Formation of the shear layer in toroidal edge plasma

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Abstract
The equilibrium and stability of axisymmetric toroidal edge plasma in the presence of an anomalous ballooninglike transport is studied. The equilibrium density evolves so that the transport rate, D⊥/Ln2 equals the sound transit time, cs/πqR. As a result, confinement is coupled to the local sound velocity such that a reduced sound velocity (larger ion mass) leads to improved confinement. A result of the large asymmetric transport is the generation of nearly sonic parallel flows which allow the system to be unstable to the Stringer spin‐up instability. For weak magnetic pumping, a large poloidal rotation of order the poloidal sound speed (acs/qR) is driven, forming a localized shear layer. The direction of rotation corresponds to a negative radial electric field in the closed flux region. For strong magnetic pumping the rotation is equal in magnitude and opposite in direction to the ion diamagnetic drift, the neoclassical result. Particle sources localized in poloidal angle can strongly impact the rotation. A source placed between the bottom and the outside of the torus enhances the rotation while the same source placed between the top and the inside retards the rotation. However, a large enough source at the latter location will drive the rotation in the other direction, reversing the sign of the electric field. The implications of the results for understanding Ohmic, L‐mode, and H‐mode discharges are discussed.