PHYSICS OF PLASMAS, vol.13, no.11, 2006 (SCI-Expanded)
Using laser induced fluorescence (LIF) and higher order spectral analysis, we present the first measurements of phase space resolved coherent nonlinear interactions among the components of low frequency density fluctuations, (omega <=omega(ci)), in a linearly magnetized device. The bicoherence calculations employing the two point correlation technique suggest that there are two different coherent nonlinear wave-wave interactions in the measured spectrum. The first one, having a short correlation length and existing for slow moving ions, for which parallel to upsilon(i parallel to)parallel to <=parallel to upsilon(ith)parallel to, is an interaction between fluctuations below the electron drift frequency, omega(*). The second one is the strongest for fast moving ions, for which parallel to upsilon(i parallel to)parallel to >=parallel to upsilon(ith)parallel to, and is a mode coupling between the azimuthal drift wave modes, m=1 and m=2. Combining these bispectral results with earlier linear analysis based on the power spectra of the fluctuations, we suggest that the nonlinear coupling observed between the spectral components below omega(*) for the case of slow moving ions is associated with the anomalous kinetic component. For slow moving ions, as we increase the neutral collision frequencies, the nonlinear interaction observed for spectral components below omega(*) decreases and the harmonic mode coupling for omega(*) takes over. (c) 2006 American Institute of Physics.