Simple models for the radial and poloidal E × B drifts in the scrape-off layer of a divertor tokamak: Effects on in/out asymmetries

The influence of radial and poloidal E × B drifts in the scrape-off layer (SOL) of a divertor tokamak is considered. For B in the `forward' direction (the ∇B ion drift towards a lower X point, single null divertor configuration), the poloidal drift tends to increase the plasma pressure and the recycling rates at the outer target, lowering the temperature there - as compared with the inner target. When the radial drift dominates, these asymmetries are reversed. The radial drift is estimated to be more important than the poloidal drift for high ne operation (the `high recycling' regime). The poloidal drift case is analysed by using a simple, isothermal model and boundary conditions at the targets which allow for the effect of drift. The radial drift case is modelled on the basis of a heuristic analysis of the viscous drag experienced by the return parallel flow along the SOL which is induced by the radial drift fluxes. Poloidal drifts can result in substantially reduced plasma contact at one target (`detachment'); simultaneous detachment at both targets due to poloidal drifts appears to be less likely. Radial drifts could result in detachment at one or both targets, but the type of detachment involved does not appear to be consistent with that experimentally observed. The experimental observations made in the Tokamak de Varennes, with divertor biasing arranged to increase/decrease the radial electric field in the SOL, relative to its natural level, are broadly consistent with the poloidal drift model