Dynamic transport regulation by zonal flow-like structures in the TJ-II stellarator

TL;DR

This paper demonstrates the detection of global, zonal-flow-like structures in the TJ-II stellarator and shows their role in modulating turbulence and particle transport, providing new insights into plasma behavior.

Contribution

It introduces a novel SVD-based method to extract and analyze global floating potential structures and their impact on transport in stellarators.

Findings

Global structures modulate turbulence spectral power

Transport into the Scraped-Off Layer is influenced by these structures

First direct evidence of transport modulation by zonal flow-like structures

Abstract

Floating potential structures that are correlated over a long distance are observed with a 2D probe array in the plasma edge of the TJ-II stellarator. We introduce a method based on the Singular Value Decomposition to extract the spatio-temporal structure of the global, fluctuating, zonal-flow-like floating potential from the combined measurements of a 2D probe array and a distant single probe. The amplitude of these global structures is seen to modulate not only the high k-poloidal spectral power of the local turbulence, but also particle transport into the unconfined Scraped-Off Layer, as observed by H-alpha monitors around the device. These observations provide the first direct evidence of the global modulation of transport by zonal flow-like structures. The ability to identify spontaneous and collective rotation events with flux surface symmetry opens up the possibility to perform unperturbative studies of the effective viscosity in stellarators and tokamaks.