Radiation transport effects in lithium divertors in NSTX-U.

Plans are developed to test lithium-based divertor power and particle handling solutions in the NSTX-U tokamak with incident heat fluxes up to 100 MW/m². The lithium divertors rely on radiative and collisional energy dissipation using high-density lithium vapor. At high lithium densities, lithium radiation trapping may lead to modifications in divertor radiated power and lithium charge state distribution. The magnitude and diagnostics of these effects are assessed for realistic NSTX-U plasma scenarios and divertor configurations using the multi-fluid transport code UEDGE, the dynamic surface response code Wall-Li, and the radiation transport code Cretin. At higher input powers (6-10 MW), NSTX-U lithium divertors enter steady-state vapor shielding regimes with high lithium densities and non-negligible opacities of ultraviolet and extreme ultraviolet emission lines.