1,721,037 research outputs found
Coupling impedance and single beam collective effects for the future circular collider (lepton option)
Get PDFIn the framework of the Future Circular Collider study, the high luminosity electron-positron collider FCC-ee will cover a beam energy range from 45.6 GeV to 182.5 GeV, thus allowing very precise measurements of all known heavy particles. The research activity presented in this Ph.D. thesis analyzes some important limitations for the operation of this machine, i.e. electron cloud (EC) and collective effects, particularly critical on the Z resonance due to the low energy and the high beam current. EC build up simulations have been performed for the main components of the machine, revealing the necessity of a NEG coating in the entire ring to lower the Secondary Electron Yield (SEY) of the surface. The presence of this coating affects the resistive wall (RW) impedance seen by the beam, representing the major source of wakefields in the machine due to its large circumference. The work presented in this thesis proves analytically and numerically that for the FCC-ee beam parameters on the Z resonance the contribution of the RW impedance can be reduced by decreasing the thickness of this layer. However, reducing the thickness of NEG coatings can affect the performance of the material itself and therefore the maximum SEY and related EC mitigation. For this reason, this thesis also includes an extensive set of measurements performed at CERN to characterize experimentally Ti-Zr-V thin films with thicknesses below 250 nm in terms of activation performance and SEY. An impedance model was also developed, through the characterization and optimization of the impedance of some important machine components. This model was crucial for a better understanding of single bunch and multi bunch instabilities, thus allowing to identify adequate mitigation techniques for ensuring beam stability during operation. This work also summarizes the impedance studies in the interaction region (IR) of FCC-ee
Comparison between theory and simulations for longitudinal instabilities of coasting beams
No full textBy using the linear kinetic theory and the particle-in-cell code SCOP-RZ/PATRIC the longitudinal instability observed in the ESR has been investigated for different values of the longitudinal impedance of the machine. The numerical investigations show that initially the instability grows exponentially as predicted by the linear theory. When the perturbation reaches large values, significant high-order harmonics are produced: steepening of the density profile as well as saturation of the instability growth appear first, and turbulence later on. Analytical and numerical studies with a small RF voltage applied at the detuned cavity gap have also been carried out in order to meet the conditions in which actual measurements were made at the ESR. (C) 1998 Elsevier Science B.V. All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Theory and simulations of intense laser cooled coasting beams
No full textFor an accurate investigation of the interaction between an intense ion beam and laser light, the dynamical effect on the ions due to laser cooling is added in the kinetic description of the beam. This leads to a modified Vlasov equation, which accounts for space-charge effects and for cooling. The numerical solution of this model provides a suitable tool to study the dynamics of a laser cooled beam. Results from simulations are used here to show both the validity of a perturbative analysis on the kinetic equation and the effects of intra-beam scattering in the cooling process. (C) 2000 Elsevier Science B.V. All rights reserved
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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