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

Abstract

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 Tipt over plasma current Ip, 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 Tipt 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. © 2012 American Institute of Physics.