Phase space resolved measurements of fast-ion density fluctuations and energy transfer induced by Alfvén eigenmodes

We present phase-space resolved measurements of energetic particle (EP) fluctuations near toroidal Alfvén eigenmode (TAE) frequencies in the DIII-D tokamak. A newly rebuilt fast channel in the imaging neutral particle analyzer (INPA) system, with a significantly improved signal-to-noise ratio, enabled the detection of fluctuations at approximately 100–130 kHz, frequencies that were previously inaccessible. Coherent fluctuations with amplitudes of about 1% were observed in localized regions of EP phase space. The secular energy exchange rate between the wave and the EP population is directly extracted from the observed INPA and electron density and temperature fluctuations, accounting for both parallel and perpendicular electric field components. Analysis reveals that poloidal energy exchange dominates by approximately two orders of magnitude over parallel and radial components, at the measurement position near the outer midplane, with energy transfer occurring through the interaction between drift velocities and perpendicular electric fields. The averaged energy exchange rates are of the order of 10 keV s − 1 . A comparison with ASCOT orbit-following simulations confirms that the observed fluctuations originate from the passively viewed (trapped) particle population rather than from passing particles. The measured TAE radial structure shows good agreement with ideal MHD simulations using the NOVA code. These measurements provide unprecedented experimental validation of wave-particle energy transfer mechanisms.