Use of Fast Ion D‐Alpha diagnostics for understanding ICRF effects

Combined neutral beam injection and fast wave heating at cyclotron harmonics accelerate deuterium fast ions in the National Spherical Torus Experiment (NSTX) and in the DIII‐D tokamak. Acceleration above the injected energy is evident in fast‐ion D‐alpha (FIDA) and volume‐average neutron data. The FIDA diagnostic measures spatial profiles of the accelerated fast ions. In DIII‐D, the acceleration is at a 4th or 5th cyclotron harmonic; the maximum enhancement in the high‐energy FIDA signal is 8–10 cm beyond the resonance layer. In NSTX, acceleration is observed at five harmonics (7–11) simultaneously; overall, the profile of accelerated fast ions is much broader than in DIII‐D. The energy distribution predicted by the CQL3D Fokker‐Planck code agrees fairly well with measurements in DIII‐D. However, the predicted profiles differ from experiment, presumably because the current version of CQL3D uses a zero‐banana‐width model.