1,720,954 research outputs found
A mechanism to accelerate the late ablation in pulsed plasma thrusters
No full textPulsed Plasma Thrusters (PPTs) are long standing electric propulsion thrusters that are reliable, relatively simple and low cost. One of the main issues with the PPT is its poor utilization of the propellant and low efficiency. Typically only 40-60% of the propellant contributes to the production significant impulse and the efficiency is around 8%. The cause of the PPT’s poor propellant utilization is the late time ablation (LTA), which has a major impact on the efficiency. LTA is the sublimation of propellant that takes place after the main discharge, due to the propellant, usually Polytetrafluoroethylene (Teflon®), being at a temperature above its sublimation point. The LTA produces a low speed gas and macro particles that do not contribute significantly to the thrust. This work presents a way of accelerating the late time ablation by employing an additional discharge after the main discharge, in a separate pair of electrodes. A new thruster, called the two-stage pulsed plasma thruster (TS-PPT) was built and tested. This thruster has two pair of electrodes: primary and secondary. The primary is placed in contact with the propellant surface, as in a regular PPT. The secondary is placed downstream, relatively far from the propellant. A new approach, dividing the PPT thrust generation in two phases, was envisioned. The first phase is responsible for the ablation process and takes place in the first pair of electrodes. The second phase takes place in the second pair of electrodes and is responsible for the acceleration process. The phase division allowed for propellant metering and better propellant utilization. Evidence was
found that a pair of electrodes placed downstream, further from the propellant surface is able to discharge in the late ablation portion of the propellant and can impart extra energy into the exhaust and improve propellant utilization. A simple analytical model was developed to predict trends. A prototype of a TS-PPT was designed and built. A vacuum facility was modified, adapted, partially designed and built. An average mass bit consumption test was carried out. A simple time-of-flight experiment revealed that the fastest portions of the plasma from the primary electrodes discharge were travelling at around 33 km/s. Several current measurements were performed and calculations of the total electrical resistance, total inductance, electromagnetic impulse bit, specific impulse, efficiency, and other parameters were calculated based on experimental data. Experimental results indicated that significant improvements in the specific impulse and efficiency are possible by utilising a two-stage PPT. Specific impulses as high as 4000s were calculated based on experimental results, indicating a better propellant utilization
Enabling low-cost high-energy missions with small spacecraft by using pulsed plasma thrusters
No full textPulsed Plasma Thrusters (PPTs) were the first Electric Propulsion devices ever to be employed in an actual space mission, and continue to be used today when simplicity, robustness and scalability to different power levels are dominant requirements. Therefore, they find a natural niche of application in small-spacecraft missions, where mass, volume and onboard power are at a premium, in spite of their low overall efficiency and not fully understood physical operating principles. While PPTs have drawn renewed attention from the international space community after a long hiatus, this has been generally limited, until now, to low Delta-V, low total impulse missions. In this paper, we investigate the possibility of performing high Delta-v, high total impulse missions, such as orbit raising or even deep-space missions, using PPTs onboard small spacecraft
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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