Three-wave interactions between fast-ion driven modes in the National Spherical Torus Experiment

Experiments in the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)] have yielded new, unique observations of nonlinear three-wave interactions between compressional Alfven eigenmodes (CAEs) and other fast-ion driven instabilities. Specifically, nonlinear interactions of CAEs have been conclusively identified with both energetic particle modes (EPMs) and toroidicity-induced Alfven eigenmodes (TAEs). These nonlinear interactions occur simultaneously with other three-wave interactions observed between the TAEs and EPMs [N. A. Crocker et al., Phys. Rev. Lett. 97, 045002 (2006)]. The interaction between the CAEs and EPMs spatially redistributes the energy of the CAEs, concentrating it into a toroidally localized wave packet in the same way that the interaction between the TAEs and EPMs spatially concentrates the energy of the TAEs. The interaction between the CAEs and TAEs has been shown to further subdivide the CAE wave packet into a train of smaller wave packets. These nonlinear interactions occur during fast-ion loss events. The spatial redistribution of CAE fluctuation energy will modify the effect of the CAEs on fast-ion transport during these events.