SPONTANEOUS POLOIDAL SPIN-UP OF TOKAMAKS AND THE TRANSITION TO H-MODE

The radial transport of toroidal angular momentum and circulation in a tokamak resulting from diffusion which is poloidally asymmetric is shown to produce an instability of the poloidal rotation. This instability, due to Stringer, sets in where the local particle-confinement time is smaller than the damping time of poloidal flow and leads to poloidal velocity shear. The nonlinear interplay between the poloidal spin-up and turbulence-driven anomalous transport is shown to lead to bifurcated equilibria of the type observed in the L-to-H-mode transition in tokamaks.