1,720,969 research outputs found
FEEP New Propellants
No full textExecutive Summary of Final Report to ESA Contract 22873/09/F-VS, "FEEP New Propellants
Theoretical and Experimental Analysis of a Two-Stage System for Classification
No full textWe consider a popular approach to multicategory classification tasks: a two-stage system based on a first (global) classifier with rejection followed by a (local) nearest-neighbor classifier. Patterns which are not rejected by the first classifier are classified according to its output. Rejected patterns are passed to the nearest-neighbor classifier together with the top-it ranking classes returned by the first classifier. The nearest-neighbor classifier, looking at patterns in the top-h classes, classifies the rejected pattern. An editing strategy for the nearest-neighbor reference database, controlled by the first classifier, is also considered. We analyze this system, showing that even if the first level and nearest-neighbor classifiers are not optimal in a Bayes sense, the system as a whole may be optimal. Moreover, we formally relate the response time of the system to the rejection rate of the first classifier and to the other system parameters. The error-response time trade-off is also discussed. Finally, we experimentally study two instances of the system applied to the recognition of handwritten digits. In one system, the first classifier is a fuzzy basis functions network, while in the second system it is a feed-forward neural network. Classification results as well as response times for different settings of the system parameters are reported for both systems
Characterization of Linear Slit FEEP Using a Ionic Liquid Propellant
No full textA ionic liquid has been used as propellant in a linear slit FEEP thruster for the first time. The test was carried out using a standard FEEP emitter fed with the ionic liquid BMIM - Tf2N. The emitter was easily ignited and operated for about 200 hours successfully. Extraction of particle beams was demonstrated both with positive and negative applied voltage, with very stable current at positive polarity and more intense, less stable emission in negative particle mode. In positive mode, emission was remarkably stable for operation at 6 kV during as much as 50 hours. Some dependancy of extracted current on emitter temperature was observed. The estimated maximum thrust was 20 μN in positive mode and 150 μN in negative mode. The applicability of ionic liquid propellants in operational linear slit FEEP thrusters will have to be assessed by future investigation, in particular regarding the ionic or microdroplet nature of the emitted beam
Development of a Miniaturized Electric Propulsion System for the E-Sail Project
No full textThe Electric Sail (E-Sail) is an innovative space propulsion concept relying on the solar wind momentum for producing thrust through the interaction of solar wind protons with a number of very long, electrically charged tethers. With such long wires, centrifugal force alone is not sufficient to ensure complete deployment of the assembly. Therefore, small thrusters are needed to increase/decrease the angular velocity and, accordingly, determine centrifugal force during tether deployment. In consideration of the stringent requirements of E-Sail, a variant of the Field Emission Electric Propulsion (FEEP) system has been selected, based on the use of ionic liquids as propellant, instead of alkali metals. This paper discusses the unique challenges posed by the development of the E-Sail Remote Unit thruster system. The envisaged simplified ionic liquid FEEP has a large potential for utilization on a variety of micro- and nano- satellite platforms, from Cubesats to Earth observation small satellites, to future scientific mission
Ionic Liquid FEEP: Recent Experimental Results
No full textThe ionic liquid FEEP (IL-FEEP) is new, simplified version of the field emission thruster where the alkali metal propellant is replaced by an ionic liquid. Development of the IL-FEEP thruster is based on the heritage of the cesium FEEP and is currently funded by ESA and EU programmes. This paper presents the results of a recent experimental campaign aimed at assessing the performance of a novel linear slit FEEP emitter fed with a ionic liquid propellant. Tests were done on a newly designed, 8-mm slit FEEP emitter made of glass, fed with EMI-BF4. The test setup allowed for continuous monitoring of the thruster electrical parameters, as well as for direct scanning of the exhaust beam profiles using electrostatic wire probes. The test results essentially validate the possibility to operate the linear slit FEEP emitter with ionic liquid propellant in pure ionic mode and confirm that the glass emitter is suited for operation with the selected propellant. Experimental results show very regular and repeatable performance in positive mode, while operation in negative polarity showed some irregularities in beam profiles. Thruster performance in a later test in alternate polarity mode was remarkably stable over a very long period, in spite of the predominance of positive mode firing that could have affected the propellant composition in the reservoir due to electrochemical degradation. Alternate polarity is therefore regarded as a promising operating mode for the simplified FEEP thruster
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
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