Tokamak-edge toroidal rotation due to inhomogeneous transport and geodesic curvature

In a model kinetic ion transport equation for the pedestal and scrape-off layer, passing-ion drift orbit excursions interact with spatially inhomogeneous but purely diffusive transport to cause the orbit-averaged diffusivities to depend on the sign of ν_∥, preferentially transporting counter-current ions for realistic parameter values. The resulting pedestal-top intrinsic rotation is typically co-current, reaches experimentally relevant values, and is proportional to pedestal-top ion temperature T_ipt over plasma current I_p, as observed in experiment. The rotation drive is independent of the toroidal velocity and its radial gradient, representing a residual stress. Co-current spin-up at the L-H transition is expected due to increasing T_ipt and a steepening of the turbulence intensity gradient. A more inboard (outboard) X-point leads to additional co- (counter-) current rotation drive. Beyond intrinsic rotation, comparison of heat and momentum transport reveals that neutral beam injection must be significantly unbalanced in the counter-current direction to cause zero toroidal rotation at the pedestal top.