Decay of geodesic acoustic modes due to the combined action of phase mixing and Landau damping

TL;DR

This paper investigates how phase mixing and Landau damping collaboratively cause rapid decay of geodesic acoustic modes in tokamak plasmas, highlighting their combined effect on energy redistribution and mode lifetime.

Contribution

It introduces an analytical and numerical study of the synergistic damping mechanisms affecting GAMs, emphasizing their impact under realistic tokamak conditions.

Findings

GAM decay time is significantly reduced by combined phase mixing and Landau damping.

Radial mode structure evolution matches predictions from initial value simulations.

Decay times can be comparable to nonlinear GAM drive times in certain plasma regimes.

Abstract

Geodesic acoustic modes (GAMs) are oscillations of the electric field whose importance in tokamak plasmas is due to their role in the regulation of turbulence. The linear collisionless damping of GAMs is investigated here by means of analytical theory and numerical simulations with the global gyrokinetic particle-in-cell code ORB5. The combined effect of the phase mixing and Landau damping is found to quickly redistribute the GAM energy in phase-space, due to the synergy of the finite orbit width of the passing ions and the cascade in wave number given by the phase mixing. When plasma parameters characteristic of realistic tokamak profiles are considered, the GAM decay time is found to be an order of magnitude lower than the decay due to the Landau damping alone, and in some cases of the same order of magnitude of the characteristic GAM drive time due to the nonlinear interaction with an ITG mode. In particular, the radial mode structure evolution in time is investigated here and reproduced quantitatively by means of a dedicated initial value code and diagnostics.