Effective Mass in Rosenbluth-Hinton type zonal flows

TL;DR

This paper investigates the effective mass in Rosenbluth-Hinton zonal flows in tokamaks, revealing that a phase space coordinate shift explains the large inertia and resolves paradoxes related to angular momentum and particle flows.

Contribution

It introduces a drift kinetic calculation showing that a phase space coordinate shift accounts for the large effective mass and resolves existing paradoxes in Rosenbluth-Hinton flows.

Findings

The effective mass arises from a shift in energy coordinates proportional to the electric field.

The phase space shift explains the discrepancy between expected and observed angular momentum.

Reverse circulating particle flows are identified, balancing the trapped particle precession flows.

Abstract

An initial radial electric field, $E_r(0)$, in an axisymmetric tokamak, results in geodesic acoustic mode (GAM) oscillations. The GAMs Landau damp, resulting in a much smaller final residual electric field, $E_r(\infty)$, and accompanying parallel zonal flows (Rosenbluth and Hinton, 1998 PRL 80, 724, hereafter RH). The phenomenon exhibits a large effective mass (inertia due to flows), with an enhancement of order the well-known RH factor. In apparent paradox, the final angular momentum in the RH parallel zonal flow is much smaller than the angular momentum expected from the well-known rapid precession of the trapped particle population in the final electric field. In addition, an effective mass calculated naively based on the rapid trapped particle (TP) precession is much larger than the RH factor. A drift kinetic calculation is presented showing that the mathematical origin of the extra mass factor is a shift, proportional to $E_r$, of the usual energy coordinates in phase space. Importantly, this shift contributes to the effective mass even if the system is linearized in $E_r$, and can be interpreted as a first order linear shift in the Jacobian. Further, the Jacobian shift recovers the large TP precession flow and also uncovers the presence of reverse circulating particle flows that, to lowest order, are equal and opposite to the TP precession. A detailed calculation is presented. Work supported by DOE.