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## Controle da turbulência em plasmas; Control of turbulence in plasmas

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

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#caos (sistemas dinâmicos)#chaos (dynamic systems)#eletrostatic turbulence#experimental physics#fenômenos de transporte#física de plasmas#física experimental#plasma physics#transport phenomena#turbulência eletrostática

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...

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

Fonte: American Institute of Physics (AIP)
Publicador: American Institute of Physics (AIP)

Tipo: Artigo de Revista Científica

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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.

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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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...

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 22/05/2013

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/04/2014

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 03/06/2004

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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

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## Stochastic modelling of intermittent scrape-off layer plasma fluctuations

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/06/2015

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 09/07/2013

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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

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## Numerical Simulations of Intermittent Transport in Scrape-Off Layer Plasmas

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/09/2003

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 14/07/2014

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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.

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 30/10/2007

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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.

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 11/06/2014

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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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 09/06/2015

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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.

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/08/2014

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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.

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 17/08/2015

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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...

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/10/2004

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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...

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 22/10/2004

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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...

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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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

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