Potential Vorticity Transport in Weakly and Strongly Magnetized Plasmas

TL;DR

This paper develops a new mean field theory to understand how tangled magnetic fields influence potential vorticity mixing, turbulence, and momentum transport in magnetized plasmas, with implications for phenomena like the L-H transition.

Contribution

It introduces a novel mean field theory for potential vorticity mixing in magnetized plasmas, highlighting the impact of stochastic magnetic fields on turbulence and flow dynamics.

Findings

Stochastic magnetic fields reduce Reynolds stress coherence.

Potential vorticity flux decoherence suppresses momentum transport.

A dimensionless parameter quantifies the impact on the L-H transition.

Abstract

Tangled magnetic fields, often coexisting with an ordered mean field, have a major impact on turbulence and momentum transport in many plasmas, including those found in the solar tachocline and magnetic confinement devices. We present a novel mean field theory of potential vorticity mixing in $\beta$-plane magnetohydrodynamic (MHD) and drift wave turbulence. Our results show that mean-square stochastic fields strongly reduce Reynolds stress coherence. This decoherence of potential vorticity flux due to stochastic field scattering leads to suppression of momentum transport and zonal flow formation. A simple calculation suggests that the breaking of the shear-eddy tilting feedback loop by stochastic fields is the key underlying physics mechanism. A dimensionless parameter that quantifies the increment in power threshold is identified and used to assess the impact of stochastic field on the L-H transition. We discuss a model of stochastic fields as a resisto-elastic network.