Destabilization of fast magnetoacoustic waves by circulating energetic ions in toroidal plasmas

An instability of fast magnetoacoustic waves driven by circulating energetic ions in axisymmetric toroidal plasmas and characterized by frequencies below the ion gyrofrequency is considered. An important role of the l=0 resonance (l is the number of a cyclotron harmonic) in the wave-particle interaction is revealed: It is shown that this resonance considerably extends an unstable region in the space of the pitch-angles of the energetic ions and the wave frequencies. The analysis is carried out for a "slow" instability, which has the growth rate less than the bounce frequency of the energetic ions. Specific examples relevant to the National Spherical Torus Experiment [Spitzer , Fusion Technol. 30, 1337 (1996)], where instabilities of this kind were observed, are considered. (C) 2003 American Institute of Physics.