High-temperature, liquid lithium plasma-facing component research for NSTX-U and next-step devices

Liquid metals offer significant potential advantages as plasma-facing component materials including the elimination of net-reshaping due to plasma erosion. Large erosive fluxes may also result in a ``vapor-shielded'' regime to further reduce the incident heat flux. Engineering design studies indicate that near-term cooling technologies will enable cooling of liquid lithium PFC surfaces to temperatures of 700-900C with $10 MW/m^2$ incident where the lithium vapor pressure is comparable to scrape-off layer plasma pressures. Experiments conducted on the Magnum-PSI linear plasma device provide the first data in a quasi-steady, divertor-like plasma ($T_e$~2eV, $N_e~2\times10^{20}m^{-3}$). Lithium layers tested in the plasma indicate very high redeposition fractions exist near the target surface with lifetimes of 3-4s. These experiments provide the first feasibility assessments of a continuously-vapor shielded regime for use in the NSTX-U.