On fast radial propagation of parametrically excited geodesic acoustic mode

Z Qiu,L Chen,F Zonca

doi:10.1063/1.4918944

Abstract

The spatial and temporal evolution of parametrically excited geodesic acoustic mode (GAM) initial pulse is investigated both analytically and numerically. Our results show that the nonlinearly excited GAM propagates at a group velocity which is, typically, much larger than that due to finite ion Larmor radius as predicted by the linear theory. The nonlinear dispersion relation of GAM driven by a finite amplitude drift wave pump is also derived, showing a nonlinear frequency increment of GAM. Further implications of these findings for interpreting experimental observations are also discussed.

Highlights

Geodesic Acoustic Modes (GAM) [1, 2] are finite-frequency components of zonal structures (ZS) [3, 4] unique in toroidal plasmas, which could be spontaneously excited by microscopic drift wave (DW) type turbulence [5], including drift Alfven waves (DAW); and in turn, are capable of scattering DW/DAW into stable short radial-wavelength domain [2, 6,7,8,9]
In-depth analysis of the experimentally obtained dispersion relation leads to the conclusion that, even though a quadratic dependence of geodesic acoustic mode (GAM) frequency on its radial wavevector, qualitatively consistent with linear theory of KGAM [19], is obtained, the coefficient for finite ion Larmor radius (FILR) effects is much larger than that predicted by linear theory [14, 21]
It is found that the parametrically excited GAM propagates at a nonlinear group velocity, which is the mean of the linear group velocities of GAM and DW, and is much larger than that predicted by linear theory of kinetic GAM

Summary

INTRODUCTION

Geodesic Acoustic Modes (GAM) [1, 2] are finite-frequency components of zonal structures (ZS) [3, 4] unique in toroidal plasmas, which could be spontaneously excited by microscopic drift wave (DW) type turbulence [5], including drift Alfven waves (DAW); and in turn, are capable of scattering DW/DAW into stable short radial-wavelength domain [2, 6,7,8,9]. In-depth analysis of the experimentally obtained dispersion relation leads to the conclusion that, even though a quadratic dependence of GAM frequency on its radial wavevector, qualitatively consistent with linear theory of KGAM [19], is obtained, the coefficient for FILR effects is much larger than that predicted by linear theory [14, 21] This discrepancy has been found in numerical simulations [15], where the measured radial propagation velocity of the DW driven GAM is used to determine the coefficients of FILR effect, and is found to be much larger than unity.

THEORETICAL MODEL

SPATIALTEMPORAL EVOLUTION OF THE COUPLED DW-GAM SYSTEM

CONCLUSIONS AND DISCUSSIONS