Effects of orbit squeezing on neoclassical toroidal plasma viscosity in tokamaks

The effects of orbit squeezing are important to neoclassical and anomalous transport fluxes in the region where plasma confinement is improved. This occurs in the edge region after the transition from the low confinement mode (L-mode) to the high confinement mode (H-mode) or in the vicinity of low-order rational surfaces. Neoclassical toroidal viscosity in tokamaks induced by the broken toroidal symmetry resulting from the activity of magnetohydrodynamic instabilities or error fields is calculated to include orbit squeezing effects. It is found that in the 1/ν regime, the magnitude of the neoclassical toroidal viscosity is enhanced by a factor of |S|^3/2, where ν is the collision frequency and S is the orbit squeezing factor; while in the ν regime, it is reduced by a factor of |S|^1/2. A boundary layer analysis is performed to remove the singularity in the vicinity of the trapped-circulating boundary in the ν regime. As a result, the well-known ν regime is recovered. Orbit squeezing has little effect in the ν regime. The implications on the physics of H-mode confinement are also discussed.