Geodesic Acoustic Mode Measurements in DIII-D

Geodesic acoustic modes (GAMs) are nonlinearly driven, axisymmetric ($m=0$, $n=0$) $E\times B$ flows, which may play an important role in establishing the saturated level of turbulence in tokamaks. Doppler backscattering (DBS) measures the flow of turbulent structures and the level of intermediate-k ($k_\perp\rho_s\sim 1-4$) density fluctuations. Measurements have been made with multichannel DBS systems at toroidal locations separated by 180$^\circ$. Both linear characteristics of the mode and its nonlinear interactions have been studied. Observations include cases where the GAM exists as a persistent mesoscale structure, coherent over $\sim$1/3 of the minor radius; measurements in repeat shots indicate a poloidal dependence of the GAM's radial wavenumber; and bicoherence analysis between the toroidally separated DBS systems has revealed a relationship between the GAM and low frequency zonal flows.