The B2-Eirene code package was used to investigate the density limit in ASD
EX-Upgrade, both in the old, open divertor configuration and the new, more
closed configuration. A combination of time-dependent (directly simulating
the experimental scenario of raising the plasma density with an external ga
s puff) and time-independent runs for discrete densities were performed, wi
th carbon produced self-consistently via physical and chemical sputtering.
In addition some time-independent calculations were done without impurities
to further elucidate the physics. Good agreement was found with the experi
ment. In addition the role of flow reversal in the carbon contamination of
the main plasma arising from divertor generated sources was clarified. (C)
1999 Elsevier Science B.V. All rights reserved.