Neutral pressure and separatrix density related models for seed impurity divertor radiation in ASDEX Upgrade

Analytical expressions for the divertor radiation around the onset of detachment based on the divertor neutral pressure and the upstream separatrix density are tested against measurements on strongly nitrogen seeded high confinement mode (H-mode) discharges in ASDEX Upgrade. The divertor pressure is used as a key experimental parameter as far as possible, since it can be regarded as engineering parameter based on the balance of puffing and pumping. For a tentative extrapolation to future tokamak power plants, limits set by the maximum core impurity concentration via the burn conditions are related to the divertor impurity content via the divertor enrichment factor. Combined with the H-mode density limit, which is most likely a limit for the separatrix density nesep/nGreenwald ≤ 0.5, estimates for the maximum separatrix power Psep compliant with the achievement of detachment are presented for a density related 2-point model and a flux related radiation model. The detachment, which is assumed necessary for the achievement of sufficiently long target lifetimes, requires in H-mode the dissipation of a major fraction of the power flowing to the divertor by impurity radiation. Favorable conditions for power exhaust feature a high Greenwald density / high divertor neutral pressure. The latter also facilitates a faster helium exhaust, which increases the margin for the seed impurity concentration in the core. Major uncertainties in the extrapolation of seeded scenarios regard the divertor impurity enrichment and the power width λ in the divertor for detached conditions.