Recent Fast-ion Experiments

Diagnostic improvements and operational flexibility have facilitated new fast-ion studies on \hbox{DIII-D}. The Balmer$_\alpha$ light from deuterium ions that neutralize as they pass through a neutral beam yields the fast-ion energy distribution and spatial profile; neutral particle, neutron, and beam-ion loss diagnostics corroborate the D$_\alpha$ diagnostic. In plasmas with fast-wave heating at the fourth, fifth, and sixth cyclotron harmonic, fast ions are accelerated above the injection energy; the profile data show that the acceleration is greatest near the cyclotron harmonic resonance layer. Internal density and external magnetic fluctuations produced by fast-ion instabilities with MHz frequencies are now measured. Experiments with magnetic fields as low as 0.5~T indicate that compressional Alfv\'en eigenmodes are readily excited in conventional tokamaks and will probably be excited in ITER. A study of Alfv\'en cascade modes provides our first simultaneous measurements of internal fluctuations and the resulting impact on the fast-ion profile.