During plasma excitation by high power Alfven waves in TCA, signals at harmonics of the generator frequency are observed in the plasma scrape-off layer. In this paper we report experimental investigations of the sheath effect and the excitation efficiency and dispersion properties of these harmonics. The results indicate that the harmonics arise either directly or indirectly through the driven Alfven waves and not the sheath effect at the exciting antenna. The RF ion saturation current is observed a non-negligible peak amplitude in comparison to the time averaged ion saturation current and may provide evidence of non-linear evolution of the driven Alfven waves. (author) 8 figs., 8 refs.