Investigation of scrape-off layer up-down asymmetries in diverted plasmas in TEXT-Upgrade

Abstract

In equatorial single null diverted discharges in TEXT-Upgrade, up-down asymmetries have been noted, the sign and magnitude of which depend on plasma conditions such as electron density and X point position. Geometrical effects do not play a role in these plasmas, which are topologically up-down symmetric, so the asymmetry must come from physical driving mechanisms. Several such mechanisms will be investigated, both computationally and experimentally, and compared with data from tokamak discharges. An analysis is included of the E × B , diamagnetic and ∇ T × B drifts as well as plasma rotation. The B2-EIRENE plasma simulation code is used to evaluate the respective influences of the ∇ T × B driving terms. Scans of the X point position relative to the divertor plates and of the electron density are performed. The X point is brought in all the way to the divertor plates so as to assess the importance of the private region in the drift flow patterns and to break the flow loops. The low recycling regime typical of TEXT-Upgrade single null plasmas allows separation and identification of the influence of individual flows. It is concluded that E × B drift is the dominant effect and that it qualitatively explains the reported behaviour.