1,721,224 research outputs found
Superconductor-based passive shielding and screening systems
Full text linkThe attenuation of magnetic fields is crucial for various application fields, including health,
space exploration, and fundamental physics, to name just a few. Superconductors are key materials for addressing this challenge. In this review, we mainly focus on the shielding and screening of quasi-static magnetic fields using superconductor-based passive layouts. After providing a brief overview of the principles of magnetic shielding and screening using superconductors, we outline commonly used procedures for measuring the field attenuation. Next, we give an insight into analytical and numerical models able to reproduce experimental results and predict the performances of new designs. Key challenges and achievements in employing low temperature or high temperature superconducting bulk and tape-based structures for reducing a given applied field are then discussed. Additionally, hybrid designs combining superconducting and ferromagnetic materials, aimed at enhancing the shielding ability or fabricating magnetic cloaks, are described. Finally, we highlight future challenges and potential
advancements in this technology
Investigation of methods for combining several trapped field superconducting magnets to obtain large magnetic field gradients
Full text linkThe unique magnetic and electric properties of type II superconductors make these materials extremely attractive in applications where large magnetic flux density gradients are desired. Indeed, these gradients can be exploited to exert remotely a magnetic force which is of interest for magnetic levitation systems, magnetic drug delivery systems or brake systems to enumerate a few. The ability of type II superconductors to trap permanent superconducting current loops allows them to act as permanent magnets with the significant advantage that they can generate magnetic flux densities up to one order of magnitude larger than the saturation magnetization of conventional ferromagnetic materials. This thesis aims to determine how several trapped-field magnets with non-parallel magnetization directions can be combined efficiently to generate large magnetic field gradients and surpass the limits of ferromagnetic materials. Starting from configurations inspired by those involving conventional magnets, the work elucidates the
advantages and potential limitations associated with superconducting assemblies. Then, several modifications, either in the geometry of the assembly or in the combination procedure, are proposed and explored to enhance the performance of the final structure. To this aim, two cryogenic experimental setups are designed, assembled, calibrated and used extensively.
The first setup allows the assembly of up to five pre-magnetized samples arranged linearly
through a precisely controlled translation, at liquid nitrogen temperature. This system is
used to investigate how superconductors magnetized independently can be efficiently arranged in a Halbach array configuration. Trapped-field measurements show that a superconducting Halbach array of three YBaCuO samples produces, at a distance of 20 mm, a magnetic flux density gradient 30% higher than an isolated sample. Since the magnetizations of neighbouring trapped-field magnets are perpendicular in such arrangements, each superconductor of the array inevitably experiences a time-varying field component perpendicular to its main magnetization direction during the assembly process. Considering that magnetized superconductors are prone to partial demagnetization under these circumstances, a specific emphasis is placed on developing
methods to mitigate the detrimental impact of such partial demagnetization. To this aim, the use of superconductors with triangular cross-sections as peripheral samples in a three-sample configuration is shown to significantly reduce the demagnetization and to maintain better performances than stand-alone superconductors. Alternatively, using the trapped magnetic field of an additional sample positioned above the central superconductor of the array, and extracted after the assembly process, is found to be an effective re-magnetization technique.
The second setup is an insertion tool compatible with the sample chamber of a Physical Property Measurement System. It allows for the in-situ magnetization of two samples and for the controlled rotation of one sample by an angle of up to 190°. With this system, the magnetic flux density gradient achieved with either a single or a pair of magnetized YBaCuO superconductors in the presence of a uniform background DC magnetic field is investigated at 59 K, 65 K and 77 K. Although the background field reduces the trapped field ability of individual samples, it is shown that substantial gradients can still be generated in such conditions. This investigation reveals that the distance between the samples is sufficient to avoid any mutual demagnetization effect during the rotational motion. As a result, the combined contribution of each sample generates a higher magnetic flux density gradient in comparison to the one produced with a stand-alone superconductor
Investigation of passive magnetic shielding methods combining bulk superconductors and coated conductors
Full text linkA lot of engineering applications require the development of electrical devices with high power densities and involve large magnetic fields. These devices are generally character- ized by high values of stray flux densities potentially detrimental to their environment. In this context, it is necessary to screen these devices by means of effective magnetic shields. Large flux densities in the low frequency regime (typically < 100 Hz) cannot be efficiently shielded by conventional ferromagnetic or conducting materials and require the use of superconductors. The goal of this thesis is to investigate methods to combine the properties of bulk high-temperature superconductors and second generation coated conductors in order to design sizable passive magnetic shields.
In order to develop efficient magnetic shields, it is necessary to obtain information on the properties of the individual superconducting samples. Accordingly, the first part of this thesis consists in characterizing the electrical and magnetic properties of bulk sam- ples and coated conductors. In addition to providing critical information to interpret experimental results, these properties can also be used to implement numerical models. In this work, bulk superconductors are characterized in terms of their critical current den- sity and flux trapping ability. Some of the coated conductors are deposited on a slightly ferromagnetic substrate. An experimental set-up is thus developed to characterize the ferromagnetic properties of these substrates.
Then, hybrid magnetic shields (combining bulk superconductors and coated conduc- tors) are investigated experimentally. The study is performed in configurations of in- creasing complexity both in terms of the uniformity of the applied magnetic field and in terms of the symmetry of the shield structure itself. In particular, a cylindrical shield made of coated conductor annuli stacked together is characterized under different config- urations of external applied field. Methods to improve the efficiency of such a shield are also presented. Next, we demonstrate the ability to screen the stray flux density over a significant volume by using arrays of bulk superconducting samples. The influence of the non-superconducting joints between the different samples can be mitigated by combining these arrays with coated conductors in closed loop configuration.
Finally, an experimental prototype of hybrid magnetic screen is tested in the vicinity
of a large field superconducting magnet exhibiting a significant stray flux density. These conditions are commonly encountered in large scale engineering applications. This work is performed in the Department of Applied Science and Technology at Politecnico di Torino (Italy).
The various experimental results obtained throughout this study can be successfully reproduced by 2D and 3D numerical models. These models are useful to interpret the experimental data and estimate the influence of the superconducting parameters on the efficiency of the shield. Consequently, they allow for a deeper understanding of the shield- ing mechanisms. The results obtained in the framework of this thesis demonstrate the relevance of combining bulk superconductors and coated conductors for low frequency magnetic shielding. The intent is that these results can provide insights to design a variety of efficient magnetic shields combining both kinds of superconductors
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
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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