223 research outputs found
Intermittent fluctuations in the Alcator C-Mod scrape-off layer for ohmic and high confinement mode plasmas
Plasma fluctuations in the scrape-off layer of the Alcator C-Mod tokamak in
ohmic and high confinement modes have been analyzed using gas puff imaging
data. In all cases investigated, the time series of emission from a single
spatially-resolved view into the gas puff are dominated by large-amplitude
bursts, attributed to blob-like filament structures moving radially outwards
and poloidally. There is a remarkable similarity of the fluctuation statistics
in ohmic plasmas and in edge localized mode-free and enhanced D-alpha high
confinement mode plasmas. Conditionally averaged wave forms have a two-sided
exponential shape with comparable temporal scales and asymmetry, while the
burst amplitudes and the waiting times between them are exponentially
distributed. The probability density functions and the frequency power spectral
densities are self-similar for all these confinement modes. These results are
strong evidence in support of a stochastic model describing the plasma
fluctuations in the scrape-off layer as a super-position of uncorrelated
exponential pulses. Predictions of this model are in excellent agreement with
experimental measurements in both ohmic and high confinement mode plasmas. The
stochastic model thus provides a valuable tool for predicting
fluctuation-induced plasma-wall interactions in magnetically confined fusion
plasmas.Comment: 17 pages, 10 figure
Comparison between mirror Langmuir probe and gas puff imaging measurements of intermittent fluctuations in the Alcator C-Mod scrape-off layer
Statistical properties of the scrape-off layer (SOL) plasma fluctuations are
studied in ohmically heated plasmas in the Alcator C-Mod tokamak. For the first
time, plasma fluctuations as well as parameters that describe the fluctuations
are compared across measurements from a mirror Langmuir probe (MLP) and from
gas-puff imaging (GPI) that sample the same plasma discharge. This comparison
is complemented by an analysis of line emission time-series data, synthesized
from the MLP electron density and temperature measurements. The fluctuations
observed by the MLP and GPI typically display relative fluctuation amplitudes
of order unity together with positively skewed and flattened probability
density functions. Such data time series are well described by an established
stochastic framework which model the data as a superposition of uncorrelated,
two-sided exponential pulses. The most important parameter of the process is
the intermittency parameter, {\gamma} = {\tau}d / {\tau}w where {\tau}d denotes
the duration time of a single pulse and {\tau}w gives the average waiting time
between consecutive pulses. Here we show, using a new deconvolution method,
that these parameters can be consistently estimated from different statistics
of the data. We also show that the statistical properties of the data sampled
by the MLP and GPI diagnostic are very similar. Finally, a comparison of the
GPI signal to the synthetic line-emission time series suggests that the
measured emission intensity can not be explained solely by a simplified model
which neglects neutral particle dynamics
Intermittent electron density and temperature fluctuations and associated fluxes in the Alcator C-Mod scrape-off layer
The Alcator C-Mod mirror Langmuir probe system has been used to sample data
time series of fluctuating plasma parameters in the outboard mid-plane far
scrape-off layer. We present a statistical analysis of one second long time
series of electron density, temperature, radial electric drift velocity and the
corresponding particle and electron heat fluxes. These are sampled during
stationary plasma conditions in an ohmically heated, lower single null diverted
discharge.
The electron density and temperature are strongly correlated and feature
fluctuation statistics similar to the ion saturation current. Both electron
density and temperature time series are dominated by intermittent,
large-amplitude burst with an exponential distribution of both burst amplitudes
and waiting times between them.
The characteristic time scale of the large-amplitude bursts is approximately
15{\mu}s. Large-amplitude velocity fluctuations feature a slightly faster
characteristic time scale and appear at a faster rate than electron density and
temperature fluctuations.
Describing these time series as a superposition of uncorrelated exponential
pulses, we find that probability distribution functions, power spectral
densities as well as auto-correlation functions of the data time series agree
well with predictions from the stochastic model.
The electron particle and heat fluxes present large-amplitude fluctuations.
For this low-density plasma, the radial electron heat flux is dominated by
convection, that is, correlations of fluctuations in the electron density and
radial velocity. Hot and dense blobs contribute approximately 6% of the total
fluctuation driven heat flux
A Minimalist Turbulent Boundary Layer Model
We introduce an elementary model of a turbulent boundary layer over a flat
surface, given as a vertical random distribution of spanwise Lamb-Oseen vortex
configurations placed over a non-slip boundary condition line. We are able to
reproduce several important features of realistic flows, such as the viscous
and logarithmic boundary sublayers, and the general behavior of the first
statistical moments (turbulent intensity, skewness and flatness) of the
streamwise velocity fluctuations. As an application, we advance some heuristic
considerations on the boundary layer underlying kinematics that could be
associated with the phenomenon of drag reduction by polymers, finding a
suggestive support from its experimental signatures.Comment: 5 pages, 10 figure
The First Results for a New Layout of the Stay Cables for Great Span Bridges
AbstractThe air-elastic vibrations of structures induce fluctuating stresses that lead to fatigue damage accumulation and may determine structural failure without exceeding ultimate strength This paper proposes a new layout of stay cables to be used in the construction or the retrofitting of long span bridges, capable of mitigating the air-elastic problems due to environmental vibrations such as the rain-wind excitations. The structural scheme adopted was derived from the critical conditions in terms of stability obtained by referring to lateral suspension cables stayed bridge with two planes of fan pattern stay cables. The new layout consists in implementing an additional plane of cable stays placed symmetrically just under the deck bridge. The final layout of the cable stays was identified as “duplex”. The numerical investigation was carried out in the frequency domain. The results obtained show a sensible increasing of stiffness, as well as a reduction of the natural period of vibrations. In the analysis the deck was considered as thin and very light. The Duplex layout had, also, permitted to mitigate the wind effects, because the presence of the inferior stay cables simulate the viscous dampers
Fluctuation statistics in the scrape-off layer of Alcator C-Mod
We study long time series of the ion saturation current and floating
potential, sampled by Langmuir probes dwelled in the outboard mid-plane scrape
off layer and embedded in the lower divertor baffle of Alcator C-Mod. A series
of ohmically heated L-mode plasma discharges is investigated with line-averaged
plasma density ranging from n_e/n_G = 0.15 to 0.42, where n_G is the Greenwald
density.
All ion saturation current time series that are sampled in the far scrape-off
layer are characterized by large-amplitude burst events. Coefficients of
skewness and excess kurtosis of the time series obey a quadratic relationship
and their histograms coincide partially upon proper normalization. Histograms
of the ion saturation current time series are found to agree well with a
prediction of a stochastic model for the particle density fluctuations in
scrape-off layer plasmas.
The distribution of the waiting times between successive large-amplitude
burst events and of the burst amplitudes are approximately described by
exponential distributions. The average waiting time and burst amplitude are
found to vary weakly with the line-averaged plasma density.
Conditional averaging reveals that the radial blob velocity, estimated from
floating potential measurements, increases with the normalized burst amplitude
in the outboard mid-plane scrape-off layer. For low density discharges, the
conditionally averaged waveform of the floating potential associated with large
amplitude bursts at the divertor probes has a dipolar shape. In detached
divertor conditions the average waveform is random, indicating electrical
disconnection of blobs from the sheaths at the divertor targets.Comment: 45 pages, 20 figure
Relationship between frequency power spectra and intermittent, large-amplitude bursts in the Alcator C-Mod scrape-off layer
This is the peer reviewed version of the following article: Theodorsen, A., Garcia, O.E., Kube, R., LaBombard, B. & Terry, J.L. (2017). Relationship between frequency power spectra and intermittent, large-amplitude bursts in the Alcator C-Mod scrape-off layer. Nuclear Fusion, 57(114004). https://doi.org/10.1088/1741-4326/aa7e4c, which has been published in final form at https://doi.org/10.1088/1741-4326/aa7e4c.Fluctuations in the boundary region of the Alcator C-Mod tokamak have been analyzed using gas puff imaging data from a set of Ohmically heated plasma density scan experiments. It is found that the relative fluctuation amplitudes are modest and close to normally distributed at the separatrix but become increasingly larger and skewed towards the main chamber wall. The frequency power spectra are nevertheless similar for all radial positions and line-averaged densities. Predictions of a stochastic model, describing the plasma fluctuations as a super-position of uncorrelated pulses, are shown to be in excellent agreement with the measurements. This implies that the pulse duration is the same, while the degree of pulse overlap decreases radially outwards in the scrape-off layer. The universal frequency power spectral density is thus determined by the shape and duration of the large-amplitude bursts associated with blob-like structures. The model also describes the rate of threshold level crossings, for which the exponential tails underline the intermittency of the fluctuations in the far scarpe-off layer
Strongly intermittent far scrape-off layer plasma fluctuations in Alcator C-Mod plasmas close to the empirical discharge density limit
Intermittent plasma fluctuations in the boundary region of the Alcator C-Mod
device were comprehensively investigated using data times-series from gas puff
imaging and mirror Langmuir probe diagnostics. Fluctuations were sampled during
stationary plasma conditions in ohmically heated, lower single null diverted
configurations with scans in both line-averaged density and plasma current,
with Greenwald density fractions up to . Utilizing a stochastic model, we
describe the plasma fluctuations as a super-position of uncorrelated pulses,
with large-amplitude events corresponding to blob-like filaments moving through
the scrape-off layer. A deconvolution method is used to estimate the pulse
arrivals and amplitudes. The analysis reveals a significant increase of pulse
amplitudes and waiting times between pulses as the line-averaged density
approaches the empirical discharge density limit. Broadened and flattened
average radial profiles are thus accompanied by strongly intermittent and
large-amplitude fluctuations. Although these filaments are arriving less
frequently at high line-averaged densities, we show that there are significant
increases in radial far-SOL particle and heat flux which will further enhance
plasma--wall interactions. The stochastic model has been used as the framework
for study of the scalings in the intermittency, flux and mean waiting times and
mean amplitudes and is being used to inform predictive capability for the
effects of filamentary transport as a function of Greenwald fraction
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