Página 1 dos resultados de 83 itens digitais encontrados em 0.007 segundos

Multifractality in plasma edge electrostatic turbulence

RODRIGUES NETO, Camilo; GUIMARES FILHO, Zwinglio de Oliveira; CALDAS, Iberê Luiz; NASCIMENTO, Ivan Cunha; KUZNETSOV, Yurii
Fonte: AMER INST PHYSICS; NEW YORK Publicador: AMER INST PHYSICS; NEW YORK
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
56.2%
Plasma edge turbulence in Tokamak Chauffage Alfven Bresilien (TCABR) [R. M. O. Galvao et al., Plasma Phys. Contr. Fusion 43, 1181 (2001)] is investigated for multifractal properties of the fluctuating floating electrostatic potential measured by Langmuir probes. The multifractality in this signal is characterized by the full multifractal spectra determined by applying the wavelet transform modulus maxima. In this work, the dependence of the multifractal spectrum with the radial position is presented. The multifractality degree inside the plasma increases with the radial position reaching a maximum near the plasma edge and becoming almost constant in the scrape-off layer. Comparisons between these results with those obtained for random test time series with the same Hurst exponents and data length statistically confirm the reported multifractal behavior. Moreover, the persistence of these signals, characterized by their Hurst exponent, present radial profile similar to the deterministic component estimated from analysis based on dynamical recurrences. (C) 2008 American Institute of Physics.

Bicoherence in electrostatic turbulence driven by high magnetohydrodynamic activity in Tokamak Chauffage Alfven Bresilien

Lima, Gustavo Zampier dos Santos; Guimarães Filho, Zwinglio de Oliveira; Batista, Antonio Marcos; Caldas, Ibere Luiz; Lopes, Sergio Roberto; Viana, Ricardo Luiz; Nascimento, Ivan Cunha; Kuznetsov, Yurii
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
66.25%
During some discharges in Tokamak Chauffage Alfven Bresilien [R. M. O. Galvao et al., Plasma Phys. Controlled Fusion 43, 1181 (2001)] high magnetohydrodynamic activity may appear with a peaked frequency spectrum. Whenever this peak occurs, the ambient broadband electrostatic turbulence is remarkably modified, synchronizing into the dominant magnetic fluctuation frequency and presenting high bicoherence in the whole plasma edge with a maximum bicoherence inside the plasma. A phenomenological model is introduced to investigate this driven turbulence bicoherence, consisting of nonlinearly coupled phase-randomized drift modes with time-periodic external driving at the dominant magnetic fluctuation frequency. The bicoherence spectrum of this model can mimic features of the experimental results. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3099701]; FAPESP; CNPq; Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES); FINEP/CNEN

Electrostatic turbulence driven by high magnetohydrodynamic activity in Tokamak Chauffage Alfven Bresilien

Guimarães Filho, Zwinglio de Oliveira; Caldas, Ibere Luiz; Viana, Ricardo Luiz; Heller, Maria Vittoria Adelina Prario; Nascimento, Ivan Cunha; Kuznetsov, Yurii; Bengtson, Roger D
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
66.24%
In Tokamak Chauffage Alfven Bresilien [R. M. O. Galvao , Plasma Phys. Controlled Fusion 43, 1181 (2001)], high magnetohydrodynamic (MHD) activity may appear spontaneously or during discharges with a voltage biased electrode inserted at the plasma edge. The turbulent electrostatic fluctuations, measured by Langmuir probes, are modulated by Mirnov oscillations presenting a dominant peak with a common frequency around 10 kHz. We report the occurrence of phase locking of the turbulent potential fluctuations driven by MHD activity at this frequency. Using wavelet cross-spectral analysis, we characterized the phase and frequency synchronization in the plasma edge region. We introduced an order parameter to characterize the radial dependence of the phase-locking intensity. (c) 2008 American Institute of Physics.

Self-organized criticality in MHD driven plasma edge turbulence

Lima, Gustavo Zampier dos Santos; Iarosz, Kelly Cristiane; Batista, Antonio Marcos; Caldas, Ibere Luiz; Guimarães Filho, Zwinglio de Oliveira; Viana, Ricardo Luiz; Lopes, Sergio Roberto; Nascimento, Ivan Cunha; Kuznetsov, Yurii
Fonte: ELSEVIER SCIENCE BV; AMSTERDAM Publicador: ELSEVIER SCIENCE BV; AMSTERDAM
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
66.21%
We analyze long-range time correlations and self-similar characteristics of the electrostatic turbulence at the plasma edge and scrape-off layer in the Tokamak Chauffage Alfven Bresillien (TCABR), with low and high Magnetohydrodynamics (MHD) activity. We find evidence of self-organized criticality (SOC), mainly in the region near the tokamak limiter. Comparative analyses of data before and during the MHD activity reveals that during the high mHD activity the Hurst parameter decreases. Finally, we present a cellular automaton whose parameters are adjusted to simulate the analyzed turbulence SOC change with the MHD activity variation. (C) 2011 Published by Elsevier B.V.; CNPq; CNPq; CAPES; CAPES; FAPESP; FAPESP; Fundacao Araucaria; Fundacao Araucaria

Turbulência Eletrostática e Magnética em Tokamaks; Electrostatic and magnetic turbulence in Tokamaks

Castro, Raul Murete de
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 22/10/1996 PT
Relevância na Pesquisa
56.38%
Neste trabalho foi realizado um estudo experimental da turbulência e do transporte de partículas e energia induzido por flutuações na borda do plasma do tokamak TBR-1. Para isto, foi utilizado um conjunto de sondas eletrostáticas (incluindo uma sonda tripla de resposta rápida) e magnéticas, especialmente construído para este fim. Técnicas de análise espectral foram aplicadas aos dados e permitiram verificar a influência das flutuações da temperatura nos parâmetros de transporte. Os resultados mostram que o nível relativo das flutuações da temperatura é da ordem de 10% e os da densidade e do potencial variam de ~10 a ~ 30 %. As flutuações eletrostáticas possuem faixas largas de freqüência e números de onda, o que caracteriza a borda do plasma como um meio turbulento. Estas flutuações se propagam no sentido da deriva diamagnética dos íons. A correção das flutuações das grandezas do plasma utilizando as flutuações da temperatura causou mudanças significativas nos transportes de partículas e energia induzidos por flutuações, nas posições mais internas da borda do plasma. O tempo de confinamento de partículas, calculado a partir deste transporte, está na faixa de ~1 a ~ 1,5 ms, que é da mesma ordem do tempo de confinamento obtido por outros métodos...

Análise espectral por wavelet da turbulência no tokamak TCABR; Wavelet spectral analysis of turbulence Tokamak TCABR

Lima, Gustavo Zampier dos Santos
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 07/10/2005 PT
Relevância na Pesquisa
56.15%
Desenvolvemos uma aplicação das análises espectrais por Fourier e wavelet para o estudo de flutuações intermitentes e não estacionárias. Para isso, elaboramos algoritmos adequados, que revelam os modos principais presentes nas flutuações analisadas e as suas evoluções. Baseados nesses algoritmos, apresentamos um método para a seleção de "bursts" (irrupções) em meio ao background da série intermitente anali­sada. Inicialmente, aplicamos essa análise às flutuações intermitentes obtidas inte­grando numericamente as equações de Lorenz. Dessa forma, obtivemos a evolução do espectro das frequências dessas flutuações e selecionamos os seus bursts. Simi­larmente, obtivemos a evolução dos espectros de frequência de flutuações elétricas turbulentas medidas no tokamak TCABR. Confirmamos que a análise por wavelet é adequada para observar a modulação da turbulência pelas oscilações magnéticas. Para essas flutuações, selecionamos os bursts intermitentes presentes nas flutuações turbulentas e obtivemos as distribuições dos intervalos de tempo entre bursts sucessivos. Obtivemos, ainda, as distribuições das medidas da flutuação da turbulência no TCABR. Com a modulação mencionada...

Recurrence quantification analysis of turbulent fluctuations in the plasma edge of Tokamak Chauffage Alfveacuten Breacutesilien tokamak

Guimarães Filho, Zwinglio de Oliveira; Caldas, Ibere Luiz; Viana, Ricardo Luiz; Nascimento, Ivan Cunha; Kuznetsov, Yurii; Kurths, Jurgen
Fonte: AMER INST PHYSICS Publicador: AMER INST PHYSICS
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
56.06%
Recurrences are close returns of a given state in a time series, and can be used to identify different dynamical regimes and other related phenomena, being particularly suited for analyzing experimental data. In this work, we use recurrence quantification analysis to investigate dynamical patterns in scalar data series obtained from measurements of floating potential and ion saturation current at the plasma edge of the Tokamak Chauffage Alfveacuten Breacutesilien [R. M. O. Galva approximate to o , Plasma Phys. Controlled Fusion 43, 1181 (2001)]. We consider plasma discharges with and without the application of radial electric bias, and also with two different regimes of current ramp. Our results indicate that biasing improves confinement through destroying highly recurrent regions within the plasma column that enhance particle and heat transport.; FAPESP; CNPq; Fundacao Araucaria; Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES); FINEP-RNP (Brazilian Fusion Network); Deutsche Forschungsgemeinschaft - DFG[SFB 555]; Universite de Provence and PACA (Provence Alpes Cote d'Azur) Region

Statistical features of edge turbulence in RFX-mod from Gas Puffing Imaging

Sattin, F.; Scarin, P.; Agostini, M.; Cavazzana, R.; Serianni, G.; Spolaore, M.; Vianello, N.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 30/06/2006
Relevância na Pesquisa
46.17%
Plasma density fluctuations in the edge plasma of the RFX-mod device are measured through the Gas Puffing Imaging Diagnostics. Statistical features of the signal are quantified in terms of the Probability Distribution Function (PDF), and computed for several kinds of discharges. The PDFs from discharges without particular control methods are found to be adequately described by a Gamma function, consistently with the recent results by Graves et al [J.P. Graves, et al, Plasma Phys. Control. Fusion 47, L1 (2005)]. On the other hand, pulses with external methods for plasma control feature modified PDFs. A first empirical analysis suggests that they may be interpolated through a linear combination of simple functions. An inspection of the literature shows that this kind of PDFs is common to other devices as well, and has been suggested to be due to the simultaneous presence of different mechanisms driving respectively coherent bursts and gaussian background turbulence. An attempt is made to relate differences in the PDFs to plasma conditions such as the local shift of the plasma column. A simple phenomenological model to interpret the nature of the PDF and assign a meaning to its parameters is also developed.; Comment: 27 pages. Published in PPCF

Impurity and Trace Tritium Transport in Tokamak Edge Turbulence

Naulin, Volker
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 26/03/2004
Relevância na Pesquisa
55.99%
The turbulent transport of impurity or minority species, as for example Tritium, is investigated in drift-Alfv\'en edge turbulence. The full effects of perpendicular and parallel convection are kept for the impurity species. The impurity density develops a granular structure with steep gradients and locally exceeds its initial values due to the compressibility of the flow. An approximate decomposition of the impurity flux into a diffusive part and an effective convective part (characterized by a pinch velocity) is performed and a net inward pinch effect is recovered. The pinch velocity is explained in terms of Turbulent Equipartition and is found to vary poloidally. The results show that impurity transport modeling needs to be two-dimensional, considering besides the radial direction also the strong poloidal variation in the transport coefficients.; Comment: 12 Pages, 5 Figures

Statistical model for intermittent plasma edge turbulence

Sattin, F.; Vianello, N.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 23/05/2005
Relevância na Pesquisa
66.22%
The Probability Distribution Function of plasma density fluctuations at the edge of fusion devices is known to be skewed and strongly non-Gaussian. The causes of this peculiar behaviour are, up to now, largely unexplored. On the other hand, understanding the origin and the properties of edge turbulence is a key issue in magnetic fusion research. In this work we show that a stochastic fragmentation model, already successfully applied to fluid turbulence, is able to predict an asymmetric distribution that closely matches experimental data. The asymmetry is found to be a direct consequence of intermittency. A discussion of our results in terms of recently suggested BHP universal curve [S.T. Bramwell, P.C.W. Holdsworth, J.-F. Pinton, Nature (London) 396, 552 (1998)], that should hold for strongly correlated and critical systems, is also proposed; Comment: 13 pages. Physica Review E, accepted

Nonlinear gyrofluid computation of edge localised ideal ballooning modes

Kendl, A.; Scott, B. D.; Ribeiro, T.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 16/02/2010
Relevância na Pesquisa
46.17%
Three dimensional electromagnetic gyrofluid simulations of the ideal ballooning mode blowout scenario for tokamak edge localized modes (ELMs) are presented. Special emphasis is placed on energetic diagnosis, examining changes in the growth rate in the linear, overshoot, and decay phases. The saturation process is energy transfer to self generated edge turbulence which exhibits an ion temperature gradient (ITG) mode structure. Convergence in the decay phase is found only if the spectrum reaches the ion gyroradius. The equilibrium is a self consistent background whose evolution is taken into account. Approximately two thirds of the total energy in the edge layer is liberated in the blowout. Parameter dependence with respect to plasma pressure and the ion gyroradius is studied. Despite the violent nature of the short-lived process, the transition to nonlinearity is very similar to that found in generic tokamak edge turbulence.; Comment: The following article has been submitted to Physics of Plasmas. After it is published, it will be found at http://pop.aip.org/

Modifications of turbulence and turbulent transport associated with a bias-induced confinement transition in LAPD

Carter, T. A.; Maggs, J. E.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.21%
Azimuthal flow is driven in the edge of the Large Plasma Device (LAPD) [W. Gekelman, {\itshape et. al}, Rev. Sci. Instr. {\bfseries 62}, 2875 (1991)] through biasing a section of the vacuum vessel relative to the plasma source cathode. As the applied bias exceeds a threshold, a transition in radial particle confinement is observed, evidenced by a dramatic steepening in the density profile, similar to the L- to H-mode transition in toroidal confinement devices. The threshold behavior and dynamic behavior of radial transport is related to flow penetration and the degree of spatial overlap between the flow shear and density gradient profiles. An investigation of the changes in turbulence and turbulent particle transport associated with the confinement transition is presented. Two-dimensional cross-correlation measurements show that the spatial coherence of edge turbulence in LAPD changes significantly with biasing. The azimuthal correlation in the turbulence increases dramatically, while the radial correlation length is little altered. Turbulent amplitude is reduced at the transition, particularly in electric field fluctuations, but the dominant change observed is in the cross-phase between density and electric field fluctuations. The changes in cross-phase lead to a suppression and then apparent reversal of turbulent particle flux as the threshold is exceeded.; Comment: 11 pages...

A Self-Organized-Criticality model consistent with statistical properties of edge turbulence in a fusion plasma

Sattin, Fabio; Baiesi, Marco
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.11%
The statistical properties of the intermittent signal generated by a recent model for self-organized-criticality (SOC) are examined. A successful comparison is made with previously published results of the equivalent quantities measured in the electrostatic turbulence at the edge of a fusion plasma. This result re-establishes SOC as a potential paradigm for transport in magnetic fusion devices, overriding shortcomings pointed out in earlier works [E. Spada, et al, Phys. Rev. Lett. 86, 3032 (2001); V. Antoni, et al, Phys. Rev. Lett. 87, 045001 (2001)].; Comment: 4 pages, 4 figures

Impurity transport in plasma edge turbulence

Naulin, Volker; Wood, Martin Priego; Rasmussen, Jens Juul
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 14/10/2004
Relevância na Pesquisa
66.17%
The turbulent transport of minority species/impurities is investigated in 2D drift-wave turbulence as well as in 3D toroidal drift-Alfven edge turbulence. The full effects of perpendicular and -- in 3D -- parallel advection are kept for the impurity species. Anomalous pinch effects are recovered and explained in terms of Turbulent EquiPartition (TEP); Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France)

Observation of self-organized criticality (SOC) behavior during edge biasing experiment on TEXTOR

Xu, Yuhong H.; Jachmich, S.; Weynants, R. R.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 21/10/2004
Relevância na Pesquisa
46.16%
The self-organized criticality (SOC) behavior of the edge plasma transport has been investigated using the fluctuation data measured in the plasma edge and the scrape-off layer of TEXTOR tokamak before and during the edge electrode biasing experiments. In the "nonshear" discharge phase before biasing, both the potential and density fluctuations clearly exhibit some of the characteristics associated with SOC: (1) existence of f -1 power-law dependence in the frequency spectrum, (2) slowly decaying long tails in the autocorrelation function, (3) values of Hurst parameters larger than 0.5 at all the detected radial locations, (4) non-Gaussian PDF of fluctuations and (5) radial propagation of avalanche-like events in the edge plasma area. During the biasing phase, with the generation of an edge radial electric field Er and hence a sheared ErxB flow, the local turbulence is found to be well decorrelated by the ErxB velocity shear, consistent with theoretical predictions. Nevertheless, it is concomitantly found that the Hurst parameters are substantially enhanced in the negative flow shear region and in the scrape-off layer as well, which is contrary to theoretical expectation. Implication of these observations to our understanding of plasma transport mechanisms is discussed.; Comment: 12th International Congress on Plasma Physics...

Anomalous diffusion, clustering, and pinch of impurities in plasma edge turbulence

Priego, M.; Garcia, O. E.; Naulin, V.; Rasmussen, J. Juul
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 07/01/2005
Relevância na Pesquisa
76.23%
The turbulent transport of impurity particles in plasma edge turbulence is investigated. The impurities are modeled as a passive fluid advected by the electric and polarization drifts, while the ambient plasma turbulence is modeled using the two-dimensional Hasegawa--Wakatani paradigm for resistive drift-wave turbulence. The features of the turbulent transport of impurities are investigated by numerical simulations using a novel code that applies semi-Lagrangian pseudospectral schemes. The diffusive character of the turbulent transport of ideal impurities is demonstrated by relative-diffusion analysis of the evolution of impurity puffs. Additional effects appear for inertial impurities as a consequence of compressibility. First, the density of inertial impurities is found to correlate with the vorticity of the electric drift velocity, that is, impurities cluster in vortices of a precise orientation determined by the charge of the impurity particles. Second, a radial pinch scaling linearly with the mass--charge ratio of the impurities is discovered. Theoretical explanation for these observations is obtained by analysis of the model equations.; Comment: This article has been submitted to Physics of Plasmas. After it is published, it will be found at http://pop.aip.org/pop/

Bifurcation in electrostatic resistive drift wave turbulence

Numata, Ryusuke; Ball, Rowena; Dewar, Robert L.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 31/08/2007
Relevância na Pesquisa
46.18%
The Hasegawa-Wakatani equations, coupling plasma density and electrostatic potential through an approximation to the physics of parallel electron motions, are a simple model that describes resistive drift wave turbulence. We present numerical analyses of bifurcation phenomena in the model that provide new insights into the interactions between turbulence and zonal flows in the tokamak plasma edge region. The simulation results show a regime where, after an initial transient, drift wave turbulence is suppressed through zonal flow generation. As a parameter controlling the strength of the turbulence is tuned, this zonal flow dominated state is rapidly destroyed and a turbulence-dominated state re-emerges. The transition is explained in terms of the Kelvin-Helmholtz stability of zonal flows. This is the first observation of an upshift of turbulence onset in the resistive drift wave system, which is analogous to the well-known Dimits shift in turbulence driven by ion temperature gradients.; Comment: 21 pages, 11 figures

Non-perturbative models of intermittency in edge turbulence

Anderson, Johan; Kim, Eun-jin
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 15/01/2009
Relevância na Pesquisa
66.15%
A theory of the probability distribution function (PDF) tails of the blob density in plasma edge turbulence is provided. A simplified model of the fast convective radial transport is used. The theoretically predicted PDF tails corroborate earlier measurements of edge transport, further confirming the strongly non-Gaussian feature of edge transport. It is found that increasing the cross sectional spatial scale length ($L_x$ and $L_y$) of the blob results in larger transport whereas increasing the toroidal scale length ($L_z$) decreases the PDF. The results imply that the PDF decreases for larger blob speed $v_b$.; Comment: 19 pages, 2 figures

Flux surface shaping effects on tokamak edge turbulence and flows

Kendl, Alexander; Scott, Bruce D.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 22/09/2005
Relevância na Pesquisa
46.09%
Shaping of magnetic flux surfaces is found to have a strong impact on turbulence and transport in tokamak edge plasmas. A series of axisymmetric equilibria with varying elongation and triangularity, and a divertor configuration are implemented into a computational gyrofluid turbulence model. The mechanisms of shaping effects on turbulence and flows are identified. Transport is mainly reduced by local magnetic shearing and an enhancement of zonal shear flows induced by elongation and X-point shaping.; Comment: 10 pages, 11 figures. Submitted to Physics of Plasmas

Turbulent Edge Structure Formation in Complex Configurations

Kendl, A; Scott, B D; Ball, Rowena; Dewar, Robert
Fonte: American Institute of Physics (AIP) Publicador: American Institute of Physics (AIP)
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.07%
Simulations of edge turbulence structures in the geometrics of stellarator and tokamak configurations were analyzed. The approximation procedure to include three-dimensional magnetic field configurations into flux tube models of the geometry was outlined. Turbulent structure formation in terms of parallel and perpendicular mode and radial zonal flow structures was discussed. The turbulent transport showed considerably different levels, which depended strongly on the various discussed geometric aspects of the magnetic field configuration.