The nonlinear saturation of beam-driven instabilities: Irregular bursting in the DIII-D tokamak

Intense fast-ion populations created by neutral-beam injection into a tokamak can destabilize fishbone modes and other instabilities. Regular, periodic bursts of fishbones often occur but, when another magnetohydrodynamic (MHD) mode is also unstable, the burst cycle is irregular. The complexity of the burst cycle over a long time scale correlates with the purity of the Fourier spectrum during a single burst (a short time scale). The data are consistent with a simple predator–prey model in which the second MHD mode introduces periodic perturbations into the evolution equations.