During plasma excitation by high power Alfven waves at TCA, signals at harmonics of the generator frequency are observed in the plasma scrape-off layer. In this paper, experimental investigations of the sheath effect and the excitation efficiency and dispersion properties of the harmonics are reported. The results indicate that the harmonics arise, either directly or indirectly, through the driven Alfven waves and not through the sheath effect at the exciting antenna. The RF ion saturation current is observed to have a non-negligible peak amplitude in comparison with the time averaged ion saturation current and may provide evidence of a non-linear evolution of the driven Alfven waves.