Measurements of fast-ion acceleration at cyclotron harmonics using Balmer-alpha spectroscopy

Combined neutral beam injection and fast wave heating at the fourth and fifth cyclotron harmonics accelerate fast ions in the DIII-D tokamak. Measurements with a nine-channel fast-ion D-alpha (FIDA) diagnostic indicate the formation of a fast-ion tail above the injection energy. Tail formation correlates with enhancement of the d–d neutron rate above the value that is expected in the absence of fast-wave acceleration. FIDA spatial profiles and fast-ion pressure profiles inferred from the equilibrium both indicate that the acceleration is near the magnetic axis for a centrally located resonance layer. The enhancement is largest 8–10 cm beyond the radius where the wave frequency equals the cyclotron harmonic, probably due to a combination of Doppler-shift and orbital effects. The fast-ion distribution function calculated by the CQL3D Fokker–Planck code is fairly consistent with the data.