BIFURCATION OF POLOIDAL ROTATION AND SUPPRESSION OF TURBULENT FLUCTUATIONS - A MODEL FOR THE L-H TRANSITION IN TOKAMAKS

The poloidal momentum balance equation in tokamaks is shown to have bifurcated solutions; the poloidal flow velocity Up can suddenly become more positive when the ion collisionality decreases. The corresponding radial electric field Er becomes more negative, suppresses turbulent fluctuations, and improves plasma confinement. A heuristic argument is employed to illustrate the effects of Er on turbulent fluctuations. A more negative value of Er and/or a more positive value of dE r/dr can suppress the fluctuation amplitudes, if dP/dr<0 (with r the local minor radius and P the plasma pressure). The theory is employed to explain the L-H transition observed in tokamaks. © 1990 American Institute of Physics.