Theory of Enhanced Reversed Shear Mode in Tokamaks

It is shown that toroidal-magnetic-field ripple-induced particle flux can drive poloidal $\mathit{E}\times \mathit{B}$ speed to bifurcate over the local maximum of the nonlinear poloidal (or parallel) viscosity. Here, $\mathit{E}$ is the electric field, and $\mathit{B}$ is the magnetic field. This mechanism, together with the turbulence suppression due to the radial gradient of the $\mathit{E}\times \mathit{B}$ and diamagnetic angular velocity, is employed to explain the enhanced reversed shear mode observed in the core region of tokamaks.