Interplay between toroidal rotation and flow shear in turbulence stabilisation

The interplay between toroidal rotation u, parallel flow shear u′, and perpendicular flow shear γ_E in the stabilisation of tokamak turbulence is investigated in non-linear flux-tube gyrokinetic simulations. The simulations are performed for a reference L-mode DIII-D plasma (the so-called shortfall case) at $r/a=0.8$ , varying the flow parameters around their nominal values. Depending on the respective signs of u, u′, and γ_E, turbulence is found to be enhanced, reduced, or unchanged. When the coupling is favorable, the overall effect on the non-linear heat fluxes can be very large, even at moderate flow values. The ion heat flux is, for instance, decreased by a factor of 3 when the direction of the parallel flow shear is reversed with respect to its nominal value. Even more surprising, keeping u′ and γ_E at their nominal values, the ion heat flux decreases by more than 50% when the toroidal flow is reversed. The relevance of this mechanism in the experiments which depends on the ability to decouple u, u′, and γ_E is discussed. The interplay between u and u′ observed in the non-linear simulations qualitatively follows the linear stability results and is interpreted in the frame of a simple fluid model.