Turbulence saturation via fine-scale profile shearing in fusion plasmas

Microturbulence can produce stationary fine-scale radial corrugations on the plasma density and temperature gradients in magnetic confinement fusion devices. We study the effect of these corrugations, focussing on electron temperature gradient (ETG) transport in the tokamak pedestal, and report three main findings. 1) In the presence of a sinusoidal background temperature gradient corrugation, each ETG mode splits into three distinct eigenvalues, with one being the original, one being more unstable and one being less unstable. 2) Despite the presence of more unstable linear modes, nonlinear gyrokinetic simulations show a significant reduction in fluxes. 3) Profile shearing associated with the fine-scale background corrugations is identified as the saturation mechanism explaining the reduction in fluxes. It originates from the radial variation of the mode’s own phase velocity (proportional to the local diamagnetic drift velocity and the pressure gradient), and not from externally generated flows or E × B zonal flows. Fine-scale profile shearing could be a ubiquitous turbulence saturation mechanism in fusion plasmas.