Poloidal flows and enhanced reverse shear bifurcation in tokamaks

Abstract

The effect of poloidal flow shear on transition to enhanced reverse shear (ERS) mode in tokamaks is studied. A model is examined where the flow-fluctuation equilibrium evolves in time as a function of pressure gradients. Under some conditions the transition to the ERS phase occurs via as ss-fold catastrophe and is accompanied by a spike in poloidal flow generation where poloidal flow shear is reversed. The nature of the transition is found to depend critically on the ratio of factors controlling the damping and generation of poloidal flow shear. The turbulence level shows a periodic bursting behavior which is suppressed in the ERS phase. The relevance of these results to recent observations from the Tokamak Fusion Test Reactor [R. E. Bell et al., Phys. Rev. Lett. 81, 1429 (1998)] is briefly discussed.