Maxwell Prize lecture on measuring the phase-space transport of energetic particles in magnetic fusion experiments

This paper is a personal review of experiments we performed that culminated in the measurement of the fast-ion flows in phase space that are induced by Alfvén eigenmodes (AEs). Along the way, I review the salient properties of energetic particles (EPs), the diagnostics that measure them in magnetic fusion experiments, AEs, and early work that showed that EP transport is small for plasma instabilities that do not resonate with orbital frequencies. The importance of the diagnostic “weight function” to relate signals to the fast-ion distribution function is emphasized throughout. Modulation of the source of fast ions enables measurement of flows into and out of the phase-space volume detected by the weight function. In plasmas with many unstable AEs, resonant EP orbits become stochastic, resulting in phase-space flows that follow the trajectories predicted by theory. The final section contains my perspective on the next stage of magnetic fusion EP research.