Gyrokinetic theory of parametric decays of kinetic Alfvén waves
Institute for Fusion Theory and Simulation, Zhejiang University - Hangzhou 310027, PRC
2 Department of Physics and Astronomy, University of California - Irvine, CA 92697-4575, USA
3 Associazione Euratom-ENEA sulla Fusione, C.R. Frascati - C.P. 65, 00044 Frascati, Italy, EU
Accepted: 9 September 2011
The fundamental parametric decay processes of kinetic Alfvén waves (KAW) have been reexamined by employing the nonlinear gyrokinetic equations. Dispersion relations, valid for arbitrary k⊥ρi, are derived for parametric decays to KAW and ion sound waves. Here, k⊥ and ρi are, respectively, the wave number perpendicular to the magnetic field and the ion Larmor radius. It is found that, contrary to the small k⊥ρi drift-kinetic results, nonlinear ion Compton scatterings also contribute significantly to the nonlinear ion Landau damping. Furthermore, for k⊥ρi>|ω0/Ωi|1/2, with ω0 and Ωi being, respectively, the KAW and ion cyclotron frequencies, the decay processes are significantly enhanced over and qualitatively different from the ideal-magnetohydrodynamic (MHD) results. These findings are relevant for collisionless plasma transports, as well as non-local wave energy transports. In particular, they question the applicability of ideal-MHD–based theories for the prediction of saturated Alfvén wave spectra and the corresponding fluctuation-induced transports in space and laboratory plasmas, suggesting that gyrokinetic theories are necessary for realistic comparisons with experimental measurements and observations.
PACS: 52.30.Gz – Gyrokinetics / 52.35.Bj – Magnetohydrodynamic waves (e.g., Alfven waves) / 52.35.Mw – Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
© EPLA, 2011