Replicating measured upstream and high-recycling divertor conditions in tokamak edge fluid simulations

The previously observed tendency of plasma edge fluid codes (including SOLPS-ITER and EDGE2D-EIRENE, across multiple devices and plasma species) to underpredict the ion current to the low-field side (LFS) divertor by a factor of 2–3 in high-recycling L-mode plasmas is investigated and resolved by revising the imposed cross-field transport and radial boundary conditions. The combination of higher input power at the core boundary, increased decay length for the electron density at the radial outermost boundary, and reduced cross-field transport coefficients in the far scrape-off layer (SOL) enables code-experiment agreement on the upstream and LFS target plasma profiles in JET within experimental uncertainties. The original discrepancy is traced to divertor power starvation due to excessive power losses in the main chamber (MC) SOL. The revised EDGE2D-EIRENE simulations demonstrate that high-recycling conditions are replicated without modifications to the atomic/molecular data or plasma geometry extension to the first wall. Experimental validation using deuterium Balmer-α emission to infer the ratio of ionisation sources in the MC and divertor supports the revised boundary conditions. These results accentuate the importance of carefully constraining the boundary conditions and transport parameters in predictive edge fluid simulations, particularly for high-recycling and semi-detached SOL regimes.