Characterization of local turbulence in magnetic confinement devices

Abstract

A multifractal analysis based on evaluation and interpretation of large deviation spectra is applied to plasma edge turbulence data from different devices (MAST and Tore Supra). It is demonstrated that in spite of some universal features there are unique characteristics for each device as well as for different confinement regimes. In the second part of the exposition the issue of estimating the variable power law behavior of spectral densities is addressed. The analysis of this issue is performed using fractional Brownian motion (fBm) as the underlying stochastic model whose parameters are estimated locally in time by wavelet scale spectra. In this manner information about the inertial range as well as variability of the fBm parameters is obtained giving more information important for understanding edge turbulence and intermittency.

Time-Resolved Measurements of Suprathermal Ion Transport Induced by Intermittent Plasma Blob Filaments

“…In fusion experiments and astrophysical plasmas, measurements are limited to a few positions by high-temperature and diagnostics accessibility. In those cases, it is not possible to characterize the transport by the temporal evolution of the variance of displacements and information about the transport has to be inferred from the time trace statistics [11][12][13].In this Letter, we present first time-resolved measurements of the cross-field transport of suprathermal ions in a turbulent magnetized plasma. Previously, in the TORPEX device [14], by using three-dimensional time-averaged measurements of the width of a suprathermal ion beam in combination with numerical simulations (an example is shown in Fig.…”

“…In fusion experiments and astrophysical plasmas, measurements are limited to a few positions by high-temperature and diagnostics accessibility. In those cases, it is not possible to characterize the transport by the temporal evolution of the variance of displacements and information about the transport has to be inferred from the time trace statistics [11][12][13].…”

Characterization of Intermittency in Plasma Edge Turbulence

“…The multifractal spectrum is the one-dimensional curve f (α) where the abscissa represents Hölder (regularity) exponent α and the ordinate is the fractal co-dimension related to the probability of finding the singularities with a given exponent α within the analyzed signal. Details on the calculation of the large deviation spectrum (LDS), a specific type of the multifractal spectrum, may be found in [11] and [8]. The following measure is used in the calculation of LDS…”

“…www.cpp-journal.org instead of the ones proposed in [4] while detailed justification of this choice is given in [8]. The LDS for three different modes of the MAST device and on five different scales, namely ∆t = 2 3 , 2 4 ...2 7 are presented in Figs.…”

“…The most striking feature of these spectra is their departure from a pure bell-shape and concavity and is a good example where LDS provide more information than traditionally used Legendre spectra. As explained in detail in [8], there are more multiplicative laws underlying the cascade process so that there is a lumping of measures whose supports are disjoint. Note the tendency in the spectra to increasingly separate out the concave part as the confinement regimes changes from L-to H-mode.…”