Bispectral analysis as a tool for the detection of zonal flows

Mirko Ramisch
Institut für Experimentelle und Angewandte Physik, Universität Kiel, Germany

In turbulent toroidal plasmas large-scale poloidal flows can be generated spontaneously. The shear of these so called zonal flows can lead to a radial decorrelation of turbulence and therefore to a reduction of the fluctuation amplitudes and turbulent transport. To understand the physics of zonal flows, the driving mechanism is investigated. Theory has proposed different models of three wave interactions to describe the generation mechanisms. Since bispectral analysis is a tool for the detection of three mode coupling it may be used to support the theory experimentally. In experiments, the full set of data needed to calculate the appropriate quantities is in not available. Therefore simplified forms of the bicoherence are applied to data. In this presentation simulated data from the DALF turbulence code are used to test the different simplifications and to compare them with the full expression for the bicoherence.

A short introduction into bispectral analysis is given. The bispectral analysis of simulation data is also compared with the appearance of zonal flows in the simulations.