  Starting from the assumption that saturation of plasma turbulence driven by
temperature-gradient instabilities in fusion plasmas is achieved by a local
energy cascade between a long-wavelength outer scale, where energy is injected
into the fluctuations, and a small-wavelength dissipation scale, where
fluctuation energy is thermalized by particle collisions, we formulate a
detailed phenomenological theory for the influence of perpendicular flow shear
on magnetized-plasma turbulence. Our theory introduces two distinct regimes,
called the weak-shear and strong-shear regimes, each with its own set of
scaling laws for the scale and amplitude of the fluctuations and for the level
of turbulent heat transport. We discover that the ratio of the typical radial
and poloidal wavenumbers of the fluctuations (i.e., their aspect ratio) at the
outer scale plays a central role in determining the dependence of the turbulent
transport on the imposed flow shear. Our theoretical predictions are found to
be in excellent agreement with numerical simulations of two paradigmatic models
of fusion-relevant plasma turbulence: (i) an electrostatic fluid model of slab
electron-scale turbulence, and (ii) Cyclone-base-case gyrokinetic ion-scale
turbulence. Additionally, our theory envisions a potential mechanism for the
suppression of electron-scale turbulence by perpendicular ion-scale flows based
on the role of the aforementioned aspect ratio of the electron-scale
fluctuations.
