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).

Generalized self-similarity of intermittent plasma turbulence in space and laboratory plasmas

“…(2006), L-2 by Skvortsova et al. (2006), TJ-II by Alonso et al. (2006), on linear machines by Carter (2006), Chiu & Sen (2000), on reversed field pinches by Antoni et al.…”

“…Numerous Langmuir probe measurements were made on tokamaks: T-10 by Vershkov, Grashin & Chankin (1987), D-IIID by Boedo et al (2001), TEXTOR by Huber et al (2005), JET by Gonçalves et al (2005), Tore Supra by Antar, Counsell & Yu (2003), JT-60U by Asakura et al 2000, Asdex-U by Endler et al (1995), MAST by Antar, Counsell & Ahn (2005), on stellarators and helical devices: LHD by Ohno et al (2006), L-2 by Skvortsova et al (2006), TJ-II by Alonso et al (2006), on linear machines by Carter (2006), Chiu & Sen (2000), on reversed field pinches by Antoni et al (1998), on plasma device NAGDIS-II by Ohno et al (2004), PISCES by Schmitz et al (1990) and others fusion devices. The plasma fluctuations have been experimentally observed to be highly correlated and of constant amplitude along the magnetic field lines (over more than one toroidal turn in toroidal devices).…”

Evolution of statistical properties of microturbulence during transient process under electron cyclotron resonance heating of the L-2M stellarator plasma

“…This approach allows one to analyze nonstationary chaotic processes, including the process of plasma transition into the H-mode. It was found in [15] that the PDF of the fluctuation increments (first order differences) before the transition into the H-mode is described by a mixture of three Gaussian distributions and only by two ones after the transition.…”

Characterization of Intermittency in Plasma Edge Turbulence

“…It is well known that the probability density function (pdf) of plasma density fluctuations or the particle flux at the edge of fusion devices is skewed and non-Gaussian. Additional insight into the nature of intermittency may be obtained if statistical analysis of amplitudes of the density fluctuations (or the flux) is replaced by the analysis of its incremental values (essentially its first time derivative) [9]. One of the main objectives for the use of the original process (density or flux) at equidistant points in time for evaluating pdfs is that the obtained data do not represent homogeneous independent samples due to their long-range dependence thus hampering the interpretation of relevant statistical results.…”