THE EXCITATION OF OBLIQUELY PROPAGATING FAST ALFVEN WAVES AT FUSION ION-CYCLOTRON HARMONICS

The theory of the magnetoacoustic cyclotron instability, which has bee n proposed as a mechanism for suprathermal ion cyclotron harmonic emis sion observed in large tokamaks, is generalized to include finite para llel wave number k(parallel-to). This extension introduces significant new physics: the obliquely propagating fast Alfven wave can undergo c yclotron resonant interactions with thermal and fusion ions, which aff ects the instability driving and damping mechanisms. The velocity-spac e distribution of the fusion ions is modeled by a drifting ring, which approximates the distribution calculated for the emitting region in t ritium experiments on the Joint European Torus (JET) [Cottrell et al., Nucl. Fusion 33, 1365 (1993)]. Linear instability can occur simultane ously at the fusion ion cyclotron frequency and all its harmonics when the fusion ion concentration is extremely low, because the finite k(p arallel-to) gives rise to a Doppler shift, which decouples cyclotron d amping due to thermal ions from wave growth associated with fusion ion s. Doppler shifts associated with finite k(parallel-to) may also be re lated to the observed splitting of harmonic emission lines.