Magnetic fluctuation spectra and non-linear MHD mode interaction in RFX

Non-linear interaction of magnetohydrodynamic (MHD) modes sustaining the reversed field pinch configuration is an important field of research with many implications in different fields of plasma physics. The dynamo mechanism can be supported by different magnetic fluctuation spectra when the same magnetic boundary is considered. In this paper we study and characterize magnetic spectra in RFX from a statistical point of view during stationary regimes both from the global equilibrium point of view and also in the sense of the distribution of the amplitude of the magnetic fluctuations. Three classes of spectra are defined according to their spectral composition. Correlation with global plasma quantities such as plasma current, density and reversal parameter are investigated in order to point out the parameters controlling the number of interacting modes. It is found that field errors also play an important role in the evolution towards large or narrow spectra. To investigate the nonlinear interaction of magnetic fluctuations, bispectral analysis proved to be a powerful tool. A new relation between spontaneous rotation of some modes (always phase and wall locked in RFX standard discharges) and narrow spectra is obtained and discussed.