On the detection of Reynolds stress as a driving and damping mechanism of geodesic acoustic modes and zonal flows

Bispectral analysis has been proposed as a diagnostic for Reynolds stress (RS) as a driving mechanism of zonal flows (ZF) in toroidally confined plasmas. A turbulence simulation code was used to test this technique on a well-defined system. It turned out that the geodesic acoustic mode dominates the poloidal flow spectrum and that it reduces radial transport in the same way as does a low-frequency ZF. Using the total cross-bicoherence, a correlation between RS and large-scale poloidal flows could be detected. The experimentally more accessible auto-bicoherences did not prove to be a useful quantity for studying this interaction. RS was not observed as a precursor of the flow; rather it appears simultaneously in the region of radial flow shear.