B. A. Carreras

Probabilistic transport models for plasma transport in the presence of critical thresholds: Beyond the diffusive paradigm

It is argued that the modeling of plasma transport in tokamaks may benefit greatly from extending the usual local paradigm to accommodate scale-free transport mechanisms. This can be done by combining Levy distributions and a nonlinear threshold condition within the continuous time random walk concept. The advantages of this nonlocal, nonlinear extension are illustrated by constructing a simple particle density transport model that, as a result of these ideas, spontaneously exhibits much of nondiffusive phenomenology routinely observed in tokamaks. The fluid limit of the system shows that the kind of equations that are appropriate to capture these dynamics are based on fractional differential operators. In them, effective diffusivities and pinch velocities are found that are dynamically set by the system in response to the specific characteristics of the fueling source and external perturbations. This fact suggests some dramatic consequences for the extrapolation of these transport properties to larger size systems.

Zonal flows and long-distance correlations during the formation of the edge shear layer in the TJ-II stellarator

A theoretical interpretation is given for the observed long-distance correlations in potential fluctuations in TJ-II. The value of the correlation increases above the critical point of the transition for the emergence of the plasma edge shear flow layer. Mean (i.e. surface averaged, zero-frequency) sheared flows cannot account for the experimental results. A model consisting of four envelope equations for the fluctuation level, the mean flow shear, the zonal flow amplitude shear and the averaged pressure gradient is proposed. It is shown that the presence of zonal flows is essential to reproduce the main features of the experimental observations.

Topological structures of the resistive pressure gradient turbulence with averaged poloidal flow

When a significant averaged poloidal flow is generated by the resistive pressure-gradient-driven turbulence the topological properties of the flow structures can change in some radial regions where the shear flow is large. We have applied the topological analysis approach that we have developed (2013 J. Phys. A: Math. Theor. 46 375501) to this situation and found that in addition to the filamentary vortex structures there are deformed toroidal structures that seem to act as transport barriers. Analysis of all these structures is presented here.

Three-dimensional MHD analysis of heliotron plasma with RMP

The interaction between pressure driven modes and magnetic islands generated by a resonant magnetic perturbation (RMP) in the large helical device (LHD) is numerically analyzed. In this analysis, three-dimensional treatment is essential in the equilibrium and dynamics calculations, because the equilibrium pressure profile is deformed by the RMP. The deformation changes the linear mode structure from the interchange type to the ballooning-like type that is localized around the X-point of the island in the equilibrium magnetic field including the RMP. This mode causes a pressure collapse in the nonlinear evolution, which spreads from the X-point to the core. Therefore, the spatial phase of the collapse is fixed to the island geometry. The fixed phase agrees with the LHD experimental results with a natural error field.

The impact of rational surfaces on radial heat transport in TJ-II

In this work, we study the outward propagation of temperature perturbations. For this purpose, we apply an advanced analysis technique, transfer entropy, to ECE measurements performed in ECR heated discharges at the low-shear stellarator TJ-II. We observe that the propagation of these perturbations is not smooth, but is slowed down at specific radial positions, near trapping zones characterized by long time lags with respect to the perturbation origin. We also detect instances of rapid or instantaneous (non-local) propagation, in which perturbations appear to jump over specific radial regions. The analysis of perturbations introduced in a resistive magneto-hydrodynamic model of the plasma leads to similar results. The radial regions corresponding to slow radial transport are identified with maxima of the flow shear associated with rational surfaces (mini-transport barriers). The non-local interactions are ascribed to MHD mode coupling effects.

A topological analysis of plasma flow structures

A new topological analysis of the plasma flow structures is presented for some pressure-gradient-driven turbulence results. The analysis is done by separating the structures into radial layers and studying each layer separately. This allows for the identification of flow cycles and flow filaments and the determination of the life of the cycles and length of the filaments.

Self-organized criticality in a cold plasma

We present direct evidence for the existence of self-organized critical behavior in cold plasma. A multiple anodic double layer structure generated in a double discharge plasma setup shows critical behavior for the anode bias above a threshold value. Analysis of the floating potential fluctuations reveals the existence of long-range time correlations and power law behavior in the tail of the probability distribution function of the fluctuations. The measured Hurst exponent and the power law tail in the rank function are strong indication of the self-organized critical behavior of the system and hence provide a condition under which complexities arise in cold plasma.

A possible mechanism for confinement power degradation in the TJ-II stellarator

This work uses the outward propagation of spontaneously generated fluctuations of the electron temperature to study heat transport in the TJ-II stellarator. Data from a set of experiments in which the heating power was scanned systematically are analyzed using the transfer entropy. The transfer entropy graph suggests there are at least two modes or channels of propagation: one channel is continuous, reminiscent of diffusion, while the other is non-local, activated mainly when the heating power is large. When the heating power is increased, the region of non-locality expands outwards, leading to the ubiquitously observed deterioration of confinement with heating power.This work uses the outward propagation of spontaneously generated fluctuations of the electron temperature to study heat transport in the TJ-II stellarator. Data from a set of experiments in which the heating power was scanned systematically are analyzed using the transfer entropy. The transfer entropy graph suggests there are at least two modes or channels of propagation: one channel is continuous, reminiscent of diffusion, while the other is non-local, activated mainly when the heating power is large. When the heating power is increased, the region of non-locality expands outwards, leading to the ubiquitously observed deterioration of confinement with heating power.

Applicability of transfer entropy for the calculation of effective diffusivity in heat transport

A method has been proposed to study heat transport in magnetically confinement plasmas, based on the transfer entropy (TE). In this work, we study this method by introducing perturbations in simulations made using a resistive Magneto-HydroDynamic model. The evolution of radial heat transport is monitored using the TE, and these results are used to compute an effective heat diffusivity. This effective diffusivity is then compared to estimates from other methods. The analysis is applied to several numerical simulations and in various radial ranges. It is shown that the transfer entropy is a suitable technique to analyze heat transport and evaluate an effective diffusivity in fusion plasmas.A method has been proposed to study heat transport in magnetically confinement plasmas, based on the transfer entropy (TE). In this work, we study this method by introducing perturbations in simulations made using a resistive Magneto-HydroDynamic model. The evolution of radial heat transport is monitored using the TE, and these results are used to compute an effective heat diffusivity. This effective diffusivity is then compared to estimates from other methods. The analysis is applied to several numerical simulations and in various radial ranges. It is shown that the transfer entropy is a suitable technique to analyze heat transport and evaluate an effective diffusivity in fusion plasmas.

Multi-scale MHD analysis of LHD plasma with background field changing

The mechanism of the partial collapse observed in the experiment with the background magnetic field changing in the Large Helical Device (LHD) is numerically investigated with a nonlinear magnetohydrodynamics (MHD) simulation. Since the different timescales of the perturbations and the background field changing have to be treated simultaneously for the analysis of this plasma, a multi-scale simulation scheme is developed. The effect of the perturbation dynamics on the equilibrium pressure and rotational transform is taken into account in this scheme. The result indicates that the collapse is caused by the destabilization of an infernal-like mode due to the magnetic hill enhanced by the change of the background field. The mechanism of the reduction of the central beta observed after the partial collapse in the experiment is also analysed in relation to the effect of the background field changing.