Characterization of Turbulence in Terms of Probability Density Function

Abstract

A new approach, based on multi-field probability density function statistical analysis, is proposed to describe transport in non-equilibrium systems and to unravel the overall picture connecting transport, gradients and flows in fusion plasmas. Based on this approach, significant improvement in our understanding of the statistical properties of fluctuations, the non-linear link between transport and gradients and the physics of flows have been recently achieved in fusion plasmas. An empirical similarity in the scaling properties of the probability distribution function (PDF) of turbulent transport has been observed in the plasma edge region in fusion plasmas. The investigation of the dynamical interplay between fluctuation in gradients, turbulent transport and radial electric fields has shown that these parameters are strongly coupled. The bursty behavior of turbulent transport is linked with a departure from the most probable radial gradient. The dynamical relation between fluctuations in gradients and transport is strongly affected by the presence of sheared poloidal flows which organized themselves near marginal stability. Experimental results show that there is a dynamical relationship between transport and parallel flows, showing that turbulent transport can drive parallel flows in the plasma boundary of fusion plasmas. These results emphasize the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the traditional way to characterize transport based on the computation of effective transport coefficients.