Modeling the Alfvén eigenmode induced fast-ion phase-space flow measured by an imaging neutral particle analyzer

An imaging neutral particle analyzer (INPA) provides energy and radially resolved measurements of the confined fast-ion population at the DIII-D tokamak. In recent experiments, it was used to diagnose fast-ion flow driven by multiple, marginally unstable Alfvén Eigenmodes (AEs). Different models are applied to reproduce such measurements. Ad hoc energetic particle diffusivity modeling by TRANSP significantly deviates from the observations. Comparably, reduced modeling, i.e. a combination of NOVA-K and ASCOT5 codes, reproduce some key features of the observed flow, but largely fail to interpret the observed fast ion depletion near the plasma axis. At last, hybrid simulations predict an RSAE consistent with the experiment that redistributes the injected ions. The resulting synthetic INPA images are in good agreement with the measurement near the injection energy. These simulations confirm that the measured flow follows streamlines defined by the intersection of phase-space surfaces of constant magnetic moment μ and constant E' = nE+⍵P_ɸ. Nonperturbative effects are required to reproduce the depletion of fast ions near the magnetic axis at the injection energy.