Observation of global Alfvén eigenmode avalanche events on the National Spherical Torus Experiment

Instabilities excited by the fast-ion population on NSTX (Ono et al 2000 Nucl. Fusion 40 557) extend from low-frequency energetic particle modes (EPMs) at tens of kHz through toroidal Alfvén eigenmodes (TAEs) in the range 50-150 kHz to global and compressional Alfvén eigenmodes (GAE and CAE) in the frequency range 0.3-2.5 MHz, or roughly 0.1ω _ci to 0.7ω _ci. The GAE instabilities exhibit complex non-linear behaviour, including onset of strong growth above an amplitude threshold. This is conjectured to occur when resonance regions in phase space start to overlap, resulting in enhanced rapid growth and redistribution of energetic particles, a process referred to as an 'avalanche' (Berk et al 1995 Nucl. Fusion 35 1661). The GAE are suppressed following the avalanche, suggesting depletion of the fast-ion population resonantly driving the modes, and in some instances the GAE bursts appear to trigger lower frequency TAE avalanches or EPMs, suggesting some significant redistribution of fast ions in phase space has occurred. These are the first reported observations of avalanching behaviour for an instability driven through the Doppler-shifted cyclotron resonance. This paper also provides internal measurements of GAE structure showing that the mode amplitude peaks towards the plasma core.