Zonal flows and long-distance correlations during the formation of the edge shear layer in the TJ-II stellarator

TL;DR

This paper provides a theoretical interpretation of long-distance potential fluctuation correlations in TJ-II stellarator edge shear layer formation, emphasizing the role of zonal flows in the transition.

Contribution

It introduces a model with four coupled envelope equations demonstrating the essential role of zonal flows in plasma edge shear layer formation.

Findings

Correlation increases above the transition point

Mean sheared flows alone cannot explain observations

Zonal flows are crucial for reproducing experimental features

Abstract

A theoretical interpretation is given for the observed long-distance correlations in potential fluctuations in TJ-II. The value of the correlation increases above the critical point of the transition for the emergence of the plasma edge shear flow layer. Mean (i.e. surface averaged, zero-frequency) sheared flows cannot account for the experimental results. A model consisting of four envelope equations for the fluctuation level, the mean flow shear, the zonal flow amplitude shear, and the averaged pressure gradient is proposed. It is shown that the presence of zonal flows is essential to reproduce the main features of the experimental observations.