Evidence for regions of nearly suppressed velocity space diffusion caused by finite Larmor radius effects during ICRF heating

During high power ICRF heating, the distribution function of the resonating ions becomes non-Maxwellian and an anisotropic tail develops mainly in the perpendicular velocity direction. The evolution of the distribution function can be described by a quasi-linear diffusion operator in phase space. Owing to finite Larmor radius (FLR) effects, the diffusion coefficient in this operator varies with the gyro-radius normalized to the wavelength. At certain ion energies the diffusion coefficient can become strongly reduced, effectively preventing resonating ions from reaching higher energies. Measurements are reported which show a significant difference in the distribution function between different scenarios for heating of hydrogen ions in JET. Comparison with theoretical calculations shows that these differences can be explained by higher order FLR effects on velocity space diffusion during ICRF heating. The importance of these effects on the power deposition is also emphasized.