1,721,815 research outputs found
On the potential of time delay neural networks to detect indirect coupling between time series
Full text linkDetermining the coupling between systems remains a topic of active research in the field of complex science. Identifying the proper causal influences in time series can already be very challenging in the trivariate case, particularly when the interactions are non-linear. In this paper, the coupling between three Lorenz systems is investigated with the help of specifically designed artificial neural networks, called time delay neural networks (TDNNs). TDNNs can learn from their previous inputs and are therefore well suited to extract the causal relationship between time series. The performances of the TDNNs tested have always been very positive, showing an excellent capability to identify the correct causal relationships in absence of significant noise. The first tests on the time localization of the mutual influences and the effects of Gaussian noise have also provided very encouraging results. Even if further assessments are necessary, the networks of the proposed architecture have the potential to be a good complement to the other techniques available in the market for the investigation of mutual influences between time serie
Causality detection methods applied to the investigation of malaria epidemics
Full text linkMalaria, a disease with major health and socio-economic impacts, is driven by multiple factors, including a complex interaction with various climatic variables. In this paper, five methods developed for inferring causal relations between dynamic processes based on the information encapsulated in time series are applied on cases previously studied in literature by means of statistical methods. The causality detection techniques investigated in the paper are: a version of the kernel Granger causality, transfer entropy, recurrence plot, causal decomposition and complex networks. The methods provide coherent results giving a quite good confidence in the conclusions
Development and tests of a simple multifoil spectrometer for highly time-resolved line intensity measurements in the RFX experiment
No full textA simple and cost-effective spectrometer dedicated to line-intensity monitoring in the soft X-ray range has been realized, installed and tested in the RFX reversed field pinch experiment for the study of magnetically confined fusion plasmas. The technique of photon energy selection by means of multilayer material filters has been applied to the instrument described here, which, together with the associated electronics, has been designed in an original compact version for RFX. This present version allows on-line monitoring with good time resolution (10 kHz bandwidth) of line emission from carbon and oxygen in their H- and He-line states (which fall within the soft X-ray energy range), but, with modifications of the set of material filters, different wavelength ranges can be chosen and then other lines can be monitored. A detailed description of the instrument is given here and the data analysis technique is presented. Particular attention is dedicated to a critical analysis of the technique and to the discussion of the experimental errors which may be associated with this measurement; new results on the estimate of systematic errors and experimental improvements to eliminate the problem of continuum radiation are presented. As a demonstration of the proper operation of the instrument, data taken under a wide range of conditions in 0.5-0.6 MA RFX plasmas are shown and discussed. Good agreement of the measurements with other spectroscopic diagnostics already operating on the machine is found; by exploiting the high time resolution of the instrument, interesting results concerning the RFP configuration`s dynamics are obtained. Based on this prototype, a multichord diagnostic has been designed for imaging the impurity emission radial profiles
Testing the consistency of multimachine databases for physical studies of regression
No full textThe investigation of various aspects of tokamak physics is performed with a combination of experiments carried out in different machines, to improve the statistical basis of the results and to cover a sufficient wide region of the operational space. Therefore, in the last decades, various multimachine databases have been built to address general and specific physical questions, particularly related to the extrapolation of present results to the next generation of devices. In this paper, a methodology of analysis is presented, to assess whether a multimachine data set is sufficiently coherent to really substantiate the conclusions, which are expected to be derived from it. A series of statistical and information theoretical tools have been refined to address the consistency of the data provided by the different devices. The developed techniques allow determination of whether it is reasonable to expect that the physics is the same in the various devices and/or that the entries do not present unacceptable bias. To exemplify the potential of the proposed approach, a systematic analysis of the ITPA database of the confinement time has been performed, using both dimensional and dimensionless quantities. The results obtained strongly suggest that better care should be taken in ensuring the coherence of data obtained from different experiments on different devices
Electron Temperature Measurements with High Time Resolution in RFX
No full textCore electron temperature measurements have been performed in the large, high-current RFX reversed field pinch experiment (a = 0.46 m, R = 2 m, target plasma current = 2 MA) with the double filter technique. With this technique the soft x-ray emission is measured by two Si detectors, covered by different Be-filters, and viewing the same region of the plasma. By choosing the filters in such a way that only the emission from the continuum part of the spectrum is measured, the ratio of the two signals is a function of the highest electron temperature along the line of sight of the detectors.
With this diagnostic, the electron temperature has been measured in RFX with a bandwidth of approximately 2 kHz. This allows the direct measurement, for the first time in a RFP on a single shot basis, of coherent oscillations, associated with relaxation events and showing under some conditions a sawtooth-like character.
The measurements of electron temperature obtained with this method are in good agreement with those performed with other diagnostics
Total radiation losses and emissivity profiles in RFX
Full text linkIn the framework of adding new information to the reversed field pinch (RFP) confinement database, the radiation emitted by RFX plasmas has been investigated using an eight chord bolometric camera, whose detectors have been calibrated absolutely. From the experimental data the emissivity profiles have been reconstructed by means of a generalized tomography reconstruction algorithm. This analysis confirms that the radiation emitted in RFX is systematically concentrated at the edge. The dependence of the emitted power on the plasma density shows that the radiation increases approaching the high density regime, but it rarely goes beyond 30% of the input power for stationary discharges. This behaviour is strongly dependent on the concentration of impurities but, in any case, in RFX there is no evidence of disruptions. A simple local energy balance allows a preliminary evaluation of the radial heat flux profile to be obtained. These measurements indicate that an active impurity screening mechanism is acting in the edge and that transport is the major energy loss mechanism in RFX
Robust scaling laws for energy confinement time, including radiated fraction, in Tokamaks
No full textIn recent years, the limitations of scalings in power-law form that are obtained from traditional
log regression have become increasingly evident in many fields of research. Given the wide
gap in operational space between present-day and next-generation devices, robustness of the
obtained models in guaranteeing reasonable extrapolability is a major issue. In this paper,
a new technique, called symbolic regression, is reviewed, refined, and applied to the ITPA
database for extracting scaling laws of the energy-confinement time at different radiated
fraction levels. The main advantage of this new methodology is its ability to determine the
most appropriate mathematical form of the scaling laws to model the available databases
without the restriction of their having to be power laws. In a completely new development,
this technique is combined with the concept of geodesic distance on Gaussian manifolds so
as to take into account the error bars in the measurements and provide more reliable models.
Robust scaling laws, including radiated fractions as regressor, have been found; they are not
in power-law form, and are significantly better than the traditional scalings. These scaling
laws, including radiated fractions, extrapolate quite differently to ITER, and therefore
they require serious consideration. On the other hand, given the limitations of the existing
databases, dedicated experimental investigations will have to be carried out to fully understand
the impact of radiated fractions on the confinement in metallic machines and in the next
generation of devices
Time Series Denoising based on Wavelet Decomposition and Cross-Correlation between the Residuals and the Denoised Signal
No full textIn this paper, a new denoising method, based on the wavelet transform of the noisy signal, is described. The method implements a variable thresholding, whose optimal value is determined by analyzing the cross-correlation between the denoised signal and the residuals and by applying different criteria depending on the particular decomposition level. The residuals are defined as the difference between the noisy signal and the denoised signal. The procedure is suitable for denoising signals in real situations when the noiseless signal is not known. The results, obtained with synthetic data generated by well-known chaotic systems, show the very competitive performance of the proposed technique
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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