Generation of parasitic axial flow by drift wave turbulence with broken symmetry: Theory and experiment

TL;DR

This paper combines theory and experiments to show how drift wave turbulence with broken symmetry generates intrinsic axial flows in plasma, driven mainly by turbulence Reynolds stresses linked to density gradients.

Contribution

It provides the first detailed experimental validation of the theory linking drift-wave turbulence asymmetry to axial flow generation in plasma devices.

Findings

Axial flow is driven by turbulence Reynolds stress linked to density gradients.

Azimuthal flow shear regulates turbulence fluctuation levels.

Axial flow is parasitic and primarily driven by turbulence, not directly by energy exchange with azimuthal flow.

Abstract

Detailed measurements of intrinsic axial flow generation parallel to the magnetic field in the controlled shear decorrelation experiment linear plasma device with no axial momentum input are presented and compared to theory. The results show a causal link from the density gradient to drift-wave turbulence with broken spectral symmetry and development of the axial mean parallel flow. As the density gradient steepens, the axial and azimuthal Reynolds stresses increase and radially sheared azimuthal and axial mean flows develop. A turbulent axial momentum balance analysis shows that the axial Reynolds stress drives the radially sheared axial mean flow. The turbulent drive (Reynolds power) for the azimuthal flow is an order of magnitude greater than that for axial flow, suggesting that the turbulence fluctuation levels are set by azimuthal flow shear regulation. The direct energy exchange between axial and azimuthal mean flows is shown to be insignificant. Therefore, the axial flow is parasitic to the turbulence-zonal flow system and is driven primarily by the axial turbulent stress generated by that system. The non-diffusive, residual part of the axial Reynolds stress is found to be proportional to the density gradient and is formed due to dynamical asymmetry in the drift-wave turbulence.