Spontaneous excitation of geodesic acoustic mode by toroidal Alfvén eigenmodes

¹ Institute for Fusion Theory and Simulation, Zhejiang University - Hangzhou 310027, PRC
² Department of Physics and Astronomy, University of California - Irvine, CA 92697-2575, USA
³ Associazione Euratom-ENEA sulla Fusione, C. R. Frascati - C. P. 65, 00044 Frascati, Italy, EU

Received: 1 December 2012
Accepted: 15 January 2013

Abstract

Spontaneous nonlinear excitation of geodesic acoustic mode (GAM) by toroidal Alfvén eigenmodes (TAE) is studied within the framework of gyrokinetic theory. The dispersion relation for the parametric decays of a pump TAE mode into a TAE lower sideband and a GAM is derived. It is shown that, in the ideal MHD first stability region, the condition for spontaneous excitation of GAM by TAEs is ω²₀ > V²_A/(4q²R²₀), in which, ω₀ is the pump TAE real frequency, V _A is the Alfvén speed, q is the safety factor and R₀ is the torus major radius. The corresponding threshold condition is also derived and suggests the decay process as an effective saturation mechanism for TAE.