Design window for open-surface lithium divertor with helium-cooled substrate

This study for an open-surface flowing lithium (Li) divertor with a thin helium-cooled substrate includes a scaling analysis, numerical computations and an analytical solution to access the design window for a low vaporization divertor option where the maximum temperature at the Li free surface is limited to 450 °C. Three heat removal regimes were identified in the scaling analysis and then accessed in numerical computations: convection-dominated, diffusion-dominated and convection-diffusion. Of them, convection-dominated regime, in which all applied heat flux is removed by a high velocity Li layer, offers the highest heat removal capability. For a generic divertor, a high surface heat flux of 10 MW/m² can be removed with the Li flowing at 7.3 m/s. Under the conditions of the US Fusion Nuclear Science Facility (FNSF), the required Li velocities are 6.5 m/s for the inner leg, and 15 m/s for the outer leg. The obtained analytical solutions for the diffusion-dominated regime, where the incident heat flux is removed by helium, suggests the lowest of the three regimes heat removal capability in the range of 0.5–1.2 MW/m².