Carlos Hidalgo - Academia.edu (original) (raw)

Papers by Carlos Hidalgo

Physics of Plasmas, 1998

The rescaled range analysis techniques are used to investigate long-range dependence in plasma ed... more The rescaled range analysis techniques are used to investigate long-range dependence in plasma edge fluctuations ͓Mandelbrot and Wallis, Water Resources Res. 4, 909 ͑1969͔͒. This technology has been applied to data from several confinement devices such as tokamaks, stellarators, and reversed-field pinch. The results reveal the self-similar character of the electrostatic fluctuations at the plasma edge with self-similarity parameters ranging from 0.62 to 0.72. These results show that the tail of the autocorrelation function decays as a power law for time lags longer than the decorrelation time and as long as times of the order of the confinement time. In cold plasma devices (T e Ͻ1 eV at the core͒, there is no evidence of algebraic tails in the autocorrelation function. Some other characteristic features of the autocorrelation function and power spectrum have been investigated. All of these features are consistent with plasma transport as characterized by self-organized criticality.

Physics of Plasmas, 1996

Fluctuation-induced fluxes have a bursty character. As a consequence, a significant part of the t... more Fluctuation-induced fluxes have a bursty character. As a consequence, a significant part of the total particle flux is carried out by sporadic, large transport bursts. The local flux distribution function is consistent with a near-Gaussian character of the fluctuations. The radial dependence of the statistical properties of plasma fluctuations and induced fluxes have been investigated in the plasma boundary region of the TJ-I tokamak ͓I. García-Cortés et al., Phys. Fluids B 4, 4007 ͑1992͔͒ and the TJ-IU torsatron ͓E.

Physics of Plasmas, 1995

A recently introduced tool for the analysis of turbulence, wavelet bicoherence [B. Ph. van Millig... more A recently introduced tool for the analysis of turbulence, wavelet bicoherence [B. Ph. van Milligen, C. Hidalgo and E. Sánchez, Phys. Rev. Lett. 16 (1995) 395], is investigated. It is capable of detecting phase coupling -nonlinear interactions of the lowest (quadratic) orderwith time resolution. To demonstrate its potential, it is applied to numerical models of chaos and turbulence and to real measurements. It detected the coupling interaction between two coupled van der Pol oscillators. When applied to a model of drift wave turbulence relevant to plasma physics, it detected a highly localized coherent structure. Analyzing reflectometry measurements made in fusion plasmas, it detected temporal intermittency and a strong increase in nonlinear phase coupling coinciding with the L/H (Low -to -High confinement mode) transition.

Physical Review Letters, 1993

Physical Review Letters, 1995

A new technique combining wavelet analysis and bispectral analysis has been developed. This analy... more A new technique combining wavelet analysis and bispectral analysis has been developed. This analysis tool permits the detection of structure in turbulent or chaotic data with time resolution, even in the presence of a significant noise contribution. Application of this technique to data obtained in fusion plasmas with Langmuir probes demonstrates its possibilities by detecting short-lived intermittent nonlinear coupling. Its application in the field of chaos analysis is indicated.

Plasma Physics and Controlled Fusion, 2005

The influence of plasma density and edge gradients on the development of perpendicular sheared fl... more The influence of plasma density and edge gradients on the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. The development of the naturally occurring velocity shear layer requires a minimum plasma density. Experimental findings have shown that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increases. Electron density profiles show a broadening evolution as density increases above the critical value where sheared flow is developed, while the temperature profile remains similar, reflecting the strong impact of plasma density in the global confinement scaling. Furthermore, the shearing rate of the spontaneous sheared flow turns out to be close to the one needed to trigger a transition to improved confinement regimes. Density ramp experiments show, within the experimental uncertainty, no evidence of hysteresis during the spontaneous shear development. Power modulation, in the proximity of the critical plasma density, allows the characterization of plasma potential and electric field relaxation during the transition. The present results have a direct impact on the understanding of the physics mechanisms underlying the generation of critical sheared flow, pointing to the important role of turbulent driven flow. This paper was first presented at the EPS04 conference.

Physical Review Letters, 2000

We show that the modulational instability growth rate of zonal flows is determined directly from ... more We show that the modulational instability growth rate of zonal flows is determined directly from the quasilinear wave kinetic equation. We also demonstrate the relation between zonal-flow growth and the cross bispectrum of the high-frequency drift-wave-driven Reynolds stress and the low-frequency plasma potential by explicit calculation. Experimental measurements of the spatiotemporal evolution of the spectrum integrated bicoherence at the L ! H transition near the edge shear layer indicate a modification in the nonlinear phase coupling, which might be linked to the generation of sheared E 3 B flows.

Physical Review Letters, 1999

The probability distribution function of the turbulence-induced particle flux at the plasma edge ... more The probability distribution function of the turbulence-induced particle flux at the plasma edge has distinct functional forms over two distinct ranges of time scales. One range corresponds to the fluctuation time scales and the other one is the mesoscale range: time scales between the turbulence decorrelation and confinement time. In the second range, the probability distribution function is selfsimilar and essentially has only the outward flux tail. This structure reflects some of the mechanisms of the underlying turbulence. PACS numbers: 52.35.Ra, From the Langmuir probe measurements in low-power Ohmically heated or electron cyclotron heated plasma discharges and for several types of confinement devices, we have concluded that the electrostatic potential and density fluctuations at the plasma edge are self-similar over a broad range of time scales . The selfsimilarity range is, in general, for time scales longer than the turbulence decorrelation times up to times of the order of confinement time, the mesoscale range. The upper bound of the self-similarity range is difficult to determine because of the lengths of time records available and the varying plasma conditions on these longer time scales. It was found that for fluctuation measurements within the plasma confinement region the self-similarity parameter varies between H 0.62 and H 0.75, a relatively small range of variation given the diversity of plasma confinement devices considered.