Measurement and simulation of edge flows induced by ergodization in Tore Supra

Test particle simulations are used to model edge flows in the ergodic divertor of Tore Supra. The field line tracing code MASTOC is used to calculate the full 3D magnetic geometry. The model assumes ballistic guiding center motion along the magnetic field lines with cross-field transport represented by a random walk process. Large flows of alternating direction are found in the laminar zone. The flow pattern displays the m=18 poloidal mode number imposed by the divertor coils. Some of the flow is due to magnetic connections to neutralizer plates, while some is due to preferential channeling of particle flux from the ergodic layer. The measured radial extent of the flow pattern and the appearance of flow reversal at the Gundestrup probe location are well described by the simulations and is simply related to the connection length map. The flows should influence impurity transport via the drag force: we present preliminary test particle simulations of impurity ion transport within the 3D flow patterns. Divertor operation leads to an improvement of impurity screening with respect to the limiter case if the impurities are ionized within the laminar zone.