[en] An analysis of tokamak density fluctuations data permits the determination of two characteristic exponents. The exponents correspond to the powers of a power-law dependence of the distributions of the long-lasting monotonic change (''flight'') of the density and the time length of these changes. Speculation based on these results leads to construction of the fractional kinetic equation for the distribution function of the flights. The asymptotic transport properties of the particle density distribution function are directly connected with the exponents obtained from the density fluctuations data. (c) 2000 American Institute of Physics

[en] Measurements of parallel and perpendicular ion temperatures in the Large Experiment on Instabilities and Anisotropies (LEIA) space simulation chamber display an inverse correlation between the upper bound on the ion temperature anisotropy and the parallel ion beta (β=8πnkT/B²). Fluctuation measurements indicate the presence of low frequency, transverse, electromagnetic waves with wave numbers and frequencies that are consistent with predictions for Alfven Ion Cyclotron instabilities. These observations are also consistent with in situ spacecraft measurements in the Earth's magnetosheath and with a theoretical/computational model that predicts that such an upper bound on the ion temperature anisotropy is imposed by scattering from enhanced fluctuations due to growth of the Alfven ion cyclotron instability. (c) 2000 American Institute of Physics

[en] Some aspects of low-frequency, long-wavelength fluctuations are considered. A stochastic model is used to show that power-law time correlations need not arise from self-organized criticality. A formula for the frequency spectrum of uncorrelated, overlapping avalanches is shown to be a special case of the spectral balance equation of renormalized statistical turbulence theory. It is argued that there need be no contradiction between the presence of long-time correlations and the existence of local transport coefficients. (c) 2000 American Institute of Physics

[en] Broadband reflectometry is a current technique that uses the round-trip group delays of reflected frequency-swept waves to measure density profiles of fusion plasmas. The main factor that may limit the accuracy of the reconstructed profiles is the interference of the probing waves with the plasma density fluctuations: plasma turbulence leads to random phase variations and magneto hydrodynamic activity produces mainly strong amplitude and phase modulations. Both effects cause the decrease, and eventually loss, of signal at some frequencies. Several data processing techniques can be applied to filter and/or interpolate noisy group delay data obtained from turbulent plasmas with a single frequency sweep. Here, we propose a more powerful algorithm performing two-dimensional regularization (in space and time) of data provided by multiple consecutive frequency sweeps, which leads to density profiles with improved accuracy. The new method is described and its application to simulated data corrupted by noise and missing data is considered. It is shown that the algorithm improves the identification of slowly varying plasma density perturbations by attenuating the effect of fast fluctuations and noise contained in experimental data. First results obtained with this method in ASDEX Upgrade tokamak are presented. copyright 1997 American Institute of Physics

[en] Plasma edge fluctuations show a degree of intermittency similar to fluid turbulence. Using fluctuation measurements obtained with Langmuir probe data from two confinement devices, it is shown that plasma fluctuations have a multifractal character over the fluctuation range of scales with intermittency levels comparable to the levels measured in neutral fluid turbulence. In the mesoscale range, that is, for time scales between 10 times the turbulence decorrelation time and plasma confinement time, plasma fluctuations have a structure closer to a monofractal with very low intermittency. (c) 2000 American Institute of Physics

[en] Recent results on multimode feedback control of magnetohydrodynamic (MHD) modes and a variety of diagnostic uses of feedback are summarized. First, is the report on reduction and scaling of transport under feedback. By controlling the fluctuation amplitudes and consequently the transport via feedback, it is found that the scaling of the diffusion coefficient is linear with root-mean-square rms fluctuation level. The scaling appears not to agree with any generic theory. A variety of other diagnostic uses of feedback have been developed. The primary goal is an experimental methodology for the determination of dynamic models of plasma turbulence, both for better transport understanding and more credible feedback controller designs. A specific motivation is to search for a low-order dynamic model, suitable for the convenient study of both transport and feedback. First, the time series analysis method is used for the determination of chaotic attractor dimension of plasma fluctuations. For ExB rotational flute modes it is found to be close to three, indicating that a low-order dynamic model may be adequate for transport prediction and feedback controller design. Second, a new method for direct experimental determination of nonlinear dynamical models of plasma turbulence using feedback has been developed. Specifically, the process begins with a standard three-wave coupling model and introduces a variable feedback gain. The power spectrum, delayed power spectrum, and bispectrum of fluctuations are then experimentally obtained. By varying the feedback gain continuously, an arbitrary number of numerical equations for a fixed number of unknowns can be generated. Their numerical solution yields the linear dispersion, as well as nonlinear coupling coefficients. This method has been successfully applied for ExB rotationally driven flute modes. (c) 2000 American Institute of Physics

[en] Nonlinear bounce-gyrocenter Hamilton equations for full electromagnetic field fluctuations in general magnetic geometry are derived by the phase-space Lagrangian Lie-perturbation method. These reduced dynamical equations can be used to follow the orbits of magnetically trapped charged particles in the presence of low-frequency electromagnetic fluctuations in magnetic-field geometries suitable for applications in fusion and space plasma physics. (c) 2000 American Institute of Physics

[en] A new version of the Brownian motion describing the motion of a dust particle in a turbulent plasma is considered. Here, the stochastic force acting on the dust particle is due to the fluctuations of the plasma potential and not due to the usual molecular collisions. Another significant difference is due to the fact that the dust electric charge is not constant but fluctuates with the potential. A four-dimensional formulation of the problem is also given. copyright 1997 American Institute of Physics

[en] The statistical properties of plasma fluctuations have been investigated in the plasma boundary region of fusion (tokamaks and stellarators) and nonfusion plasmas. Fluctuations in ion saturation current and floating potential have a near-Gaussian character in the proximity of the velocity shear layer (r_sh). However, fluctuations deviate from a Gaussian distribution when moving inside of the plasma edge (r< r_sh) or into the scrape-off layer region (r>r_sh). Furthermore, fluctuations show sporadic pulses that are asymmetric in time. The present analysis shows a coupling of those pulses and the averaged flow in the shear layer region. (c) 2000 American Institute of Physics

[en] Highly simplified models of random flows interacting with background microturbulence are analyzed. In the limit of very rapid velocity fluctuations, it is shown rigorously that the fluctuation level of a passively advected scalar is not controlled by the rms shear. In a model with random velocities dependent only on time, the level of cross-correlations between the flows and the background turbulence regulates the saturation level. This effect is illustrated by considering a simple stochastic-oscillator model, both exactly and with analysis and numerical solutions of the direct-interaction approximation. Implications for the understanding of self-consistent turbulence are discussed briefly. (c) 2000 American Institute of Physics