Nonlinear energy transfer during the transition to drift-interchange turbulence

The transition to drift-interchange turbulence is investigated by estimating the nonlinear spectral power transfer functions of density and potential fluctuations representative for the behavior of the polarization and E × B drift nonlinearities using the magnetic field strength as the control parameter. As the control parameter increases the system undergoes several changes from a quasi-periodic to a phase locked to a weakly turbulent regime. The polarization drift nonlinearity transfers kinetic energy to larger scales. The E × B drift nonlinearity provides access to the free energy by coupling density fluctuations at smaller scales to the larger scale potential structures, resulting in a formation of a very robust phase-locked regime and acting as a mechanism of self-sustainment in the weakly turbulent regime. Although the interplay between these nonlinearities is different in the different regimes, it results in a successive increase in the degrees of freedom in all regimes, which is the most important feature of a transition to turbulence.