Parasitic Momentum Flux in the Tokamak Core

TL;DR

This paper identifies a geometrical correction to E x B drift that causes outward momentum flux in tokamak cores, explaining rotation peaking and reversals without relying on specific eigenmode assumptions.

Contribution

It introduces a new geometrical correction mechanism for momentum flux that is independent of eigenmode structures and explains rotation phenomena in tokamaks.

Findings

Outward cocurrent momentum flux caused by geometrical correction.

Rotation peaking scales with electron temperature over plasma current.

Mechanism explains rotation reversals observed in experiments.

Abstract

A geometrical correction to the E x B drift causes an outward flux of cocurrent momentum whenever electrostatic potential energy is transferred to ion parallel flows. The robust symmetry breaking follows from the free energy flow in phase space and does not depend on any assumed linear eigenmode structure, acting both for axisymmetric fluctuations (such as geodesic acoustic modes) as well as more general nonaxisymmetric fluctuations. The resulting rotation peaking is countercurrent and scales as electron temperature over plasma current. This peaking mechanism can only act when fluctuations are low-frequency enough to excite ion parallel flows, which may explain some recent experimental observations related to rotation reversals.