Electron heat transport regimes in multi-scale turbulence

Electron heat transport is studied in the framework of the test particle approach using a semi-analytical method. We consider multi-scale turbulence with a range of correlation lengths lambda that covers two orders of magnitude. The model is based on numerical simulations and experiments, which show that fusion plasmas are characterized by the simultaneous presence of several types of turbulence. The electron temperature gradient drives the small scale turbulence with lambda of the order of the electron Larmor radius, while the trapped electron modes and the ion temperature gradient generate large scale potential fluctuations with lambda in the interval (rho(i), 10 rho(i)), where rho(i) is the ion Larmor radius. The transport regimes are determined for each type of turbulence and for the complex model that includes both small and large scale turbulence. We show that the electron heat transport in the multi-scale turbulence is not always the sum of the independent contributions of each component. Strong nonlinear effects can lead to diffusion coefficients that are much larger than the independent contributions. The conditions and the physical processes corresponding to these regimes are analyzed. (C) 2015 AIP Publishing LLC.