Radiation transport effects in lithium divertors.

Lithium vapor properties are key in lithium-based tokamak divertor concepts, such as the vapor box divertor and the vapor-shielding open divertor. Some of these concepts are to be tested in NSTX-U with divertor power densities up to tens of MW/m$^2$. Effects of deuterium and lithium line radiation trapping in divertor plasmas with high lithium density are studied with the radiation transport code CRETIN and the multi-fluid transport code UEDGE. Lithium atomic data from the FAC code is used. Parametric plasma temperature and density scans in deuterium and lithium plasma slabs and boxes with lithium densities $10^{18}-10^{22}$ m^${-3}$ are used to study the Li I, Li II, and Li III emission line opacities, broadband spectra, and total radiation fluxes.  A UEDGE based divertor plasma model with an NSTX lower single null divertor configuration is used with CRETIN calculations to study  lithium opacity effects: modified radiated power and radiation flux asymmetries, shifted lithium ionization balance and redistribution of lithium in the divertor.