503 research outputs found
Experimental Divertor Similarity Database Parameters
A set of experimentally-determined dimensionless parameters is proposed for
characterizing the regime of divertor operation. The objective is to be able to
compare as unambiguously as possible the operation of different divertors and
to understand what physical similarities and differences they represent.
Examples from Alcator C-Mod are given.Comment: Plain Tex (8 pages) plus 5 postscipt figure
Burst statistics in Alcator C-Mod SOL turbulence
Bursty fluctuations in the scrape-off layer (SOL) of Alcator C-Mod have been
analyzed using gas puff imaging data. This reveals many of the same fluctuation
properties as Langmuir probe measurements, including normal distributed
fluctuations in the near SOL region while the far SOL plasma is dominated by
large amplitude bursts due to radial motion of blob-like structures.
Conditional averaging reveals burst wave forms with a fast rise and slow decay
and exponentially distributed waiting times. Based on this, a stochastic model
of burst dynamics is constructed. The model predicts that fluctuation
amplitudes should follow a Gamma distribution. This is shown to be a good
description of the gas puff imaging data, validating this aspect of the model.Comment: 8 pages, 6 figure
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
Mirror Langmuir probe: A technique for real-time measurement of magnetized plasma conditions using a single Langmuir electrode
Transport and drift-driven plasma flow components in the Alcator C-Mod boundary plasma
Boundary layer flows in the Alcator C-Mod tokamak are systematically examined as magnetic topology (upper versus lower-null) and plasma density are changed. Utilizing a unique set of scanning LangmuirâMach probes, including one on the high-field side (HFS) midplane, the poloidal variation of plasma flow components in the parallel, diamagnetic and radial directions are resolved in detail. It is found that the plasma flow pattern can be decomposed into two principal parts: (1) a drift-driven component, which lies within a magnetic flux surface and is divergence-free and (2) a transport-driven component, which gives rise to near-sonic parallel flows on the HFS scrape-off layer (SOL). Toroidal rotation, PfirschâSchlĂźter and transport-driven contributions are unambiguously identified. Transport-driven parallel flows are found to dominate the HFS particle fluxes; the total poloidal-directed flow accounts for ~1/3 to all of the ion flux arriving on the inner divertor. As a result, heat convection is found to be an important player in this region, consistent with the observation of divertor asymmetries that depend on the direction of B Ă âB relative to the active x-point. In contrast, the poloidal projection of parallel flow in the low-field SOL largely cancels with E[subscript r] Ă B flow; toroidal rotation is the dominant plasma motion there. The magnitude of the transport-driven poloidal flow is found to be quantitatively consistent with fluctuation-induced radial particle fluxes on the low-field side (LFS), identifying this as the primary drive mechanism. Fluctuation-induced fluxes on the HFS are found to be essentially zero, excluding turbulent inward transport as the mechanism that closes the circulation loop in this region.United States. Dept. of Energy (Cooperative Agreement DE-FC02-99ER54512
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
Outlier classification using Autoencoders: application for fluctuation driven flows in fusion plasmas
Understanding the statistics of fluctuation driven flows in the boundary
layer of magnetically confined plasmas is desired to accurately model the
lifetime of the vacuum vessel components. Mirror Langmuir probes (MLPs) are a
novel diagnostic that uniquely allow to sample the plasma parameters on a time
scale shorter than the characteristic time scale of their fluctuations. Sudden
large-amplitude fluctuations in the plasma degrade the precision and accuracy
of the plasma parameters reported by MLPs for cases in which the probe bias
range is of insufficient amplitude. While some data samples can readily be
classified as valid and invalid, we find that such a classification may be
ambiguous for up to 40% of data sampled for the plasma parameters and bias
voltages considered in this study. In this contribution we employ an
autoencoder (AE) to learn a low-dimensional representation of valid data
samples. By definition, the coordinates in this space are the features that
mostly characterize valid data. Ambiguous data samples are classified in this
space using standard classifiers for vectorial data. This way, we avoid to
define complicate threshold rules to identify outliers, which requires strong
assumptions and introduce biases in the analysis. Instead, these rules are
learned from the data by statistical inference By removing the outliers that
are identified in the latent low-dimensional space of the AE, we find that the
average conductive and convective radial heat flux are between approximately 5
and 15% lower as when removing outliers identified by threshold values. For
contributions to the radial heat flux due to triple correlations, the
difference is up to 40%
- âŚ