Radial propagation of geodesic acoustic modes

TL;DR

This paper introduces a robust method to estimate the radial group velocity of geodesic acoustic modes (GAMs) using energy flux ratios, verified by simulations and analytical calculations, with implications for understanding GAM activity in plasma experiments.

Contribution

It presents a new approach to estimate GAM group velocity via energy flux ratios, adaptable to complex physics and magnetic geometries, improving upon previous direct calculation methods.

Findings

The method is validated with gyrokinetic and two-fluid simulations.

GAM propagation influences the interpretation of experimental GAM activity.

The approach can incorporate additional physics like magnetic geometry.

Abstract

The GAM group velocity is estimated from the ratio of the radial free energy flux to the total free energy applying gyrokinetic and two-fluid theory. This method is much more robust than approaches that calculate the group velocity directly and can be generalized to include additional physics, e.g. magnetic geometry. The results are verified with the gyrokinetic code GYRO [J. Candy and R. E. Waltz, J. Comp. Phys. 186, pp. 545-581 (2003)], the two-fluid code NLET [K. Hallatschek and A. Zeiler, Physics of Plasmas 7, pp. 2554-2564 (2000)], and analytical calculations. GAM propagation must be kept in mind when discussing the windows of GAM activity observed experimentally and the match between linear theory and experimental GAM frequencies.