Statistical analysis and modelling of turbulent fluxes in the plasma of the L-2M stellarator and the FT-2 tokamak

Abstract

A comparative analysis is done of data on turbulent fluxes measured in the edge plasma in the L-2M stellarator and the FT-2 tokamak. This analysis is performed using the estimation-maximization algorithm to determine finite mixtures of normal distributions that give the best fit to the probability distributions (PDFs) of the increments of turbulent fluxes. The resulting PDFs indicate non-Brownian motion of particles, and the weight of rare transport events (heavy tails of PDFs of fluxes) can be calculated. The number of the normal processes in model mixtures, as well as their weights in the PDFs of the increments of the turbulent flux, change during L–H transition in FT-2. Characteristically, the number of normal processes for the H-regime of FT-2 is the same as for the L-2M stellarator, and the parameters of Gaussians are also similar. This fact may be interpreted as an indirect indication that the L-2M stellarator operates in the mode of improved plasma confinement (which is consistent with the neoclassical theory for stellarators).