Closed divertor operation in ASDEX Upgrade and JET

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Abstract
Operation in ASDEX Upgrade and JET with different divertor geometries is compared with emphasis upon energy and particle exhaust and the impact of the divertor on core plasma performance. Closing the divertor increases the ratio of the neutral flux density in the divertor to that in the main chamber and facilitates improved pumping. The influence of closure on H-mode performance turns out to be weak, the main chamber physics dominates. The electron density/neutral hydrogen flux in the scrape-off layer is found to be a key parameter for particle and energy confinement as well as for the radiative losses. The density in the outer scrape-off layer is related to main chamber recycling, while the separatrix density is primarily affected by divertor geometry. Reduced target plate power load due to an increased divertor radiation level is obtained in ASDEX Upgrade Div II. The radiated power fraction in JET is systematically lower, which is partly explained by the lower edge densities. Core plasma Zeff and carbon concentrations are similar in both machines and do not depend on divertor geometry.