Spectral energy transfer and generation of turbulent structures in toroidal plasma

Nonlinear energy cascades in turbulent spectra are studied in the H-1 toroidal heliac [S. M. Hamberger et al., Fusion Technol. 17, 123 (1990)] using the spectral energy transfer estimation and the amplitude correlation technique. An inverse energy cascade of the spectral energy from the unstable range is shown to be responsible for the generation of the large-scale coherent structures dominating turbulence spectra. Among such structures are zonal flows which are also found to be generated via the inverse cascade. The generation of zonal flows is correlated with the increased strength in the nonlinear energy transfer. The onset of the strongly sheared radial electric field across the low–high (L–H) transition dramatically changes the energy transfer in the spectra and the spectral power of the fluctuations.