Angeles Pedrosa - Academia.edu (original) (raw)
Papers by Angeles Pedrosa
Physical Review Letters, 2013
The regulation of turbulent transport by zonal flows is studied experimentally on a flux surface ... more The regulation of turbulent transport by zonal flows is studied experimentally on a flux surface of the stellarator experiment TJ-K. Data of 128 Langmuir probes at different toroidal and poloidal positions on a single flux surface enable to measure simultaneously the zonal flow activity and the turbulent transport in great detail. A reduction of turbulent transport by 30 % during the zonal flow phase is found. It is shown that the reduction process is initiated by a modification in the cross phase between density and electric field followed by a reduction in the fluctuation levels, which sustain low transport levels on larger time scales than the zonal flow life time.
Nuclear Fusion, 2015
First observations of electrostatic potential variations within the flux surfaces of a toroidal m... more First observations of electrostatic potential variations within the flux surfaces of a toroidal magnetic confinement device are presented. Measurements are taken in the TJ-II stellarator with two distant Langmuir probe arrays. The edge floating potentials display differences of several tens of Volts in electron-root wave-heated plasmas. The differences are reduced for higher densities and lower electron temperatures after the ion-root electric field forms at the plasma edge. Neoclassical Monte Carlo simulations estimate the correct order of magnitude for the overall variation in potential and predict the trend observed with the radial electric field. However, for the specific location of the probes, the simulations give differences smaller than those observed experimentally.
Experimental evidence of E×B sheared flows linked to rational surfaces has been obtained in the p... more Experimental evidence of E×B sheared flows linked to rational surfaces has been obtained in the plasma edge region of the TJ-II stellarator. A possible explanation of the flow structure near the rational surface is the nonlinear beating of the magnetic field component of a vacuum field island with a plasma instability. To simulate the main characteristics of the experimental results, we use a resistive interchange model with the rotational transform profile determined by the vacuum magnetic field calculations.
The influence of the magnetic topology on plasma profiles and turbulence has been investigated in... more The influence of the magnetic topology on plasma profiles and turbulence has been investigated in ECH plasmas in the stellarator TJ-II, taking advantage of the flexibility of this almost shearless device. A wide range of edge rotational transform values can be attained, but the rotational transform profile can also be tailored by inducing currents using both ECCD and two sets of OH coils. In this way it is possible to introduce rational surfaces inside the plasma and to modify the magnetic shear to examine their effect on confinement. Kinetic effects and flux changes due to the presence of resonances and ECRH are responsible of the formation of barriers in the plasma core, while the shear flow is a key ingredient in the plasma edge. The results here shown offer wide and valuable information to assess multiple mechanisms based on neoclassical/turbulent bifurcations and kinetic effects as candidates to explain the impact of magnetic topology on radial electric fields and confinement.
Czechoslovak Journal of Physics, 2003
Apresenta-se uma descrição estatística dos processos de transporte em plasmas de fusão, em altern... more Apresenta-se uma descrição estatística dos processos de transporte em plasmas de fusão, em alternativa à abordagem tradicional baseada no cálculo dos coeficientes efectivos de transporte (coeficientes de difusão) e quantidades médias (comprimentos de correlação médios). As experiências foram realizadas na periferia dos tokamaks JET e ISTTOK e do stellarator TJ-II usando sondas de Langmuir. Esta descrição foi escolhida para evidenciar a relação dinâmica entre flutuação em gradientes, transporte turbulento e campo eléctrico radial. Estas quantidades estão fortemente acopladas e a sua relação é afectada pela presença de correntes poloidais de cisalhamento, potência de aquecimento e proximidade a limiares de instabilidade. Os resultados demonstram que, em plasmas de modo-L, o transporte e os gradientes se organizam de forma a estarem no limiar de um ponto de bifurcação, sendo o resultado consistente com o conceito de transporte turbulento auto-regulado por flutuações próximas da estabilidade marginal. Foram também encontradas evidências que o transporte turbulento está fortemente acoplado com flutuações nos fluxos paralelos, reflectindo que os fluxos paralelos terão, parcialmente, a sua origem em mecanismos turbulentos. Apresenta-se também o desenvolvimento do Detector Matricial Multicanal para o Diagnóstico de Feixe de Iões Pesados do stellarator TJ-II. Este detector permitirá a medida de flutuações de densidade permitindo a extensão dos métodos desenvolvidos ao interior do plasma.
Plasma Physics and Controlled Fusion, 2012
The dynamics of fluctuating electric field structures in the edge of the TJ-II stellarator, that ... more The dynamics of fluctuating electric field structures in the edge of the TJ-II stellarator, that display zonal flow-like traits, is studied. These structures have been shown to be global and affect particle transport dynamically [1]. In this article we discuss possible drive (Reynolds stress) and damping (Neoclassical viscosity, geodesic transfer) mechanisms for the associated E × B velocity. We show that: (a) while the observed turbulence-driven forces can provide the necessary perpendicular acceleration, a causal relation could not be firmly established, possibly because of the locality of the Reynolds stress measurements, (b) the calculated neoclassical viscosity and damping times are comparable to the observed zonal flow relaxation times, and (c) although an accompanying density modulation is observed to be associated to the zonal flow, it is not consistent with the excitation of pressure side-bands, like those present in geodesic acoustic oscillations, caused by the compression of the E × B flow field.
Plasma Physics and Controlled Fusion, 1999
ECR (electron cyclotron resonance) heated plasmas have been studied in the low magnetic shear TJ-... more ECR (electron cyclotron resonance) heated plasmas have been studied in the low magnetic shear TJ-II stellarator (R = 1.5 m, a < 0.22 m, B = 1 T, f = 53.2 GHz, P ECRH = 300 kW, power density = 1-25 W cm −3). Recent experiments have explored the flexibility of the TJ-II across a wide range of plasma volumes with different rotational transforms and rational surface densities. In this paper, the main results of this campaign are presented and, in particular, the influence of iota and rational surfaces on plasma profiles is discussed.
Plasma Physics and Controlled Fusion, 1998
The structure of fluctuations and turbulent transport have been investigated in the plasma bounda... more The structure of fluctuations and turbulent transport have been investigated in the plasma boundary region of the L2-M stellarator. Normalized fluctuation levels are in the range (3-20)% and fluctuations are dominated by frequencies below 300 kHz. In the edge plasma region located inside the last closed magnetic flux surface the radial coherence of fluctuations is due to high-frequency fluctuations (>100 kHz). The poloidal coherence is dominated by low frequencies. Linear coupling of resistive interchange modes is considered a candidate to explain the existence of highly radially correlated fluctuations in the high-frequency range.
Plasma Physics and Controlled Fusion, 2000
A view of the latest experimental results and progress in the understanding of the role of poloid... more A view of the latest experimental results and progress in the understanding of the role of poloidal flows driven by fluctuations via Reynolds stress is given. Reynolds stress shows a radial gradient close to the velocity shear layer location in tokamaks and stellarators, indicating that this mechanism may drive significant poloidal flows in the plasma boundary. Observation of the generation of E × B sheared flows via Reynolds stress at the ion Bernstein resonance layer has been noticed in toroidal magnetized plasmas. The experimental evidence of sheared E × B flows linked to the location of rational surfaces in stellarator plasmas might be interpreted in terms of Reynolds stress sheared driven flows. These results show that E × B sheared flows driven by fluctuations can play an important role in the generation of transport barriers.
Plasma Physics and Controlled Fusion, 1999
First plasmas have been successfully achieved in the TJ-II stellarator using electron cyclotron r... more First plasmas have been successfully achieved in the TJ-II stellarator using electron cyclotron resonance heating (f = 53.2 GHz, P ECRH = 250 kW). Initial experiments have explored the TJ-II flexibility in a wide range of plasma volumes, different rotational transform and magnetic well values. In this paper, the main results of this campaign are presented and, in particular, the influence of plasma wall interaction phenomena on TJ-II operation is discussed briefly.
Physics of Plasmas, 2005
͑1990͔͒ are shown to obey a non-Gaussian but apparently universal ͑i.e., not dependent on device ... more ͑1990͔͒ are shown to obey a non-Gaussian but apparently universal ͑i.e., not dependent on device and discharge parameters͒ probability density distribution ͑pdf͒. The fact that a specific shape acts as an attractor for the pdf seems to suggest that emergent behavior and self-regulation are relevant concepts for these fluctuations. This shape is closely similar to the so-called Bramwell, Holdsworth, and Pinton distribution, which does not have any free parameters.
Physics of Plasmas, 1999
Frequency spectra of fluctuations for the ion saturation current, floating potential, and turbule... more Frequency spectra of fluctuations for the ion saturation current, floating potential, and turbulent transport measured in the plasma edge of plasma confinement experiments ͑tokamaks and stellarators͒ have been analyzed to identify the frequency ranges characterized by a power dependence. Three main regions can be identified. For the intermediate frequency region, the decay of the spectra is close to 1/f , as is expected in self-organized criticality systems. This region is particularly important for the role that it plays in plasma transport and the self-similarity of the fluctuations and fluxes. The effect of plasma rotation on the decay indices has also been studied.
Physics of Plasmas, 2000
The statistical properties of plasma fluctuations have been investigated in the plasma boundary r... more The statistical properties of plasma fluctuations have been investigated in the plasma boundary region of fusion ͑tokamaks and stellarators͒ 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 (rϽr sh) or into the scrape-off layer region (rϾr 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.
Physics of Plasmas, 1999
To better understand long time transport dynamics, techniques to investigate long-range dependenc... more To better understand long time transport dynamics, techniques to investigate long-range dependences in plasma fluctuations have been applied to data from several confinement devices including tokamaks, stellarators, and reversed field pinch. The results reveal the self-similar character of the edge plasma fluctuations. This implies that the tail of the autocorrelation function decays as a power law and suggests that there is a superdiffusive component of the anomalous transport. Rescaled fluctuation and turbulent flux spectra from different devices also show a strong similarity. For a range of parameters corresponding to the tokamak ohmic regime and equivalent power for other devices, the spectral decay index may show a universal character.
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. Ascasibar et al., in Plasma Physics and Controlled Fusion Research, Proceedings of the 15th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville ͑International Atomic Energy Agency, Vienna, in press͔͒. There is a striking similarity between the statistical properties of turbulent transport in both devices.
Physics of Plasmas, 2001
There is some experimental evidence that the EϫB flows have radial structure that may be linked t... more There is some experimental evidence that the EϫB flows have radial structure that may be linked to rational surfaces. This flow structure may result from a self-organization process involving nonlinear flow amplification through Reynolds stress and fluctuation reduction by sheared flows. In stellarators, a large contribution to the Reynolds stress comes from the coupling of the magnetic field component of a vacuum field island with a plasma instability. In this process, the self-organization principle seems to be marginal stability for the fluctuations driving the flow.
Physical Review Letters, 1998
Analysis of the edge plasma fluctuation in several confinement devices reveals the self-similar c... more Analysis of the edge plasma fluctuation in several confinement devices reveals the self-similar character of the fluctuations through the presence of long-range time correlations. 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 on the order of the particle diffusion time. The algebraic decay of the longrange time correlations is consistent with plasma transport characterized by self-organized criticality.
Physical Review Letters, 1999
Frequency spectra of fluctuations of the ion saturation current, floating potential, and turbulen... more Frequency spectra of fluctuations of the ion saturation current, floating potential, and turbulent transport measured in the plasma edge of different fusion devices (tokamaks and stellarators) have been compared. All of the spectra show the same behavior over the whole frequency range investigated, which supports universality of plasma turbulence or turbulent transport. The results obtained are an indication of edge-plasma turbulence evolving into a critical state, independent of the size and plasma characteristics of the device. [S0031-9007(99)09024-9]
Physical Review Letters, 2013
The regulation of turbulent transport by zonal flows is studied experimentally on a flux surface ... more The regulation of turbulent transport by zonal flows is studied experimentally on a flux surface of the stellarator experiment TJ-K. Data of 128 Langmuir probes at different toroidal and poloidal positions on a single flux surface enable to measure simultaneously the zonal flow activity and the turbulent transport in great detail. A reduction of turbulent transport by 30 % during the zonal flow phase is found. It is shown that the reduction process is initiated by a modification in the cross phase between density and electric field followed by a reduction in the fluctuation levels, which sustain low transport levels on larger time scales than the zonal flow life time.
Nuclear Fusion, 2015
First observations of electrostatic potential variations within the flux surfaces of a toroidal m... more First observations of electrostatic potential variations within the flux surfaces of a toroidal magnetic confinement device are presented. Measurements are taken in the TJ-II stellarator with two distant Langmuir probe arrays. The edge floating potentials display differences of several tens of Volts in electron-root wave-heated plasmas. The differences are reduced for higher densities and lower electron temperatures after the ion-root electric field forms at the plasma edge. Neoclassical Monte Carlo simulations estimate the correct order of magnitude for the overall variation in potential and predict the trend observed with the radial electric field. However, for the specific location of the probes, the simulations give differences smaller than those observed experimentally.
Experimental evidence of E×B sheared flows linked to rational surfaces has been obtained in the p... more Experimental evidence of E×B sheared flows linked to rational surfaces has been obtained in the plasma edge region of the TJ-II stellarator. A possible explanation of the flow structure near the rational surface is the nonlinear beating of the magnetic field component of a vacuum field island with a plasma instability. To simulate the main characteristics of the experimental results, we use a resistive interchange model with the rotational transform profile determined by the vacuum magnetic field calculations.
The influence of the magnetic topology on plasma profiles and turbulence has been investigated in... more The influence of the magnetic topology on plasma profiles and turbulence has been investigated in ECH plasmas in the stellarator TJ-II, taking advantage of the flexibility of this almost shearless device. A wide range of edge rotational transform values can be attained, but the rotational transform profile can also be tailored by inducing currents using both ECCD and two sets of OH coils. In this way it is possible to introduce rational surfaces inside the plasma and to modify the magnetic shear to examine their effect on confinement. Kinetic effects and flux changes due to the presence of resonances and ECRH are responsible of the formation of barriers in the plasma core, while the shear flow is a key ingredient in the plasma edge. The results here shown offer wide and valuable information to assess multiple mechanisms based on neoclassical/turbulent bifurcations and kinetic effects as candidates to explain the impact of magnetic topology on radial electric fields and confinement.
Czechoslovak Journal of Physics, 2003
Apresenta-se uma descrição estatística dos processos de transporte em plasmas de fusão, em altern... more Apresenta-se uma descrição estatística dos processos de transporte em plasmas de fusão, em alternativa à abordagem tradicional baseada no cálculo dos coeficientes efectivos de transporte (coeficientes de difusão) e quantidades médias (comprimentos de correlação médios). As experiências foram realizadas na periferia dos tokamaks JET e ISTTOK e do stellarator TJ-II usando sondas de Langmuir. Esta descrição foi escolhida para evidenciar a relação dinâmica entre flutuação em gradientes, transporte turbulento e campo eléctrico radial. Estas quantidades estão fortemente acopladas e a sua relação é afectada pela presença de correntes poloidais de cisalhamento, potência de aquecimento e proximidade a limiares de instabilidade. Os resultados demonstram que, em plasmas de modo-L, o transporte e os gradientes se organizam de forma a estarem no limiar de um ponto de bifurcação, sendo o resultado consistente com o conceito de transporte turbulento auto-regulado por flutuações próximas da estabilidade marginal. Foram também encontradas evidências que o transporte turbulento está fortemente acoplado com flutuações nos fluxos paralelos, reflectindo que os fluxos paralelos terão, parcialmente, a sua origem em mecanismos turbulentos. Apresenta-se também o desenvolvimento do Detector Matricial Multicanal para o Diagnóstico de Feixe de Iões Pesados do stellarator TJ-II. Este detector permitirá a medida de flutuações de densidade permitindo a extensão dos métodos desenvolvidos ao interior do plasma.
Plasma Physics and Controlled Fusion, 2012
The dynamics of fluctuating electric field structures in the edge of the TJ-II stellarator, that ... more The dynamics of fluctuating electric field structures in the edge of the TJ-II stellarator, that display zonal flow-like traits, is studied. These structures have been shown to be global and affect particle transport dynamically [1]. In this article we discuss possible drive (Reynolds stress) and damping (Neoclassical viscosity, geodesic transfer) mechanisms for the associated E × B velocity. We show that: (a) while the observed turbulence-driven forces can provide the necessary perpendicular acceleration, a causal relation could not be firmly established, possibly because of the locality of the Reynolds stress measurements, (b) the calculated neoclassical viscosity and damping times are comparable to the observed zonal flow relaxation times, and (c) although an accompanying density modulation is observed to be associated to the zonal flow, it is not consistent with the excitation of pressure side-bands, like those present in geodesic acoustic oscillations, caused by the compression of the E × B flow field.
Plasma Physics and Controlled Fusion, 1999
ECR (electron cyclotron resonance) heated plasmas have been studied in the low magnetic shear TJ-... more ECR (electron cyclotron resonance) heated plasmas have been studied in the low magnetic shear TJ-II stellarator (R = 1.5 m, a < 0.22 m, B = 1 T, f = 53.2 GHz, P ECRH = 300 kW, power density = 1-25 W cm −3). Recent experiments have explored the flexibility of the TJ-II across a wide range of plasma volumes with different rotational transforms and rational surface densities. In this paper, the main results of this campaign are presented and, in particular, the influence of iota and rational surfaces on plasma profiles is discussed.
Plasma Physics and Controlled Fusion, 1998
The structure of fluctuations and turbulent transport have been investigated in the plasma bounda... more The structure of fluctuations and turbulent transport have been investigated in the plasma boundary region of the L2-M stellarator. Normalized fluctuation levels are in the range (3-20)% and fluctuations are dominated by frequencies below 300 kHz. In the edge plasma region located inside the last closed magnetic flux surface the radial coherence of fluctuations is due to high-frequency fluctuations (>100 kHz). The poloidal coherence is dominated by low frequencies. Linear coupling of resistive interchange modes is considered a candidate to explain the existence of highly radially correlated fluctuations in the high-frequency range.
Plasma Physics and Controlled Fusion, 2000
A view of the latest experimental results and progress in the understanding of the role of poloid... more A view of the latest experimental results and progress in the understanding of the role of poloidal flows driven by fluctuations via Reynolds stress is given. Reynolds stress shows a radial gradient close to the velocity shear layer location in tokamaks and stellarators, indicating that this mechanism may drive significant poloidal flows in the plasma boundary. Observation of the generation of E × B sheared flows via Reynolds stress at the ion Bernstein resonance layer has been noticed in toroidal magnetized plasmas. The experimental evidence of sheared E × B flows linked to the location of rational surfaces in stellarator plasmas might be interpreted in terms of Reynolds stress sheared driven flows. These results show that E × B sheared flows driven by fluctuations can play an important role in the generation of transport barriers.
Plasma Physics and Controlled Fusion, 1999
First plasmas have been successfully achieved in the TJ-II stellarator using electron cyclotron r... more First plasmas have been successfully achieved in the TJ-II stellarator using electron cyclotron resonance heating (f = 53.2 GHz, P ECRH = 250 kW). Initial experiments have explored the TJ-II flexibility in a wide range of plasma volumes, different rotational transform and magnetic well values. In this paper, the main results of this campaign are presented and, in particular, the influence of plasma wall interaction phenomena on TJ-II operation is discussed briefly.
Physics of Plasmas, 2005
͑1990͔͒ are shown to obey a non-Gaussian but apparently universal ͑i.e., not dependent on device ... more ͑1990͔͒ are shown to obey a non-Gaussian but apparently universal ͑i.e., not dependent on device and discharge parameters͒ probability density distribution ͑pdf͒. The fact that a specific shape acts as an attractor for the pdf seems to suggest that emergent behavior and self-regulation are relevant concepts for these fluctuations. This shape is closely similar to the so-called Bramwell, Holdsworth, and Pinton distribution, which does not have any free parameters.
Physics of Plasmas, 1999
Frequency spectra of fluctuations for the ion saturation current, floating potential, and turbule... more Frequency spectra of fluctuations for the ion saturation current, floating potential, and turbulent transport measured in the plasma edge of plasma confinement experiments ͑tokamaks and stellarators͒ have been analyzed to identify the frequency ranges characterized by a power dependence. Three main regions can be identified. For the intermediate frequency region, the decay of the spectra is close to 1/f , as is expected in self-organized criticality systems. This region is particularly important for the role that it plays in plasma transport and the self-similarity of the fluctuations and fluxes. The effect of plasma rotation on the decay indices has also been studied.
Physics of Plasmas, 2000
The statistical properties of plasma fluctuations have been investigated in the plasma boundary r... more The statistical properties of plasma fluctuations have been investigated in the plasma boundary region of fusion ͑tokamaks and stellarators͒ 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 (rϽr sh) or into the scrape-off layer region (rϾr 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.
Physics of Plasmas, 1999
To better understand long time transport dynamics, techniques to investigate long-range dependenc... more To better understand long time transport dynamics, techniques to investigate long-range dependences in plasma fluctuations have been applied to data from several confinement devices including tokamaks, stellarators, and reversed field pinch. The results reveal the self-similar character of the edge plasma fluctuations. This implies that the tail of the autocorrelation function decays as a power law and suggests that there is a superdiffusive component of the anomalous transport. Rescaled fluctuation and turbulent flux spectra from different devices also show a strong similarity. For a range of parameters corresponding to the tokamak ohmic regime and equivalent power for other devices, the spectral decay index may show a universal character.
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. Ascasibar et al., in Plasma Physics and Controlled Fusion Research, Proceedings of the 15th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville ͑International Atomic Energy Agency, Vienna, in press͔͒. There is a striking similarity between the statistical properties of turbulent transport in both devices.
Physics of Plasmas, 2001
There is some experimental evidence that the EϫB flows have radial structure that may be linked t... more There is some experimental evidence that the EϫB flows have radial structure that may be linked to rational surfaces. This flow structure may result from a self-organization process involving nonlinear flow amplification through Reynolds stress and fluctuation reduction by sheared flows. In stellarators, a large contribution to the Reynolds stress comes from the coupling of the magnetic field component of a vacuum field island with a plasma instability. In this process, the self-organization principle seems to be marginal stability for the fluctuations driving the flow.
Physical Review Letters, 1998
Analysis of the edge plasma fluctuation in several confinement devices reveals the self-similar c... more Analysis of the edge plasma fluctuation in several confinement devices reveals the self-similar character of the fluctuations through the presence of long-range time correlations. 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 on the order of the particle diffusion time. The algebraic decay of the longrange time correlations is consistent with plasma transport characterized by self-organized criticality.
Physical Review Letters, 1999
Frequency spectra of fluctuations of the ion saturation current, floating potential, and turbulen... more Frequency spectra of fluctuations of the ion saturation current, floating potential, and turbulent transport measured in the plasma edge of different fusion devices (tokamaks and stellarators) have been compared. All of the spectra show the same behavior over the whole frequency range investigated, which supports universality of plasma turbulence or turbulent transport. The results obtained are an indication of edge-plasma turbulence evolving into a critical state, independent of the size and plasma characteristics of the device. [S0031-9007(99)09024-9]