Measurement of velocity turbulence and application to zonal flow detection in tokamak plasmas

A time delay estimation (TDE) analysis technique for calculating velocity fluctuations from advecting turbulent density fluctuation data in high temperature tokamak plasmas has been developed and deployed. The method relies on calculating the time-resolved time delay of turbulent density eddy structures advecting between two points in the field of view of density fluctuation diagnostics due to an equilibrium plasma flow. The technique allows practical and direct access to the turbulent velocity field, which in turn relates to the electrostatic potential fluctuations associated with electrostatic drift wave turbulence.; Two techniques to extract velocity fluctuation from a pair of spatially-separated but adjacent density fluctuation measurements are applied and compared. The first method is a correlation analysis in the time domain in which the time delay, and thus velocity, is obtained as the point of maximum correlation between the two density fluctuation signals. The second method is a wavelet analysis, which relies on the extraction of a time delay from the phase of a complex wavelet cross power spectrum of the two density signals. The frequency response limit using the wavelet method is near the frequency cutoff of the density fluctuations. This allows for velocity fluctuations to be measured on time scales relevant to the studies of turbulent plasma behavior.; These TDE techniques have been applied to localized density turbulence measurements obtained with Beam Emission Spectroscopy (BES) on the DIII-D tokamak. Signatures of radially-localized zonal flows are observed in the resulting time-resolved turbulence flow field. These features include a coherent oscillation in the poloidal flow, v˜_&thetas;, near 15 KHz, with a long poloidal wavelength (m < 3) and narrow radial extent (k _rρ_i ∼ 0.2). The estimated flow-shearing rate associated with the coherent oscillation is insufficient to account for significant reduction of turbulent transport, suggesting that other mechanisms are involved in zonal flow regulation of turbulence. The mode frequency exhibits a temperature dependence that scales approximately with the local sound speed as ω ∼ c_s/a, which is consistent with predictions for a branch of zonal flows identified as geodesic acoustic modes. Such modes have been observed in nonlinear Braginskii simulations of edge plasma turbulence.