" Superdiffusion and multifractal statistics of edge plasma turbulence in fusion devices "
V.P. Budaev (Nuclear Fusion Institute, RRC Kurchatov Institute)
Abstract:
A lot of experimental evidence in fusion devices have been observed that plasma turbulence is highly intermittent. Intermittent events are well-known to play a crucial role in transport dynamics. Intermittent transport resulted from rare, large events is accompanied by coherent structures such as vortices, zonal flows, streamers and blobs, leading to the losses above ones predicted by classic heat diffusive scaling. Specific structure of plasma near the edge in fusion device generates large-scale dynamics such as ''flights'' and ballistic transport with non-Gaussian statistics. Focusing on a probabilistic approach to plasma turbulence and transport, rather then anomalous transport coefficients, allows to consider most important questions in the prediction of transport, including self-similarity scaling and non-locality of transport in edge plasma turbulence. The turbulence in fusion device should be considered as a multi-scale phenomenon beyond the Kolmogorov's refined self-similarity (monofractal) hypothesis. To describe the turbulence, the multifractal formalism is considered. It is based on the fact that the highly nonuniform probability distribution arises from the nonuniformity of the system possessing rich scaling properties and self-similarity. In the multifractal formalism, multiplicative cascading process has "coarse" time scale from that iterating towards finer scales.
It is reported the analysis of edge plasma statistics in T-10 tokamak, linear machine NAGDIS-II and Large Helical Device (LHD), small tokamaks HYBTOK-II and CASTOR. Probability density function of fluctuation in ion saturation current and floating potential measured in scrape-off layer of these machines have been analyzed in terms of the multifractal formalism revisited with wavelets. It was observed that edge turbulence possesses multifractal statistics with typical coarse time scale ~50-100 mcs of coherent structures. Self-similarity parameters have been observed to depend on the edge plasma condition. The fractal properties of fluctuation in LHD have been analyzed regarding to the magnetic structure and profile of the connection lengths of the magnetic field lines. It was observed on T-10, multifractality level is decreased in shear region in the vicinity of Last Closed Flux Surface. Correlation properties have been analyzed regarding to the multifractality parameter.