Poloidal rotation driven by nonlinear momentum transport in strong electrostatic turbulence

TL;DR

This paper calculates the nonlinear poloidal momentum flux in strong electrostatic turbulence, revealing its significance in driving poloidal rotation and its potential impact on transport barrier formation, especially at the plasma edge.

Contribution

It introduces a method to compute nonlinear poloidal momentum flux using the Hasegawa-Mima equation and highlights its importance over quasilinear theory in strong turbulence regimes.

Findings

Nonlinear momentum flux is comparable to quasilinear Reynolds stress.

Symmetry breaking in fluctuation spectrum is not required for nonlinear flux.

Nonlinear poloidal momentum transport is significant for plasma rotation and transport barriers.

Abstract

Virtually, all existing theoretical works on turbulent poloidal momentum transport are based on quasilinear theory. Nonlinear poloidal momentum flux - $\langle \tilde{v}_r \tilde{n} \tilde{v}_{\theta} \rangle$ is universally neglected. However, in the strong turbulence regime where relative fluctuation amplitude is no longer small, quasilinear theory is invalid. This is true at the all-important plasma edge. In this work, nonlinear poloidal momentum flux $ \langle \tilde{v}_r \tilde{n} \tilde{v}_{\theta} \rangle $ in strong electrostatic turbulence is calculated using Hasegawa-Mima equation, and is compared with quasilinear poloidal Reynolds stress. A novel property is that symmetry breaking in fluctuation spectrum is not necessary for a nonlinear poloidal momentum flux. This is fundamentally different from the quasilinear Reynold stress. Furthermore, the comparison implies that the poloidal rotation drive from the radial gradient of nonlinear momentum flux is comparable to that from the quasilinear Reynolds force. Nonlinear poloidal momentum transport in strong electrostatic turbulence is thus not negligible for poloidal rotation drive, and so may be significant to transport barrier formation.