First transport study of Li and Sn for COMPASS Upgrade tokamak

Abstract One of the principal research objectives of the COMPASS Upgrade (COMPASS-U) tokamak under construction in Prague is the investigation of reliable long-term plasma power exhaust solutions for future fusion reactors. Owing to its unique operational parameters, including magnetic field of 5 T, plasma current of 2 MA, divertor plasma densities exceeding 10 21 m -3 , and a tungsten first wall, COMPASS-U is expected to generate divertor surface-normal heat fluxes of several tens of MW/m 2 , relevant to future DEMO-class reactors, in 1–10 s discharges. Prototype liquid metal (LM) divertor tiles based on lithium (Li), tin (Sn), and their alloys will be experimentally tested. To support these experiments and enable extrapolation to larger devices, transport of sputtered and vaporized LMs was simulated using the ERO2.0 code. LM erosion was evaluated with HeatLMD, LM transport inside the separatrix with Aurora, and the stationary plasma background was obtained from narrow-grid SOLPS-ITER simulations complemented by METIS. For two COMPASS-U operational scenarios, the high-performance case #5400 and the ITER-like case #24300, the upper limits of acceptable confined-plasma contamination by LMs in the fully toroidal lower outer LM divertor case were assessed. The corresponding volume-averaged impurity concentrations were estimated as 14; 21 % (meaning 14 % and 21 % for scenarios #5400 and #24300, respectively) for Li, and 10 −5 ; 10 −4 % for Sn. The associated radiation losses from the confined plasma were estimated as 5; 9 % of the input power for Li, while remaining negligible for Sn. These estimates do not include the effects of edge-localized modes (ELMs), which will be investigated in future work. They also neglect Sn spitting, which may increase the risk of plasma disruptions unless Sn droplet transport is sufficiently suppressed by divertor design, and the possibility of runaway Li self-sputtering under conditions of thermally enhanced erosion yield exceeding unity combined with strong redeposition.