Nonlinear Splitting of Fast Particle Driven Waves in a Plasma: Observation and Theory

The measured spectra of fast particle driven Alfv{acute e}n eigenmodes in the JET tokamak plasma are interpreted on the basis of a nonlinear model of near-threshold kinetic instabilities. The observed mode splitting is a manifestation of the general nonlinear phenomena due to the combination of resonant wave-particle interaction and collisionlike relaxation of the resonant particles. The instability growth rate and the effective collision frequency of the resonant particles are determined. {copyright} {ital 1998} {ital The American Physical Society }