1,721,021 research outputs found
Design and test of a module of a breathable Electrostatic Shield for the MITICA 1 MV negative ion Beam Source
Full text linkThe electrical insulation of the MITICA Beam Source at 1 MV is a challenging issue, which has not been fully addressed so far on the basis of experimental results and of theoretical models available in literature. Being MITICA the full-size prototype of the Heating Neutral Beam Injector for the ITER fusion experiment, its electrical insulation is constituted just by vacuum gaps and alumina insulators, since other insulating materials such as SF6 gas or fibreglass-reinforced plastic (FRP) would be quickly degraded by the expected neutron flux produced by fusion reaction. Extrapolations based on HV tests on reduced-scale models have recently indicated the risk of electrical breakdowns in the vacuum gap between electrodes nominally operating at −1 MV and the vacuum vessel (at ground potential). The risk of electrical breakdown can be mitigated by introducing an intermediate Electrostatic Shield (ES), which essentially is an equipotential (metallic) enclosure surrounding the HV electrode, so as to divide the vacuum gap in two independent insulating gaps of 400 kV and 600 kV respectively. However, for optimal negative ion production, the ion source shall operate in H2 or D2 at a pressure of ∼ 0.3 Pa and unavoidably produces a flow of gas leaking out in the surrounding vacuum. Thus, the presence of an intermediate shield can substantially increase the background gas pressure in the vacuum gaps, and, due to the large gap length (0.6 m), exacerbate the risk of breakdown when the pressure approaches the conditions of Paschen-type discharges. In addition to this, RF-induced breakdowns were found on the rear side of the ion source during the operation of the prototype source SPIDER, which were somewhat correlated to a relatively high hydrogen pressure in that area. For these reasons, a structure capable of constituting a full equipotential barrier all around the BS and, at the same time, having sufficient gas conductivity (breathability) to allow efficient pumping of background gas, has been designed. In the first part of the paper, the requirements and design optimization of a breathable module of the intermediate ES are described. Then, an experimental campaign for the validation of the electrode implementation the test configurations and the experimental procedure is discussed
Optical Diagnostics for High-Voltage Tests in MITICA
No full textThe Neutral Beam Test Facility (NBTF), Padua, Italy, is a facility devoted to the test and optimization of the Neutral Beam (NB) Injector for ITER. A full-scale prototype, called MITICA, is under construction at NBTF and has to demonstrate all the design parameters for ITER, among which a stable beam operation at the energy of 1 MeV. To test and optimize the stable high voltage holding of 1 MV, a dedicated campaign is foreseen for 2022–2023, utilizing realistic mockups of MITICA ion source and accelerator. During the campaign, the mockup of the ion source will be biased with an increasing potential up to −1 MV and the discharge toward the grounded electrodes will be observed. In this way, possible weak points of the geometry will be detected and possibly mitigated. As diag- nostics for arc discharge position and intensity, a system of cameras was designed, including visible cameras, high-speed camera, and IR camera, placed on the available viewports of beam source vessel, and pointed toward the areas with higher probability of electric discharge. The results of this optical diagnostics will be then compared with the electric measurements of the discharges taken on a dedicated ground electrode, for a cross check. This article describes the motivations of the choice of the type and number of diagnostic cameras adopted during the experiments, the optimization of their layout, which includes also the use of mirrors, and the design of mechanical supports
Design of electrodes for high voltage tests in MITICA
No full textThe NBTF (Neutral Beam Test Facility) in Padova, Italy, is dedicated to testing and optimizing the Neutral Beam Injector for ITER. A full-scale prototype known as MITICA is currently being constructed at NBTF to demonstrate all the design parameters for ITER, including stable beam operation at 1MeV energy. To achieve stable high voltage holding of 1MV, a dedicated campaign is planned for 2023, which will utilize realistic mockups of the MITICA ion source and accelerator. During this campaign, the mockup of the ion source will be biased with an increasing potential up to -1MV, and the discharge towards the grounded electrodes will be observed. This will help identify any potential weak points in the geometry that can be mitigated. This work briefly describes the design process of the mock-up electrodes, including electrostatic and structural analyses. The construction of the electrodes has started in 2022 and the installation is foreseen for 2023
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
A novel tool for breakdown probability predictions on multi-electrode multi-voltage systems
No full textAn innovative approach for the voltage breakdown prediction in high-voltage systems, insulated by large vacuum gaps, is presented. It is based on the correlation between the clump mechanism and a statistical approach to the breakdown probability. The aim of this paper is twofold. First, the numerical solution of 3-D electrostatic problems by a couple of complementary formulations is presented. Second, an efficient post-processing tool is introduced, based on the analytical solution of the equations of motion in a domain covered by a tetrahedral mesh, to estimate the breakdown probability associated to the electrically charged microparticles leaving one electrode and clashing to the electrode with opposite polarity with sufficient energy to get vaporization. This approach has been benchmarked on a reference configuration (sphere/plane) problem and applied to calculate the particle trajectories in a very complex multi-electrode multi-voltage system
Variations on the Author
Full text link“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
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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