Toroidicity in the tokamak SOL: Effects on poloidal asymmetries, radial current and the L-H transition

A simple isothermal model for the scrape-off layer (SOL) which includes toroidal geometry and radial electric field effects, but ignores viscosity and plasma-neutral interactions, is presented. For finite toroidicity epsilon = a/R and for the case of zero radial electric field E(r), the model predicts pressure asymmetry in favour of the outer target and a non-zero radial current j(r) due to pressure-gradient terms. For the case of non-zero E(r), j(r) is changed somewhat. The dependence of the current on the radial electric field, however, is too weak and non-monotonic against E(r) to explain experimental results on ion mobility obtained in biasing experiments on the tokamak de Varennes (TdeV). This may indicate that anomalous processes, not considered here, are dominant in TdeV. The direction of the pressure-gradient-driven current is inwards for the 'normal' toroidal field (ion del B drift towards the single null divertor), and outwards for the 'reversed' field. The current can cause significant changes in radial Bur in the SOL between the two field directions in the case when the SOL width approaches the ion poloidal Larmor radius. Its effect on the SOL is proposed as a mechanism responsible for the dependence on the direction of the toroidal field of the threshold power needed to cause the LH transition. The dependence of the current on the toroidicity [j(r) similar to epsilon(<alpha)> (alpha = 1.25-1.45)], found in the calculations, suggests that the beneficial effects of the 'normal' field direction on the confinement should be more pronounced at larger epsilon.