Statistical properties of turbulence: a new approach to characterize transport in fusion plasmas
A view of recent experimental results and progress in the characterization of the statistical properties of edge turbulence in fusion plasmas is given. The study of the dynamical interplay between fluctuations in gradients, turbulent transport and radial electric fields has shown that these parameters are strongly coupled both in tokamak and stellarator plasmas. The size of turbulent events increases when the plasma deviates from the average density gradient. The radial velocity of fluctuations is of the order of 20 m/s for transport events associated with a small deviation from the most probable gradient. On the contrary, the effective radial velocity increases up to 500 m/s for transport events in which the local gradient increases significantly above the most probable gradient. These results suggest a link between the size of transport events and the nature of transport (diffusive versus non-diffusive) in the plasma boundary region. The dynamical relationship between fluctuations in gradients and transport is strongly affected by the presence of sheared poloidal flows, heating power and the proximity to instability thresholds. There is also experimental evidence that the turbulent transport is strongly coupled with fluctuations in parallel flows, reflecting that parallel flows might, at least partially, be driven by turbulent mechanisms. Using the same methods developed for the study of turbulence, the statistical properties of the radial propagation of edge localized modes (ELMs) in the SOL of JET have also been investigated. ELM events propagate radially with effective velocities in the range of 1000 m/s, again showing a link between the radial velocity and the size of transport events. At such high velocities, the radial propagation competes strongly with parallel loss to the divertor plates and may therefore be an important mechanism transporting particles to the main chamber walls of fusion devices. This work emphasizes the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the traditional way of characterizing transport based on the computation of effective transport coefficients (i.e. diffusion coefficients) and on average quantities (i.e., average correlation lengths).