Coherent losses of neutral beam ions are observed at frequencies corresponding to toroidal and reversed-shear Alfvén eigenmodes (RSAEs) in DIII-D. Reversed-shear profiles are created by injecting beam power during the plasma current ramp. Beam ion losses stemming from Alfvén eigenmode activity contribute to flattening of the energetic ion density profile in such discharges. This is the first observation of convective beam ion losses due to RSAEs. The energies and pitch angles of lost ions are measured and found to exist within a well-defined region of phase space. Loss flux signals decrease in time as current penetrates and Alfvén eigenmode activity becomes more core localized. Preliminary Monte Carlo simulations of energetic ion interactions with measured mode structures show the dominant loss mechanism is a transition from a counter-passing orbit to a trapped orbit that is lost to the wall.
Beam modulation and bump-on-tail effects on Alfvén eigenmode stability in DIII-D