Parallel and perpendicular turbulence correlation length in the TJ-II Stellarator

TL;DR

This study measures long-range plasma correlations in the TJ-II stellarator, revealing how magnetic configuration and density influence drift wave and zonal flow contributions to turbulence.

Contribution

It introduces a combined model of drift wave and zonal flow correlations, validated by experimental data across different densities in the TJ-II stellarator.

Findings

Correlation lengths vary with magnetic configuration.

A critical density triggers additional zonal flow correlations.

Total correlation is a sum of drift wave and zonal flow effects.

Abstract

Long range correlations were measured using two remote reciprocating Langmuir probe systems at TJ-II. The influence of the rotational transform on the correlation was studied by scanning the magnetic configuration. A simple drift wave correlation model, assuming an exponential decay of the correlation with different correlation lengths in the directions parallel and perpendicular to the field lines, was found to describe the observations well at low densities. The experiment was repeated at gradually higher densities, and an additional correlation was detected at a critical value of the density. In accordance with previous work, this additional correlation was ascribed to zonal flows associated with a confinement transition. Thus, the total long range correlation is found to be a sum of the drift wave and zonal flow contributions.