Noble gas exhaust with a strongly baffled divertor in ASDEX-Upgrade

Abstract

Since spring of 1997 ASDEX Upgrade has operated with the new divertor II (LYRA), which is characterised by vertical target plates, tight baffling towards the main chamber, and a dome baffle in the private flux region. The main goal of this divertor modification was to increase the divertor density (of the plasma and of the neutrals) for a given plasma density, whereas the neutral density in the main chamber should be similar or even lower. Pumping occurs now from the private flux region which is linked to the pump chamber by finite conductance below the outer target. A cryo pump (about 100 m 3/s) has been installed in the outer divertor chamber in addition to the turbo pumps. This divertor scheme is very similar to the one planned for ITER. First experiments with the new divertor indicate that the compression of helium, C He (the ratio of neutral He density in the divertor to the He density at the plasma edge) is much higher in the new divertor, while the neon exhaust efficiency has decreased. The cryo pump can strongly enhance the scrape-off layer flow of deuterium (by a factor of about 3), resulting in a small improvement of the He exhaust rate.