The characteristics of density fluctuations induced by geodesic acoustic modes in the edge of plasmas

The geodesic acoustic mode (GAM) induced density fluctuations were measured by the toroidally and poloidally departed Langmuir triple probe arrays in the edges of HL-2A (Chengdu) and HT-7 (Hefei) tokamaks. Some theoretical predictions about the mode features of GAM density fluctuations are verified in our experiments: the toroidal mode number of GAM density fluctuations is $n=0$; the amplitude is consistent with the theoretical prediction in a factor of 2; the GAM density and potential fluctuations are in anti-phase at the top of plasma cross-section. The nonlinear interactions between GAM and ambient turbulence (AT) are also investigated. The cross phase between envelope of high frequency AT and GAM density fluctuations is nearly $\pi/2$ radians, which supports the conclusions that the envelope modulation of density fluctuations is due to the GAM shear effect.