Theory of shear flow effects on long-wavelength drift wave turbulence

A simple, paradigmatic model of long-wavelength drift wave turbulence in the presence of a sheared poloidal flow is analyzed in detail. Linear theory predicts that velocity shear induces a strong stabilizing effect by shifting the eigenmode away from the k⋅B=0 resonant surface, thereby enhancing ion damping. However, multiple-helicity numerical calculations indicate that velocity shear has little or no effect on saturated fluctuation levels. Analysis suggests that this result is related to the incidence of a spiky, radially intermittent profile of the turbulent fluctuation levels, induced by low-q mode rational surfaces, and occurs when the turbulent diffusivity exceeds the product of diamagnetic frequency and gyroradius squared. An analytical theory that explains the observed suppression of velocity and magnetic shear damping is presented.