Hybrid kinetic-MHD modelling of observed fast ion migration induced by Alfvénic instabilities

In recent experiments conducted in the DIII-D tokamak, an imaging neutral particle analyzer (INPA) [1] measures fast ion transport across a broad range of local phase space in the presence of multiple Alfvén Eigenmodes with unprecedented phase space resolution. The hybrid kinetic-MHD MEGA code [2] is used to reproduce the radial location and frequency of the modes observed in the experiment, as well as the associated fast ion transport. At the INPA-interrogated pitch angle, reversed shear Alfvén eigenmodes (RSAE) are found to create phase-space islands near the location of minimum safety factor q on the low-field side of the plasma. On the high-field side, the islands cover the boundary between passing and stagnation orbits near the axis, explaining the observed prompt pile-up of the fast ions in the plasma core, which are accelerated to higher energies than the injection energy of neutral beams by a few keV. Multi-phase simulations are performed combining sequential phases of purely kinetic with hybrid modelling, including realistic neutral beam injection and collisions. These simulations cover the entire fast ion slowing-down period, enabling a direct comparison with the time-resolved phase space transport dynamics observed by the INPA.