Radial structures and nonlinear excitation of geodesic acoustic modes

¹  Associazione Euratom-ENEA sulla Fusione, C.R. Frascati - C.P. 65, 00044 Frascati, Italy, EU
²  Department of Physics and Astronomy, University of California - Irvine, CA 92697-4575, USA
³  Institute for Fusion Theory and Simulation, Zhejiang University - Hangzhou 310027, PRC

Corresponding author: zonca@frascati.enea.it

Received: 26 May 2008
Accepted: 16 June 2008

Abstract

Geodesic acoustic modes (GAM) are shown to constitute a continuous spectrum due to radial inhomogeneities. The importance and theoretical as well as experimental implications of this fact are discussed in this work. The existence of a singular layer causes GAM to mode convert to short-wavelength kinetic GAM (KGAM) via finite ion Larmor radii; analogous to kinetic Alfvén waves (KAW). Furthermore, it is shown that KGAM can be nonlinearly excited by drift-wave (DW) turbulence via 3-wave parametric interactions, and the resultant driven-dissipative nonlinear system exhibits typical prey-predator self-regulatory dynamics, consistent with recent experimental observations on HL-2A. The degeneracy of GAM/KGAM with beta-induced Alfvén eigenmodes (BAE) is demonstrated and discussed, with emphasis on its important role in the complex self-organized behaviors of burning plasmas.