Study of the interaction between diffusive and avalanche-like transport in near-critical dissipative-trapped-electron-mode turbulence

Abstract

The change in properties of turbulent transport is explored, in the context of dissipative-trapped-electron-mode (DTEM) turbulence, as a function of the relative strength of a (subdominant) diffusive transport channel to the turbulent one. If the diffusive channel is totally absent and the system is slowly driven, transport exhibits many features characteristic of self-organized-critical (SOC) systems. This finding is consistent with those reported by several authors in other situations, such as pressure-gradient-driven or ion-temperature-gradient turbulence. But we also find that the temporal persistence and spatial self-similarity characteristic of the SOC state remain present, at a level much higher than what one would naively expect, as the strength of the diffusive channel is increased. This observation, which has been previously made for numerical sandpile models, may give a partial answer to the question of why SOC features are so frequently observed in turbulent experimental data and numerical simulations in spite of the fact that the experimental conditions appear to be quite far from those required by the strict definition of the SOC state.