFR) following the drifts flow, lowering upstream impurity lithium densities. UEDGE is self‐consistently coupled with the Wall‐Li model to study plasma lithium PFC interactions depending on the local lithium sourcing based on local plasma conditions and lithium surface temperature. In these simulations, lithium evaporation shows a vapor‐shielding effect that reduces divertor heat flux and increases radiative losses once surface temperatures exceed 450°C. This research work provides a first step toward self‐consistent modeling of lithium PFCs in NSTX‐U, demonstrating the impact of drift‐driven plasma transport in SOL and divertor regions.