Toroidal momentum transport in a tokamak caused by symmetry breaking parallel derivatives

TL;DR

This paper investigates a new gyro-kinetic mechanism for toroidal momentum transport in tokamaks caused by symmetry-breaking parallel derivatives, revealing a linear dependence on normalized Larmor radius and significant impact on plasma rotation.

Contribution

It introduces an analytic model linking momentum transport to poloidal derivatives of the ballooning envelope, distinct from profile shearing effects, supported by linear gyro-kinetic simulations.

Findings

Momentum flux is linear in normalized Larmor radius (C1*)

Generates a sizeable counter-current rotation

Scales with magnetic surface aspect ratio and increases with shear, safety factor, and gradients

Abstract

A new mechanism for toroidal momentum transport in a tokamak is investigated using the gyro-kinetic model. First, an analytic model is developed through the use of the ballooning transform. The terms that generate the momentum transport are then connected with the poloidal derivative of the ballooning envelope, which are one order smaller in the normalised Larmor radius, compared with the derivative of the eikonal. The mechanism, therefore, does not introduce an inhomogeneity in the radial direction, in contrast with the effect of profile shearing. Numerical simulations of the linear ion temperature gradient mode with adiabatic electrons, retaining the finite rho* effects in the ExB velocity, the drift, and the gyro-average, are presented. The momentum flux is found to be linear in the normalised Larmor radius (\rho*) but is, nevertheless, generating a sizeable counter-current rotation. The total momentum flux scales linear with the aspect ratio of the considered magnetic surface, and increases with increasing magnetic shear, safety factor, and density and temperature gradients.