Observation of high-frequency secondary modes during strong tearing mode activity in FTU plasmas without fast ions

MHD spectroscopy in FTU has revealed high frequency (HF) oscillations between 30 and 70 kHz that accompany the development of large m=-2, n =-1 islands in ohmic plasmas. This frequency range is one order of magnitude above the island rotation frequency, one order of magnitude below the first toroidal gap in the Alfven continuum and of the same order of the low frequency gap introduced by finite beta effects. The HF spectrum is organized in pairs of modes with opposite toroidal numbers, n=1 and n=-1 for the stronger ones, which form standing waves in the island rest frame. The poloidal structure of HF modes appears to vary from vertical bar m vertical bar =2 on the low field side to |m| ∼ 5-6 on the high field side. HF lines appear when the level of poloidal field perturbation produced by the island at the plasma edge exceeds 0.2%. The absence of energetic ions in FTU ohmic plasmas, the strict correlation between HF and tearing modes and the existence of a threshold indicate that HF modes tap energy either from the tearing mode by a non-linear mechanism or from fast electrons. (author)