Saturation of magnetised plasma turbulence by propagating zonal flows

TL;DR

This paper introduces a new propagating zonal flow mode in tokamak plasmas that regulates ion-scale turbulence, leading to a threshold behavior that controls turbulence levels and matches experimental data.

Contribution

It identifies and characterizes a novel propagating zonal flow mode, the toroidal secondary mode, and derives scaling laws validated by gyrokinetic simulations and experiments.

Findings

Discovery of a propagating zonal flow mode regulating turbulence

Derivation of turbulence threshold conditions and scaling laws

Validation of theoretical predictions with simulations and experiments

Abstract

Strongly driven ion-scale turbulence in tokamak plasmas is shown to be regulated by a new propagating zonal flow mode, the toroidal secondary mode, which is nonlinearly supported by the turbulence. The mode grows and propagates due to the combined effects of zonal flow shearing and advection by the magnetic drift. Above a threshold in the turbulence level, small-scale toroidal secondary modes become unstable and shear apart turbulent eddies, forcing the turbulence level to remain near the threshold. This threshold condition is used to derive scaling laws for the turbulent heat flux, fluctuation spectra, and zonal flow amplitude, which are validated in nonlinear gyrokinetic simulations and explain previous experimental observations.