The problem of capturing marginality in model reductions of turbulence

Reduced quasilinear and nonlinear (gradient-driven) models with scale separations, commonly used to interpret experiments and to forecast turbulent transport levels in magnetised plasmas, are tested against nonlinear models without scale separations (flux-driven). Two distinct regimes of turbulence-either above threshold or near marginal stability-are investigated with Boltzmann electrons. The success of reduced models hinges in particular on the reproduction of nonlinear fluxes. Good agreement between models is found above threshold, whilst reduced models significantly underpredict fluxes near marginality, overlooking mesoscale flow organisation and turbulence self-advection. Constructive prescriptions whereby to improve reduced models are discussed.

Automated summary

This study compares reduced quasilinear and nonlinear models with scale separations to nonlinear models without scale separations by using Boltzmann electrons. The success of the reduced models lies in their ability to reproduce nonlinear fluxes. However, the reduced models significantly underestimate fluxes near marginal stability, ignoring mesoscale flow organization and turbulence self-advection. Constructive suggestions to improve the reduced models are discussed.