Symmetry breaking in MAST plasma turbulence due to toroidal flow shear

TL;DR

This paper investigates how toroidal flow shear in MAST tokamak plasmas breaks turbulence symmetry, leading to tilted correlation functions and skewed density distributions, with effects varying based on proximity to stability thresholds.

Contribution

It demonstrates experimentally and through simulations that flow shear causes symmetry breaking in plasma turbulence, affecting structure tilt and skewness.

Findings

Flow shear induces tilt in spatial correlation functions.

Flow shear causes skewness in density fluctuation distributions.

Symmetry breaking effects diminish away from the stability threshold.

Abstract

The flow shear associated with the differential toroidal rotation of tokamak plasmas breaks an underlying symmetry of the turbulent fluctuations imposed by the up-down symmetry of the magnetic equilibrium. Using experimental Beam-Emission-Spectroscopy (BES) measurements and gyrokinetic simulations, this symmetry breaking in ion-scale turbulence in MAST is shown to manifest itself as a tilt of the spatial correlation function and a finite skew in the distribution of the fluctuating density field. The tilt is a statistical expression of the "shearing" of the turbulent structures by the mean flow. The skewness of the distribution is related to the emergence of long-lived density structures in sheared, near-marginal plasma turbulence. The extent to which these effects are pronounced is argued (with the aid of the simulations) to depend on the distance from the nonlinear stability threshold. Away from the threshold, the symmetry is effectively restored.