It is shown that the poloidal momentum balance equation in tokamaks has bifurcated solutions. The poloidal flow velocity $U_p$ can suddenly become more positive when the ion collisionality decreases. The corresponding radial electric field $E_r$ becomes more negative and hence suppresses the turbulent fluctuations. Thus, plasma confinement is improved. The theory is employed to explain the L-H transition observed in tokamaks.

• Received 13 July 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.2369