Intrinsic rotation with gyrokinetic models

TL;DR

This paper investigates how intrinsic rotation in tokamaks can be modeled using gyrokinetic equations, emphasizing the importance of higher-order effects to accurately capture momentum transport.

Contribution

It highlights the necessity of including small, higher-order effects in gyrokinetic models to properly describe intrinsic rotation phenomena in tokamaks.

Findings

Lowest order gyrokinetic models cannot generate intrinsic rotation.

Higher-order effects are crucial for accurate momentum flux predictions.

Symmetry in lowest order models cancels out momentum flux.

Abstract

The generation of intrinsic rotation by turbulence and neoclassical effects in tokamaks is considered. To obtain the complex dependences observed in experiments, it is necessary to have a model of the radial flux of momentum that redistributes the momentum within the tokamak in the absence of a preexisting velocity. When the lowest order gyrokinetic formulation is used, a symmetry of the model precludes this possibility, making small effects in the gyroradius over scale length expansion necessary. These effects that are usually small become important for momentum transport because the symmetry of the lowest order gyrokinetic formulation leads to the cancellation of the lowest order momentum flux. The accuracy to which the gyrokinetic equation needs to be obtained to retain all the physically relevant effects is discussed.