Dynamics of zonal flow-like structures in the edge of the TJ-II stellarator

TL;DR

This study investigates the behavior and mechanisms of zonal flow-like electric field structures at the edge of the TJ-II stellarator, analyzing their dynamics, possible driving forces, damping processes, and associated density modulations.

Contribution

It provides new insights into the drive and damping mechanisms of edge zonal flows in stellarators, highlighting the roles of Reynolds stress and neoclassical viscosity.

Findings

Turbulence-driven forces can provide necessary acceleration but causal relations are unclear.

Neoclassical viscosity and damping times match observed zonal flow relaxation times.

Density modulations are observed but do not align with pressure side-band excitation.

Abstract

The dynamics of fluctuating electric field structures in the edge of the TJ-II stellarator, that display zonal flow-like traits, is studied. These structures have been shown to be global and affect particle transport dynamically [J.A. Alonso et al., Nucl. Fus. 52 063010 (2012)]. In this article we discuss possible drive (Reynolds stress) and damping (Neoclassical viscosity, geodesic transfer) mechanisms for the associated ExB velocity. We show that: (a) while the observed turbulence-driven forces can provide the necessary perpendicular acceleration, a causal relation could not be firmly established, possibly because of the locality of the Reynolds stress measurements, (b) the calculated neoclassical viscosity and damping times are comparable to the observed zonal flow relaxation times, and (c) although an accompanying density modulation is observed to be associated to the zonal flow, it is not consistent with the excitation of pressure side-bands, like those present in geodesic acoustic oscillations, caused by the compression of the ExB flow field.