Impurity radiation in DEMO systems modelling

• Solving the exhaust problem is crucial for DEMO.
• Here, we discuss the new impurity radiation model in the systems code PROCESS.
• Furthermore, we assess its effect on DEMO design.
• More appropriate scalings will significantly enhance predictions for DEMO.
• The controllability of highly radiative scenarios remains to be shown.

For fusion reactors with ITER divertor technology, it will be imperative to significantly reduce the heat flux into the divertor e.g. by seeded impurity radiation. This has to be done without affecting the accessibility of a high performance scenario. To assess the implications of seeded plasma impurities on DEMO design, we have developed an impurity radiation model for radiation inside the separatrix. Evaluating the validity of our model, we find the assumption of a local ionisation equilibrium to be appropriate for our purposes and the assumption of flat impurity profiles – even though not satisfactory – to represent the best currently possible. Benchmarking our model against other codes highlights the need to use up to date atomic loss function data.From the impurity radiation perspective, the main uncertainties in current DEMO design stem from the lack of confinement and L-H-threshold scalings that can be robustly extrapolated to highly radiative DEMO scenarios as well as the lack of appropriate models for the power flow from the separatrix into the divertor that include radiation in the scrape off layer. Despite these uncertainties in the model we can exclude that significant fuel dilution through seeded impurities (with Z ≥ ZAr) will be an issue for DEMO, but the controllability of highly radiative scenarios still needs to be coherently shown.