Destabilization of temperature-gradient-driven plasma turbulence by equilibrium $\vec{E}\times \vec{B}$ flow shear

Equilibrium sheared $\vec{E}\times \vec{B}$ flow, a standard cure for plasma turbulence, can backfire. In gyrokinetic simulations of a newly identified regime, imposed shear comparable to the intrinsic zonal shear destroys the self-generated zonal flows regulating the turbulence: transport rises sharply before stronger shear quenches it. A reduced fluid model traces this to the incompatibility of imposed and zonal shear layers. Simulations of spherical tokamak discharges place the inferred rotation shear at, or just below, the threshold of the sharp transport increase, implying that the toroidal rotation may be limited mainly by the heat, not momentum, injection.