Contribution of the ion diamagnetic flow to poloidal rotation and pressure asymmetries in the SOL

Under typical conditions of high recycling divertor plasmas, the ion diamagnetic flow makes the largest contribution to the ion poloidal rotation in the SOL. Owing to toroidal effects, the net poloidal rotation in the SOL gives rise to ion parallel Pfirsch-Schlüter flows, which are sustained by an up-down pressure asymmetry which reverses with toroidal field (Bϕ) reversal. The up-down pressure asymmetry in turn gives rise to net, surface averaged, radial currents r arising from the surface averaging of local diamagnetic currents jr = (1/rB)(∂p/∂θ) due to toroidal effects. The radial divergence of radial currents r can exist in the SOL because of electrical contact with the target. The resulting net toroidal r Bθ force in the direction of the main plasma current creates an in-out pressure asymmetry in favour of the ion drift side (which is the inner side in normal Bϕ and the outer side in reversed Bϕ plasmas). This pressure asymmetry should result in higher density and low temperature plasma in the inner divertor leg in normal Bϕ discharges and should also make the distribution of density and temperature between the targets more equal in reversed Bϕ discharges, consistent with the main trends observed in experiments with toroidal field reversal. The proposed mechanism can therefore provide an alternative explanation (to the effect of the E × B drifts analysed earlier) for the observed changes in target asymmetries associated with Bϕ reversal.