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Controle da turbulência em plasmas; Control of turbulence in plasmas

Toufen, Dennis Lozano
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 13/11/2012 PT
Relevância na Pesquisa
36.33%
O Texas Helimak é uma máquina toroidal de confinamento de plasma cujas linhas de campo magnético têm forma helicoidal e no qual parâmetros do plasma (como a densidade e a temperatura) são similares aos da borda e da região externa (scrape-off-layer) de um tokamak. Nesta tese foram analisados o equilíbrio e as flutuações do plasma no Texas Helimak. São apresentadas a análise e a interpretação do controle da turbulência eletrostática e do transporte turbulento de partículas pelo potencial elétrico, bias, aplicado externamente. As alterações na turbulência e no transporte causadas pela mudança do potencial elétrico externo foram investigadas em uma região do plasma com gradientes radiais uniformes. As flutuações em descargas com bias positivo ou nulo apresentam espectros de potência de banda larga e uma PDF com uma cauda acentuada que revela a ocorrência intermitente de eventos extremos. Por outro lado, as flutuações em descargas com bias negativo possuem um espectro de potência mais estreito, uma PDF mais Gaussiana e um diagrama de recorrência com mais estruturas. Um modelo de quatro ondas acopladas foi utilizado para relacionar a largura de banda da turbulência com o tempo de interação entre as ondas do modelo. Perfis radiais do transporte turbulento de partículas na direção radial foram calculados...

Coherence imaging of scrape-off-layer and divertor impurity flows in the Mega Amp Spherical Tokamak (invited)a)

Silburn, S. A.; Harrison, J. R.; Howard, J.; Gibson, K. J.; Meyer, H.; Michael, C. A.; Sharples, R. M.
Fonte: American Institute of Physics (AIP) Publicador: American Institute of Physics (AIP)
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.52%
A new coherence imaging Doppler spectroscopy diagnostic has been deployed on the UK's Mega Amp Spherical Tokamak for scrape-off-layer and divertor impurity flow measurements. The system has successfully obtained 2D images of C III, C II, and He II line-of-sight flows, in both the lower divertor and main scrape-off-layer. Flow imaging has been obtained at frame rates up to 1 kHz, with flow resolution of around 1 km/s and spatial resolution better than 1 cm, over a 40° field of view. C III data have been tomographically inverted to obtain poloidal profiles of the parallel impurity flow in the divertor under various conditions. In this paper we present the details of the instrument design, operation, calibration, and data analysis as well as a selection of flow imaging results which demonstrate the diagnostic's capabilities.; This work was part-funded by the RCUK Energy Programme under Grant No. EP/I501045 and the European Communities under the contract of Association between EURATOM and CCFE.

Fluctuation statistics in the scrape-off layer of Alcator C-Mod

Kube, Ralph; Garcia, Odd Erik; LaBombard, Brian; Terry, James L.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.63%
We study long time series of the ion saturation current and floating potential, sampled by Langmuir probes dwelled in the far-scrape off layer and embedded in the lower divertor baffle of Alcator C-Mod, for a series of discharges with line-averaged plasma density in between 0.15 and 0.42 in terms of the Greenwald density. The dynamics of all ion saturation current time series is governed by the intermittent arrival of large amplitude burst events. Coefficients of skewness and excess kurtosis of the time series obey a quadratic relation and their histograms collide upon normalization. Best fits of several proposed models for the distribution of the scrape-off layer plasma density fluctuations are compared to ion saturation current time series and are found to agree well over several decades in normalized probability with the sample histograms. Histograms of the waiting time between successive large amplitude burst events and of the burst amplitudes are well described by an exponential distribution. The best fit on the waiting time and amplitude scale length are found to vary weakly with the plasma line-averaged density. Conditional averaging reveals that the radial blob velocity increases with the normalized burst amplitude in the outboard mid plane scrape-off layer. For low density discharges...

Experimental and numerical characterisation of the turbulence in the Scrape-Off Layer of MAST

Militello, F.; Tamain, P.; Fundamenski, W.; Kirk, A.; Naulin, V.; Nielsen, A. H.; team, the MAST
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 22/05/2013
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66.33%
Numerical simulations of interchange turbulence in the Scrape-Off Layer are performed in a regime relevant for a specific L-mode MAST (Mega Ampere Spherical Tokamak) discharge. Such a discharge was diagnosed with a reciprocating arm equipped with a Gundestrup probe. A detailed comparison of the average and statistical properties of the simulated and experimental ion saturation current is performed. Good agreement is found in the time averaged radial profile, in the probability distribution functions (PDFs) and in qualitative features of the signals such as the shape, duration and separation of burst events. These results confirm the validity of the simple interchange model used and help to identify where it can be improved. Finally, the simulated data are used to assess the importance of the temperature fluctuations on plasma potential and radial velocity measurements acquired with Langmuir probes. It is shown that the correlation between the actual plasma quantities and the signal of the synthetic diagnostics is poor, suggesting that accurate measurements of the temperature fluctuations are needed in order to obtain reliable estimates of the perpendicular fluxes.; Comment: 23 pages, 16 figures. This is an author-created, un-copyedited version of an article accepted for publication in Plasma Physics and Controlled Fusion. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it

A Gyrokinetic 1D Scrape-Off Layer Model of an ELM Heat Pulse

Shi, E. L.; Hakim, A. H.; Hammett, G. W.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.33%
An electrostatic gyrokinetic-based model is applied to simulate parallel plasma transport in the scrape-off layer to a divertor plate. The authors focus on a test problem that has been studied previously, using parameters chosen to model a heat pulse driven by an edge-localized mode (ELM) in JET. Previous work has used direct particle-in-cell equations with full dynamics, or Vlasov or fluid equations with only parallel dynamics. With the use of the gyrokinetic quasineutrality equation and logical sheath boundary conditions, spatial and temporal resolution requirements are no longer set by the electron Debye length and plasma frequency, respectively. This test problem also helps illustrate some of the physics contained in the Hamiltonian form of the gyrokinetic equations and some of the numerical challenges in developing an edge gyrokinetic code.; Comment: 10 pages, 4 figures

Peeling-off of the external kink modes at tokamak plasma edge

Zheng, L. J.; Furukawa, M.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 15/04/2014
Relevância na Pesquisa
46.13%
It is pointed that there is a current jump between the edge plasma inside the last closed magnetic surface and the scrape-off layer and the current jump can lead the external kink modes to convert to the tearing modes, due to the current interchange effects [L. J. Zheng and M. Furukawa, Phys. Plasmas 17, 052508 (2010)]. The magnetic reconnection in the presence of tearing modes subsequently causes the tokamak edge plasma to be peeled off to link to the diverters. In particular, the peeling or peeling-ballooning modes can become the "peeling-off" modes in this sense. This phenomenon indicates that the tokamak edge confinement can be worse than the expectation based on the conventional kink mode picture.; Comment: 16 pages, 4 figures

Stochastic and Discrete Time Models of Long-Range Turbulent Transport in the Scrape-Off Layer

Volchenkov, D.; Lima, R.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 03/06/2004
Relevância na Pesquisa
66.33%
Two dimensional stochastic time model of scrape-off layer (SOL) turbulent transport is studied. Instability arisen in the system with respect to the stochastic perturbations of both either density or vorticity reveals itself in the strong outward bursts of particle density propagating ballistically across the SOL. The stability and possible stabilization of the cross- field turbulent system depend very much upon the reciprocal correlation time between density and vorticity fluctuations. Pdf of the particle flux for the large magnitudes of flux events can be modelled with a simple discrete time toy model of random walks concluding at a boundary. The spectra of wandering times feature the pdf of particle flux in the model and qualitatively reproduce the experimental statistics of transport events.; Comment: 21 pages,11 figures

Stochastic modelling of intermittent scrape-off layer plasma fluctuations

Garcia, O. E.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.33%
Single-point measurements of fluctuations in the scrape-off layer of magnetized plasmas are generally found to be dominated by large-amplitude bursts which are associated with radial motion of blob-like structures. A stochastic model for these fluctuations is presented, with the plasma density given by a random sequence of bursts with a fixed wave form. Under very general conditions, this model predicts a parabolic relation between the skewness and kurtosis moments of the plasma fluctuations. In the case of exponentially distributed burst amplitudes and waiting times, the probability density function for the fluctuation amplitudes is shown to be a Gamma distribution with the scale parameter given by the average burst amplitude and the shape parameter given by the ratio of the burst duration and waiting times.; Comment: 11 pages, 1 figure

Characterisation of the L-mode Scrape Off Layer in MAST: decay lengths

Militello, F.; Garzotti, L.; Harrison, J.; Omotani, J. T.; Scannell, R.; Allan, S.; Kirk, A.; Lupelli, I.; Thornton, A. J.; team, the MAST
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 15/06/2015
Relevância na Pesquisa
66.33%
This work presents a detailed characterisation of the MAST Scrape Off Layer in L-mode. Scans in line averaged density, plasma current and toroidal magnetic field were performed. A comprehensive and integrated study of the SOL was allowed by the use of a wide range of diagnostics. In agreement with previous results, an increase of the line averaged density induced a broadening of the midplane density profile.; Comment: 30 pages, 11 figures

Numerical investigation of Scrape Off Layer anomalous particle transport for MAST parameters

Militello, F; Fundamenski, W; Naulin, V; Nielsen, A H
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 09/07/2013
Relevância na Pesquisa
66.55%
Numerical simulations of L-mode plasma turbulence in the Scrape Off Layer of MAST are presented. Relevant features of the boundary plasma, such as the thickness of the density layer or the statistics of the fluctuations are related to the edge density and temperature, plasma current and parallel connection length. It is found that the density profile is weakly affected by the edge density, it broadens when the current or the temperature are decreased while the connection length has the opposite effect. The statistics of the turbulence is relatively insensitive to variations of all the edge parameters and show a certain degree of universality. Effective transport coefficients are calculated for several plasma conditions and display a strong nonlinear dependence on the parameters and on the radial variable. Finally, it is shown how the perpendicular particle fluxes in the Scrape Off Layer are related to the edge parameters.; Comment: 21 pages, 6 figures

Numerical Simulations of Intermittent Transport in Scrape-Off Layer Plasmas

Garcia, Odd Erik; Naulin, Volker; Nielsen, Anders H.; Rasmussen, Jens Juul
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/09/2003
Relevância na Pesquisa
66.52%
Two-dimensional fluid simulations of interchange turbulence for geometry and parameters relevant for the scrape-off layer of confined plasmas are presented. We observe bursty ejection of particles and heat from the bulk plasma in the form of blobs. These structures propagate far into the scrape-off layer where they are lost due to transport along open magnetic field lines. From single-point recordings it is shown that the blobs have asymmetric conditional wave forms and lead to positively skewed and flat probability distribution functions. The radial propagation velocity may reach one tenth of the sound speed. These results are in excellent agreement with recent experimental measurements.; Comment: 8 pages, 7 figures

An experimental investigation on the high density transition of the Scrape-off Layer transport in ASDEX Upgrade

Carralero, D.; Birkenmeier, G.; Müller, H. W.; Manz, P.; deMarne, P.; Müller, S. H.; Reimold, F.; Stroth, U.; Wischmeier, M.; Wolfrum, E.; team, the ASDEX Upgrade
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 14/07/2014
Relevância na Pesquisa
66.37%
A multidiagnostic approach, utilizing Langmuir probes in the midplane, X-point and divertor walls, along with Lithium beam and infrared measurements is employed to evaluate the evolution of the Scrape-off Layer (SOL) of ASDEX Upgrade across the L-mode density transition leading to the formation of a density shoulder. The flattening of the SOL density profiles is linked to a regime change of filaments, which become faster and larger, and to a similar flattening of the $q_{\parallel}$ profile. This transition is related to the beginning of outer divertor detachment and leads to the onset of a velocity shear layer in the SOL. Experimental measurements are in good agreement with several filament models which describe the process as a transition from conduction to convection-dominated SOL perpendicular transport caused by an increase of parallel collisionality. These results could be of great relevance since both ITER and DEMO will feature detached divertors and densities largely over the transition values, and might therefore exhibit convective transport levels different to those observed typically in present-day devices.

Control of test particle transport in a turbulent electrostatic model of the Scrape Off Layer

Ciraolo, Guido; Ghendrih, Philippe; Sarazin, Yanick; Chandre, Cristel; Lima, Ricardo; Vittot, Michel; Pettini, Marco
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 30/10/2007
Relevância na Pesquisa
66.33%
The ${\bm E}\times{\bm B}$ drift motion of charged test particle dynamics in the Scrape Off Layer (SOL)is analyzed to investigate a transport control strategy based on Hamiltonian dynamics. We model SOL turbulence using a 2D non-linear fluid code based on interchange instability which was found to exhibit intermittent dynamics of the particle flux. The effect of a small and appropriate modification of the turbulent electric potential is studied with respect to the chaotic diffusion of test particle dynamics. Over a significant range in the magnitude of the turbulent electrostatic field, a three-fold reduction of the test particle diffusion coefficient is achieved.

Scaling of the scrape-off layer width during inter-ELM H modes on MAST as measured by infrared thermography

Thornton, A. J.; Kirk, A.; Team, MAST
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 11/06/2014
Relevância na Pesquisa
66.37%
The power load to the divertor surfaces is a key concern for future devices such as ITER, due to the thermal limits on the material surface. One factor that characterizes the heat flux to the divertor is the fall off length in the scrape-off layer (SOL), which recent empirical scalings have shown could be as small as 1 mm. These predictions are based on a multi-machine scaling of the heat flux width fitted using an expression for the divertor heat flux profile which includes a term for the exponential decay in the SOL and diffusion about the last closed flux surface (LCFS) in the private flux region.; Comment: 20 pages, 16 figures

The effects of shape and amplitude on the velocity of scrape-off layer filaments

Omotani, J. T.; Militello, F.; Easy, L.; Walkden, N. R.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 09/06/2015
Relevância na Pesquisa
66.52%
A complete model of the dynamics of scrape-off layer filaments will be rather complex, including temperature evolution, three dimensional geometry and finite Larmor radius effects. However, the basic mechanism of $\boldsymbol{E}\times\boldsymbol{B}$ advection due to electrostatic potential driven by the diamagnetic current can be captured in a much simpler model; a complete understanding of the physics in the simpler model will then aid interpretation of more complex simulations, by allowing the new effects to be disentangled. Here we consider such a simple model, which assumes cold ions and isothermal electrons and is reduced to two dimensions. We derive the scaling with width and amplitude of the velocity of isolated scrape-off layer filaments, allowing for arbitrary elliptical cross-sections, where previously only circular cross-sections have been considered analytically. We also put the scaling with amplitude in a new and more satisfactory form. The analytical results are extensively validated with two dimensional simulations and also compared, with reasonable agreement, to three dimensional simulations having minimal variation parallel to the magnetic field.

Modelling of turbulent impurity transport in fusion edge plasmas using measured and calculated ionization cross sections

Kendl, Alexander
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 21/08/2014
Relevância na Pesquisa
36.33%
Turbulent transport of trace impurities impurities in the edge and scrape-off-layer of tokamak fusion plasmas is modelled by three dimensional electromagnetic gyrofluid computations including evolution of plasma profile gradients. The source function of impurity ions is dynamically computed from pre-determined measured and calculated electron impact ionization cross section data. The simulations describe the generation and further passive turbulent E-cross-B advection of the impurities by intermittent fluctuations and coherent filamentary structures (blobs) across the scrape-off-layer.

Investigation of the Effect of Resistivity on Scrape Off Layer Filaments using Three Dimensional Simulations

Easy, Luke; Militello, Fulvio; Omotani, John; Walkden, Nick; Dudson, Benjamin
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 17/08/2015
Relevância na Pesquisa
66.33%
The propagation of filaments in the Scrape Off Layer (SOL) of tokamaks largely determine the plasma profiles in the region. In a conduction limited SOL, parallel temperature gradients are expected, such that the resistance to parallel currents is greater at the target than further upstream. Since the perpendicular motion of an isolated filament is largely determined by balance of currents that flow through it, this may be expected to affect filament transport. 3D simulations have thus been used to study the influence of enhanced parallel resistivity on the dynamics of filaments. Filaments with the smallest perpendicular length scales, which were inertially limited at low resistivity (meaning that polarization rather than parallel currents determine their radial velocities), were unaffected by resistivity. For larger filaments, faster velocities were produced at higher resistivities, due to two mechanisms. Firstly parallel currents were reduced and polarization currents were enhanced, meaning that the inertial regime extended to larger filaments, and secondly a potential difference formed along the parallel direction so that higher potentials were produced in the region of the filament for the same amount of current to flow into the sheath. These results indicate that broader SOL profiles could be produced at higher resistivities.; Comment: 14 Pages...

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
36.33%
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...

Formation of convective cells in the scrape-off layer of the CASTOR tokamak

Stockel, J.; Devynck, P.; Gunn, J.; Martines, E.; Bonhomme, G.; Van Oost, G.; Hron, M.; Adamek, J.; Brotankova, J.; Dejarnac, R.; Duran, I.; Gorler, T.; Hansen, T.; Panek, R.; Stejskal, P.; Svoboda, V.; Zacek, F.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 22/10/2004
Relevância na Pesquisa
66.33%
Understanding of the scrape-off layer (SOL) physics in tokamaks requires diagnostics with sufficient temporal and spatial resolution. This contribution describes results of experiments performed in the SOL of the CASTOR tokamak (R=40 cm, a = 6 cm) by means of a ring of 124 Langmuir probes surrounding the whole poloidal cross section. The individual probes measure either the ion saturation current of the floating potential with the spatial resolution up to 3 mm. Experiments are performed in a particular magnetic configuration, characterized by a long parallel connection length in the SOL, L_par ~q2piR. We report on measurements in discharges, where the edge electric field is modified by inserting a biased electrode into the edge plasma. In particular, a complex picture is observed, if the biased electrode is located inside the SOL. The poloidal distribution of the floating potential appears to be strongly non-uniform at biasing. The peaks of potential are observed at particular poloidal angles. This is interpreted as formation of a biased flux tube, which emanates from the electrode along the magnetic field lines and snakes q times around the torus. The resulting electric field in the SOL is 2-dimensional, having the radial as well as the poloidal component. It is demonstrated that the poloidal electric field E_pol convects the edge plasma radially due to the E_pol x B_T drift either inward or outward depending on its sign. The convective particle flux is by two orders of magnitude larger than the fluctuation-induced one and consequently dominates.; Comment: 12th International Congress on Plasma Physics...

Convergence of statistical moments of particle density time series in scrape-off layer plasmas

Kube, Ralph; Garcia, Odd Erik
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.33%
Particle density fluctuations in the scrape-off layer of magnetically confined plasmas, as measured by gas-puff imaging or Langmuir probes, are modeled as the realization of a stochastic process in which a superposition of pulses with a fixed shape, an exponential distribution of waiting times and amplitudes represents the radial motion of blob-like structures. With an analytic formulation of the process at hand, we derive expressions for the mean-squared error on estimators of sample mean and sample variance as a function of sample length, sampling frequency, and the parameters of the stochastic process. % Employing that the probability distribution function of a particularly relevant shot noise process is given by the gamma distribution, we derive estimators for sample skewness and kurtosis, and expressions for the mean-squared error on these estimators. Numerically generated synthetic time series are used to verify the proposed estimators, the sample length dependency of their mean-squared errors, and their performance. We find that estimators for sample skewness and kurtosis based on the gamma distribution are more precise and more accurate than common estimators based on the method of moments.; Comment: 31 pages, 10 figures