Turbulent transport in tokamak plasmas with rotational shear

TL;DR

This study uses nonlinear gyrokinetic simulations to explore how rotational shear affects turbulent transport in tokamak plasmas, revealing optimal shear conditions for confinement and insights into turbulence behavior.

Contribution

It demonstrates the suppression of linear instabilities by flow shear and identifies the existence of subcritical turbulence and bifurcations in plasma transport.

Findings

Identification of a local minimum in heat flux at optimal shear.

Observation of maxima in momentum fluxes indicating possible bifurcations.

Turbulent Prandtl number remains near unity regardless of gradients.

Abstract

Nonlinear gyrokinetic simulations have been conducted to investigate turbulent transport in tokamak plasmas with rotational shear. At sufficiently large flow shears, linear instabilities are suppressed, but transiently growing modes drive subcritical turbulence whose amplitude increases with flow shear. This leads to a local minimum in the heat flux, indicating an optimal E x B shear value for plasma confinement. Local maxima in the momentum fluxes are also observed, allowing for the possibility of bifurcations in the E x B shear. The sensitive dependence of heat flux on temperature gradient is relaxed for large flow shear values, with the critical temperature gradient increasing at lower flow shear values. The turbulent Prandtl number is found to be largely independent of temperature and flow gradients, with a value close to unity.