Suppression of temperature-gradient-driven turbulence by sheared flows in fusion plasmas

TL;DR

This paper develops a phenomenological theory describing how perpendicular flow shear suppresses temperature-gradient-driven turbulence in fusion plasmas, identifying regimes and key parameters that influence turbulent transport.

Contribution

It introduces a detailed model distinguishing weak and strong shear regimes, highlighting the role of fluctuation aspect ratio in turbulence suppression, validated by numerical simulations.

Findings

Excellent agreement with fluid and gyrokinetic simulations

Identification of aspect ratio as a key parameter for turbulence suppression

Potential mechanism for electron-scale turbulence suppression by ion flows

Abstract

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 thermalised by particle collisions, we formulate a detailed phenomenological theory for the influence of perpendicular flow shear on magnetised-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.