A recently introduced tool for the analysis of turbulence, wavelet bicoherence [van Milligen, Hidalgo, and Sánchez, Phys. Rev. Lett. 16, 395 (1995)], is investigated. It is capable of detecting phase coupling—nonlinear interactions of the lowest (quadratic) order—with time resolution. To demonstrate its potential, it is applied to numerical models of chaos and turbulence and to real measurements. It detected the coupling interaction between two coupled van der Pol oscillators. When applied to a model of drift wave turbulence relevant to plasma physics, it detected a highly localized coherent structure. Analyzing reflectometry measurements made in fusion plasmas, it detected temporal intermittency and a strong increase in nonlinear phase coupling coinciding with the L/H (low-to-high confinement mode) transition.

Analysis of the edge plasma fluctuation in several confinement devices reveals the self-similar character of the fluctuations through the presence of long-range time correlations. These results show that the tail of the autocorrelation function decays as a power law for time lags longer than the decorrelation time and as long as times on the order of the particle diffusion time. The algebraic decay of the long-range time correlations is consistent with plasma transport characterized by self-organized criticality.

A new technique combining wavelet analysis and bispectral analysis has been developed. This analysis tool permits the detection of structure in turbulent or chaotic data with time resolution, even in the presence of a significant noise contribution. Application of this technique to data obtained in fusion plasmas with Langmuir probes demonstrates its possibilities by detecting short-lived intermittent nonlinear coupling. Its application in the field of chaos analysis is indicated.