Density fluctuation of geodesic acoustic mode on the HL-2A tokamak

The density fluctuations of the geodesic acoustic mode (GAM) have been observed in Ohmic deuterium plasma discharges with a combination of rake-like and three-step Langmuir probe arrays on the HL-2A tokamak. The probe arrays with poloidal and toroidal separations of 36 and 1330 mm are applied to measure the spectral property and intermittency of the GAM density fluctuations. The poloidal and toroidal mode numbers of the fluctuations are simultaneously measured for the first time. The measured fluctuation amplitude is consistent with the theoretical prediction. High coherence and near zero phase shift of the GAM density fluctuations separated toroidally by 37.5° at the same magnetic flux surface was first observed, indicating the symmetric structure of the GAM in the toroidal direction. The peak time delays of the cross-correlation function of the fluctuations above and below the midplane suggest the expected sin θ dependence. The nonlinear three wave coupling between the GAM and the ambient turbulence is shown to be a plausible mechanism for the generation of the GAM density fluctuations. The significant coherence and the corresponding fixed phase shift (∼π) between the original data and the envelope of the high frequency ambient turbulence provide the experimental evidence for the envelope modulation. The GAM amplitude is out of phase with the particle flux. Most of the intermittent frequencies for particle flux are close to the GAM frequency.