Zero-turbulence manifold in a toroidal plasma

Sheared toroidal flows can cause bifurcations to zero-turbulent-transport states in tokamak plasmas. The maximum temperature gradients that can be reached are limited by subcritical turbulence driven by the parallel velocity gradient. Here it is shown that q/Ïμ (magnetic field pitch/inverse aspect ratio) is a critical control parameter for sheared tokamak turbulence. By reducing q/Ïμ, far higher temperature gradients can be achieved without triggering turbulence, in some instances comparable to those found experimentally in transport barriers. The zero-turbulence manifold is mapped out, in the zero-magnetic-shear limit, over the parameter space (γ_E, q/Ïμ, R/L_T), where γ_E is the perpendicular flow shear and R/L_T is the normalized inverse temperature gradient scale. The extent to which it can be constructed from linear theory is discussed.