Nonlinear generation of global zonal structures in gyrokinetic simulations of TCV and ASDEX Upgrade magnetic configurations

TL;DR

This paper demonstrates that global zonal structures in tokamak turbulence are non-linearly generated by high-n turbulence modes, using gyrokinetic simulations of TCV and ASDEX Upgrade configurations, enhancing understanding of turbulence saturation.

Contribution

It reveals the nonlinear generation mechanism of global zonal structures in gyrokinetic simulations, highlighting the role of high-n turbulence spectrum in tokamak plasma behavior.

Findings

Global ZSs are non-linearly generated by high-n turbulence.

Simulation with an antenna reproduces ZS generation mechanism.

Radially extended coherent ZSs are observed in simulations.

Abstract

In tokamaks, turbulence is responsible not only for the anomalous transport of heat and particles from the core to the edge, which reduces heating efficiency, but also for the generation of zonal structures (ZSs). Examples of ZSs are those with characteristic sound frequency, like the geodesic acoustic modes (GAMs). Developing a theoretical model of ZS is essential, as they contribute to the turbulence saturation and thus indirectly influence transport. In this paper, we investigate the radial structure of ZS in the frequency range of GAMs, by means of numerical simulations using the gyrokinetic particle-in-cell code ORB5. We find that radially extended coherent ZSs (labelled here as global ZSs) are non-linearly generated by the high-n part of the turbulence spectrum (with n being the toroidal mode number) by means of self-consistent simulations. We also reproduce this generation by mimicking the turbulence modes with an antenna, thus isolating the nonlinear generation mechanism.