Intrinsic rotation driven by turbulent acceleration

TL;DR

This paper investigates how turbulent acceleration along magnetic fields in tokamak plasmas can induce intrinsic rotation, revealing that this effect often results in minimal momentum transport due to underlying symmetries.

Contribution

The study implements turbulent acceleration in a gyrokinetic code and analyzes its impact on momentum transport, highlighting the role of symmetries in limiting intrinsic rotation.

Findings

Turbulent acceleration can induce intrinsic rotation in tokamak plasmas.

Momentum transport due to turbulent acceleration is generally small, comparable to ion Larmor radius effects.

Symmetries can suppress momentum transport when turbulence is strongly driven.

Abstract

Differential rotation is induced in tokamak plasmas when an underlying symmetry of the governing gyrokinetic-Maxwell system of equations is broken. One such symmetry-breaking mechanism is considered here: the turbulent acceleration of particles along the mean magnetic field. This effect, often referred to as the `parallel nonlinearity', has been implemented in the $\delta f$ gyrokinetic code $\texttt{stella}$ and used to study the dependence of turbulent momentum transport on the plasma size and on the strength of the turbulence drive. For JET-like parameters with a wide range of driving temperature gradients, the momentum transport induced by the inclusion of turbulent acceleration is similar to or smaller than the ratio of the ion Larmor radius to the plasma minor radius. This low level of momentum transport is explained by demonstrating an additional symmetry that prohibits momentum transport when the turbulence is driven far above marginal stability.