Two types of small-poloidal wavelength low frequency floating potential fluctuations (∼1 kHz, and 10–15 kHz) are observed in the edge region of ohmically heated plasmas in the JFT-2M tokamak. The higher frequency fluctuations are inferred as geodesic acoustic modes (GAMs). Significant bicoherences between both modes and the broad-band fluctuations are observed simultaneously; however, frequency ranges are different. A significant bicoherence range is from 70 to 125 kHz at the lower frequency modes (LFM) and is from 20 to 125 kHz at the GAM. Phase angles at the significant bicoherences are coherent around π. These results indicate that both modes are nonlinearly coupled to the broad-band fluctuations. Two spectral peaks are observed around the GAM frequency and the difference in peak frequencies is around the LFM frequency, suggesting nonlinear couplings between the LFM and the GAM. However, the modulation of the GAM by the LFM is not conclusive. In this paper, the results of the letter (Nagashima et al 2005 Phys. Rev. Lett.95 095002) are presented in detail.
Spectral characteristics of zonal flows in the edge plasmas of the HT-7 tokamak