Turbulent momentum transport due to the beating between different tokamak flux surface shaping effects

TL;DR

This paper investigates how up-down asymmetric shaping effects in tokamaks influence intrinsic momentum transport, revealing that certain envelope structures can significantly enhance momentum flux scaling from exponential to polynomial decay.

Contribution

It demonstrates that the presence of an up-down asymmetric envelope created by high-order shaping effects can substantially increase intrinsic momentum transport in tokamaks.

Findings

Envelope effects can change momentum flux scaling from exponential to polynomial.

Up-down asymmetric envelopes with spatial scale comparable to minor radius enhance momentum transport.

Scaling of momentum flux depends on the symmetry and scale of shaping effects.

Abstract

Introducing up-down asymmetry into the tokamak magnetic equilibria appears to be a feasible method to drive fast intrinsic toroidal rotation in future large devices. In this paper we investigate how the intrinsic momentum transport generated by up-down asymmetric shaping scales with the mode number of the shaping effects. Making use the gyrokinetic tilting symmetry (Ball et al (2016) Plasma Phys. Control. Fusion 58 045023), we study the effect of envelopes created by the beating of different high-order shaping effects. This reveals that the presence of an envelope can change the scaling of the momentum flux from exponentially small in the limit of large shaping mode number to just polynomially small. This enhancement of the momentum transport requires the envelope to be both up-down asymmetric and have a spatial scale on the order of the minor radius.