Fast wave current drive at high ion cyclotron harmonics on DIII-D

Current driven by the fast Alfven wave is measured at the fourth and eighth harmonics of the deuterium ion cyclotron frequency in identical plasmas on the DIII-D tokamak. In non-sawtoothing discharges with neutral beam injection heating, the radial profile of the fast wave current drive (FWCD) is determined by the response of the loop voltage profile to co- and counter-antenna phasings. The dimensionless current drive efficiency is a factor of two greater for the eighth harmonic case compared to the fourth harmonic case. Modelling of the fast wave absorption using a ray tracing code shows that the decrease in FWCD efficiency for the latter situation can be explained by high harmonic damping of the fast waves on energetic ions.