Combined neutral beam injection and fast wave heating at thefourth cyclotron harmonic produce an energetic deuterium beam ion tail in theDIII-D tokamak. When the concentration of thermal hydrogen exceeds ∼5%, the beam ion absorption is suppressed in favour of second harmonichydrogen absorption. As theoretically expected, the beam absorption increaseswith beam ion gyro-radius; also, central absorption at thefifth harmonic is weaker than central absorption at the fourth harmonic.For central heating at the fourth harmonic, an energetic, perpendicular,beam population forms inside the q = 1 surface. The beam ion tail transientlystabilizes the sawtooth instability but destabilizes toroidicity induced Alfvén eigenmodes (TAEs). Saturation of the central heating correlates with theonset of the TAEs. Continued expansion of the q = 1 radius eventuallyprecipitates a sawtooth crash; complete magnetic reconnection is observed.