INTERPRETATION OF ASYMMETRIES IN TOKAMAK SCRAPE-OFF LAYERS IN TERMS OF POLOIDAL VARIATION OF ANOMALOUS TRANSPORT AND DIAMAGNETIC EFFECTS

Strong inhomogeneities in the plasma parameters of the scrape-off layer are reported from many tokamaks and are ascribed to such factors as variation of the connection lengths between limiting surfaces, poloidal dependence of the cross-field transport and of course the nonsymmetrical disposition of limiters and particle sources. In addition to these, the plasma flow associated with diamagnetic effects can make a significant contribution, which is often neglected in scrape-off layer models. Such flows arise from the fact that the electric field in the scrape-off layer is directed outwards, so that ion diamagnetic and E X B drifts are in the same direction and, together with the associated Pfirsch-Schluter flow, contribute to the flow pattern determined by plasma losses to limiting surfaces. The experimental evidence for such flows is discussed with reference to data from the DITE experiment. Probe measurements of ion-saturation current to the limiters, in discharges where the plasma current and toroidal field were alternately reversed, allowed various effects to be disentangled. The observed asymmetries in flow to the ion and electron drift sides of the limiter and the up/down asymmetry associated with the direction of delB drift are compared with the expected diamagnetic effects. New data on the HFS/LFS asymmetry associated with poloidal variation of the cross field transport is also presented.