1,720,983 research outputs found
Plasma characterization in Hall thrusters by Langmuir probes
No full textOne of the main factors that limits the Hall thruster lifetime is the degradation of surfaces exposed to plasma caused by the flux of energetic ions. Since the magnetic field shapes the plasma properties inside the thruster channel, non-standard magnetic configurations have been proposed in recent years to substantially increase the thruster lifetime. The present article reports an experimental investigation of different magnetic topologies of a Hall thruster operating on xenon and krypton. Plasma properties were measured using probes installed on the channel walls and a fast diving probe. Data are analysed by means of a Bayesian methodology. Results show that magnetic shielding correctly reduces the interaction between the plasma and the channel walls, which can result in lower erosion rates. A comparison between xenon and krypton operation showed that magnetic shielding is less effective for krypton, as the grazing line presents higher temperatures. Fast probe results also show how krypton discharges present wider acceleration regions and higher temperatures inside the channel
Direct-drive system demonstration for a low-power hall thruster
No full textA study was conducted to demonstrate a direct-drive system for a low-power hall thruster. The HT-100 Hall thruster was selected for the direct-drive demonstration. The throttling range of the thruster was approximately 100-300 W discharge power, 800-1200 s specific impulse, and 5-12mN thrust, with efficiency up to 35%. The solar array configuration was characterized by a set of four terrestrial solar panels populated by thin-film amorphous silicon solar cells. Investigations revealed that thruster performance and solar array characteristics were closely related in a direct-drive system, making it important to understand which were the factors affecting the solar array I-V curve and how they influenced it. The solar array performance was affected mainly by the irradiance and the cell temperature
Flight Experiment on the Equivalence Principle with Field Emission Electric Propulsion (FEEP2)
No full text
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
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