The Experimental Study Of The Geodesic Acoustic Mode Zonal Flow In The Edge Of Tokamak Plasmas

The zonal flows in tokamak plasma, which are toroidally and poloidally symmetric with finite radial wavenumber, play a very import role in regulating the turbulence and enhancing the confinement. Zonal flows have two branches: the Low Frequency Zonal Flow (LFZF) and Geodesic Acoustic Mode (GAM). The experimental study of GAM is the subject of this thesis.The three-dimensional wavenumber and frequency spectrum for the geodesic acoustic mode (GAM) has been measured in the HuanLiuqi-2A tokamak for the first time. The spectrum provides definite evidence for the GAM, which is characterized by k_θ=k_φ= 0 and k_rρ_i≈0.04 - 0.09 with the full width at half-maximum k_rρ_i≈0.03 - 0.07. The localized GAM packet is observed to propagate outward in the radial direction with nearly the same phase and group velocity. A single peak feature is observed in S(k_r) spectrum, which implies there only exists traveling wave component in the GAM, with little standing wave component.The envelopes of the radial electric field and density fluctuations are observed to be modulated by the GAM. By comparing the experimental result with that of the envelope analysis using model signals, the mechanism of the envelope modulation has been identified. The results strongly suggest that the envelope modulation of the E_r fluctuations is dominantly caused by the direct regulation of the GAM during the GAM generation in the energy-conserving triad interaction, and the envelope modulation of the density fluctuations is induced by the GAM shearing effect, which transfers the fluctuation energy from low to high frequencies.In addition, all the cross- and auto-bicoherences for interactions between the GAM and turbulent fluctuations show a similar peaked feature that may reflect the resonant property in the nonlinear coupling between the GAM and turbulent fluctuations.The radial particle flux associated with GAM is studied in the thesis. The GAM can reduce the radial particle flux and the asymmetry in the density fluctuation of GAM possibly leads to the asymmetry of the radial particle flux.The nonlinear dynamics analysis is also applied to GAM. The nonlinear dimension decreases with the increasing of the relative amplitude of GAM.