Modeling of plasma/lithium-surface interactions in NSTX: Status and key issues

We are studying lithium sputtering, evaporation, transport, material mixing, and surface evolution for the National Spherical Torus Experiment (NSTX) for various surfaces and plasma conditions. Lithium modeling is complex, particularly for NSTX short pulse, multiple material, variable plasma conditions. Cases examined include: (1) liquid lithium divertor (LLD) with planned high heating power/low-D-recycle plasma, (2) non-pumping/high-recycle solid or liquid divertor surface, (3) Li and C impingement on a molybdenum surface. An impurity erosion/redeposition code package is the overall integration tool, with sputter yield and velocity distributions from binary collision mixed-material codes, sheath code input for NSTX boundary conditions, and inputs of plasma edge solutions from external data-calibrated plasma fluid codes. Analysis predictions are generally favorable, showing non-runaway lithium self-sputtering, acceptable net erosion (∼5 nm/s), and moderate edge (∼10%) and core plasma (∼0.1–1%) Li contamination, for the cases studied. A Mo divertor surface is significantly affected by C and Li impingement but with low core plasma contamination predicted for a high-recycle edge plasma.