1,354,450 research outputs found
Reduced-order particle-in-cell simulations of a high-power magnetically shielded Hall thruster
No full textHigh-power magnetically shielded Hall thrusters have emerged in recent years to meet the needs of the next-generation on-orbit servicing and exploration missions. Even though a few such thrusters are currently undergoing their late-stage development and qualification campaigns, many unanswered questions yet exist concerning the behavior and evolution of the plasma in these large-size thrusters that feature an unconventional magnetic field topology. Noting the complex, multi-dimensional nature of plasma processes in Hall thrusters, high-fidelity particle-in-cell (PIC) simulations are optimal tools to study the intricate plasma behavior. Nonetheless, the significant computational cost of traditional multi-dimensional PIC schemes renders simulating the high-power thrusters without any physics-altering speed-up factors unfeasible. The novel reduced-order ‘quasi-2D’ PIC scheme enables a significant reduction in the computational cost requirement of the PIC simulations. Thus, in this article, we demonstrate the applicability of the reduced-order PIC for a cost-efficient, self-consistent study of the physics in high-power Hall thrusters by performing simulations of a 20 kW-class magnetically shielded Hall thruster along the axial-azimuthal and radial-azimuthal coordinates. The axial-azimuthal quasi-2D simulations are performed for three operating conditions in a rather simplified representation of the thruster’s inherently 3D configuration. Nevertheless, we have resolved self-consistently an unprecedented 650 μs of the discharge evolution without any ad-hoc electron mobility model, capturing several breathing cycles and approximating the experimental performance parameters with an accuracy of 70%-80% across the operating conditions. The radial-azimuthal simulations, carried out at three cross-sections corresponding to different axial locations within the discharge channel, have casted further light on the evolution of the azimuthal instabilities and the resulting variations in the electrons’ cross-field mobility and the plasma-wall interactions. Particularly, we observed the development of a long-wavelength, relatively low-frequency wave mode near the exit plane of the thruster’s channel that induces a notable electron transport and a significant ion heating
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Experimental scaling laws for the discharge oscillations and performance of Hall thrusters
No full textLow-frequency (5-30 kHz) discharge current oscillations characterize the operation of Hall thrusters and represent a valuable metric not only to monitor the thruster behavior, but also to optimize the thruster performance. Two types of low-frequency oscillation modes are commonly observed: a global "breathing " mode, associated with the propellant ionization, and a local mode, typically characterized by a lower oscillation amplitude and the appearance of azimuthal spokes. The main characteristics of discharge oscillations and the transition between the two oscillation modes vary greatly with the thruster geometry and the operating condition. In this work, we present the results of an experimental campaign carried out on a 20 kW-class thruster prototype, SITAEL's HT20k, with an exchangeable discharge channel and a magnetic circuit. Three different channel sizes were tested over a wide range of operating conditions and magnetic fields. For each operating point, a high frequency measurement of the discharge current was performed, recording the main characteristics of the oscillations. The data collected were then processed to derive the influence coefficients of each thruster parameter on the discharge current characteristics, as well as their dispersion. Finally, this allowed us to formulate general, data-driven scaling laws for the discharge current salient features, such as oscillation amplitude and dominant frequency. The gathered insight sheds light on the physical processes involved in the thruster discharge. At the same time, the possibility to model with simple functional laws the main oscillatory mode of Hall thrusters offers a unique aid to the optimization of thruster design and the evaluation of thruster performance during life
Variations on the Author
No full text“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
No full textWe 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
Development of high-current hollow cathodes at SITAEL
No full textHollow cathodes are often referred to as one of the most critical elements in electric propulsion thruster units. SITAEL has been working on the development of a family of high- and low- current hollow cathodes to couple with Hall thrusters of power levels ranging from 100 W up to 25 kW. High-current hollow cathodes include the HC20 and HC60 models, which are designed for operation with 5 kW and 20 kW class Hall-Effect Thrusters, respectively. In the framework of Ital-GovSatCom Project, the HC20 EQM is currently under development and is undergoing preparation for the environmental testing. Following the EQM verification activities, a QM will be manufactured and validated through a complete qualification test campaign. In parallel, SITAEL successfully performed a stand-alone and Thruster Unit characterisation of the HC60 EM, developed in the framework of several European Space Agency and European Commission Programs. This paper describes the main strategies, methods and targets for the development of SITAEL's high-current hollow cathodes, the HC20 and the HC60, as well as an overview of the main activities performed during the years 2019-2020, together with the further development plans
Criteri di intervento distribuito per la difesa idraulica. Un caso di studio
No full textViene illustrata l'efficacia, in termini di attenuazione del rischio idraulico, delle opere di sistemazione idraulica distribuite lungo l'asta principale di un corso d'acqua. La nota trae spunto dallo studio per la sistemazione dell'asta principale del fiume Pesa, affluente sinistro del fiume Arno, proposta con il Piano di Bonifica e Tutela del Territorio della Val di Pesa. Gli interventi studiati riguardano la realizzazione di casse di espansione attrezzate, il condiziona-mento semi-strutturale di aree soggette ad inondazione naturale, interventi vincolistici di tipo normativo e relativa rinaturazione delle aree di pertinenza fluviale. Gli effetti di tali interventi hanno mostrato una attenuazione media dei colmi di piena (TR=100 anni) lungo l’asta principale del 16 %, a fronte di opere a limitato impatto ambientale, eseguibili in modo progressivo nel tempo, con un impegno economico relativamente contenuto
Dispelling the Myths Behind First-author Citation Counts
No full textWe 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
Development status of the HT5k high voltage model
No full textSince 2013, SITAEL has been developing its 5kW-class Hall thruster unit, the HT5k-TU, comprising the HT5k thruster and the HC20 hollow cathode. Through several supporting projects, the thruster unit has undergone three design iterations and is now in the Engineering Model definition phase. As the design of the thruster unit evolved towards higher levels of maturity, multiple test campaigns were performed including several characterization tests, magnetic field topology optimization for life extension, tests with alternative propellants, assessment of operation in direct-drive mode, facility background pressure effects tests at two different pressure levels, thermal cycling and endurance test. As a result, the HT5k-TU thruster family cumulated more than 700 hrs of firing, demonstrating stable and efficient thruster operation in the power range of 3 to 7 kW. One important modification in the design of the HT5k thruster was the implementation of magnetic shielding of the discharge channel walls, eliminating in this way one main failure mode of the thruster. This improvement results in increasing the thruster lifetime by at least two orders of magnitude, making the thruster design further apt for station-keeping and deep-space robotic exploration missions of long lifetime. These scenarios typically necessitate operating the thruster at relatively-high specific impulses. However, operation of the thruster unit in the high-voltage range (500-800 V) require updating several design solutions, particularly from the perspective of voltage isolation and heat management. In this respect, to better understand the implications and requirements of high-voltage Hall thruster operation, the HT5k-TU models, both unshielded and shielded, were tested in this voltage range. The aim was to investigate the performance and stability of the thruster, in addition to better understanding the impacts from the design compatibility point of view. This paper presents an overview of the development activities performed on the various models of the HT5k-TU, focusing on the characterization results in the high-voltage regime. Furthermore, we also discuss the solutions devised for addressing the technological requirements of high-voltage Hall thruster's operation
Author, publisher and bookseller : a tripartite synergy in Nigerian book industry
No full textThis work is about the roles of Author, Publisher and Bookseller in Book development in
Nigeria. The paper started by delving into the history of Book Publishing in Nigeria after
which it proceeded by defining who an author, a publisher, and a bookseller is and
expatiated on the indispensable roles of these key actors in Nigerian Book Industry and in
the emerging Information Society. Furthermore, the various constraints to book
development were identified while the paper advised on how the Book Industry can be
further promoted in Nigeria. However, the paper concluded and made recommendations
on how the Book sector can help in enhancing scholarship in the country
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