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

Since spring of 1997 ASDEX Upgrade has operated with the new divertor II (L YRA), which is characterised by vertical target plates, tight baffling towa rds the main chamber, and a dome baffle in the private flux region. The mai n 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. Pumpin g occurs now from the private flux region which is linked to the pump chamb er 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 tu rbo 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 den sity 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 s crape-off layer flow of deuterium (by a factor of about 3), resulting in a small improvement of the He exhaust rate. (C) 1999 Elsevier Science B.V. Al l rights reserved.