1,720,967 research outputs found
Optimization of the magnetic nozzle of a 50 W helicon plasma thruster
No full textLa propulsione elettrica, caratterizzata da un alto impulso specifico e da un'alta efficienza, è una soluzione molto attraente nel campo della propulsione spaziale; soprattutto nel crescente mercato dei satelliti a basso costo, dove la riduzione della massa del propellente è una delle principali questioni. Negli ultimi dieci anni, molti sforzi sono stati fatti per lo sviluppo e lo studio di propulsori elettrici chiamati Helicon Plasma Thruster (HPT), un propulsore elettrico in cui la spinta viene ottenuta accelerando, attraverso un ugello magnetico (MN), il plasma prodotto in una sorgente Helicon. T4i - Technology for Propulsion and Innovation, in collaborazione con l'Università di Padova, ha sviluppato Regulus, un propulsore HPT da 50W, la cui dimostrazione in orbita è attualmente in svolgimento. L'ottimizzazione e l'indagine fisica per identificare i parametri di guida per la progettazione della sorgente di plasma e dell'ugello magnetico di REGULUS è in corso. Le prestazioni di un Helicon Plasma Thruster (spinta e impulso specifico) sono strettamente correlate a come viene prodotto e accelerato il plasma; per ottimizzare un propulsore helicon è quindi essenziale sviluppare un codice numerico che consenta di studiare la fisica del plasma all'interno della sorgente e dell'ugello magnetico. Tuttavia, nonostante il design semplice, la fisica del plasma dietro un propulsore helicon è estremamente complessa, rendendo la modellazione numerica estremamente difficile e attualmente in studio da diversi centri di ricerca. 3D-VIRTUS, un codice fluido sviluppato internamente dall'Università di Padova e Bologna, consente di simulare la dinamica del plasma all'interno della sorgente. Questo codice fluido non è utilizzabile per lo studio del plume e dell'ugello magnetico, dove la densità del plasma scende di diversi ordini di grandezza. Solo di recente le simulazioni 3D della dinamica del plasma di un MN sono state computazionalmente accessibili: ho indagato come modellare la dinamica del plasma dell'ugello magnetico e ho sviluppato un codice PIC 3D completo a collisioni Monte Carlo, chiamato ProPic, specificamente progettato per simulare la dinamica del plasma dell'ugello magnetico di un propulsore helicon in un dominio non assialsimmetrico. La teoria, gli algoritmi e le condizioni al contorno che ho usato per simulare l'ugello magnetico sono stati studiati, sviluppati e sperimentalmente validati. Ulteriori studi hanno permesso di utilizzare ProPic per la valutazione dell'interazione reciproca tra satellite, plume di plasma e plasma ambiente. Inoltre, per la prima volta in letteratura, è stato presentato uno studio numerico della dinamica del MN in un cluster di HPT. Una suite numerica, sviluppata dall'Università di Padova e dall'Università di Bologna, include sia codici numerici che codici PIC è stato sviluppato per studiare un propulsore Helicon nella sua interezza e per ottimizzarne le prestazioni. Questa suite numerica è stata sperimentalmente validata e utilizzata da T4i per studiare le prestazioni propulsive e le interazioni di un HPT con il satellite in orbita e il plasma ambiente.Electric propulsion, characterized by a high specific impulse and high efficiency, is a very attractive solution in the field of space propulsion; especially in the growing market of low-cost satellites, where the reduction of propellant mass is one of the main issues. In the last decade, much effort has been put into the development and study of electric propulsion thrusters called Helicon Plasma Thruster (HPT), an electric propulsion thruster in which thrust is obtained by accelerating, through a magnetic nozzle (MN), the plasma produced in a Helicon source. T4i - Technology for Propulsion and Innovation, in collaboration with the University of Padova, has developed Regulus, a 50W HPT thruster, whose demonstration in orbit is currently underway. However, the optimization and physical investigation to identify the driving parameters for the plasma source and magnetic nozzle design of REGULUS is ongoing. The performance of a Helicon Plasma Thruster (thrust and specific impulse) is closely related to how the plasma is produced and accelerated; to optimize a helicon thruster it is therefore essential to develop a numerical code that allows to study the physics of the plasma inside the source and the magnetic nozzle. However, despite the simple design, the plasma physics behind a helicon thruster is extremely complex, making numerical modelling extremely difficult and currently under study by several research centers. 3D-VIRTUS, a fluid code developed internally by the University of Padova and Bologna, allows to simulate the plasma dynamics inside the source. This fluid code is not usable for the study of the plume and the magnetic noozle, where the plasma density drops by several orders of magnitude. Only recently 3D simulation of the plasma dynamics of a MN have been computationally affordable: I have investigated how to model the plasma dynamics of the magnetic noozle and I developed a full kinetic 3D Particle-In-Cell (PIC) code with Monte Carlo collisions, called ProPic, specifically designed to simulate the plasma dynamics of the magnetic noozle of a helicon thruster in a non axysimmetric domain. The theory, algorithms, and boundary conditions that I used for simulating the magnetic noozle have been studied, developed, and experimentally validated. Further studies have allowed to use ProPic for the evaluation of the mutual interaction between spacecraft, plasma plume, and ambient plasma. Furthermore, for the first time in literature, a numerical study of the MN dynamics in a cluster of HPTs has been presented. A numerical suite, developed by the University of Padua and the University of Bologna, includes both numerical codes and PIC codes was developed in order to study a Helicon thruster in its entirety and to optimize its performance. This numerical suite was experimentally validated and used by T4i to study the propulsive performance and interactions of an HPT with the satellite in orbit and the ambient plasma
Satellite surface charging in LEO with ProPIC
No full textProPIC is a fully kinetic particle-in-cell (PIC) solver developed for space electric propulsion. This work has extended its capabilities to simulate satellite surface charging and wake generation in low Earth orbit (LEO). A novel scaling approach has been implemented, decreasing computational cost by more than one order of magnitude. The methodology and scaling approach have been verified against the revised orbital-motion-limited theory. The surface charging and wake generation in LEO have been examined for a satellite that is more complex and larger than what is typically handled with a fully kinetic PIC approach in LEO, particularly due to the presence of large solar panels. Notably, the simulated wake can be used to identify the optimal position for the plasma diagnostic sensor that minimizes interference with the wake. Moreover, despite not being a failure risk, the attitude greatly influences the surface charging of a satellite with large solar arrays installed parallel to the satellite speed vector. The study suggests that, for high positive pitch angles (>45∘), the surface charging of the solar panels can increase by as much as 75% compared to low negative pitching cases. Additionally, the study highlights that the pitch angle and satellite envelope along the motion direction significantly influence the potential gradients on the solar panels.Submitted/Accepted versio
Magnetic nozzle performance in a cluster of helicon plasma thrusters
No full textA numerical study of the plasma dynamics in a Helicon Plasma Thrusters’ (HPT) cluster is presented. For the first time in the literature, the three-dimensional (3D) plasma dynamics occurring in the plume of a HPTs’ cluster is analyzed. The physical investigation relies on ProPic, a 3D particle-in-cell (PIC) code specifically designed to simulate the plasma dynamics in magnetic nozzles and in a non-axi-symmetric domain. The code has been validated against experiments reported in the literature and cross-validated with Starfish, an open-source two-dimensional PIC software. The physical investigation has revealed an interesting mutual influence between the thrusters that constitute the cluster. Three significant phenomena that affect the cluster’s performance have been identified. The first phenomenon is related to the effect that clustering has on the shape of the magnetic field lines and, in turn, on the divergence angle of the plume. The second phenomenon is related to electron currents flowing among different thrusters, which affect the potential drop across the plume. The third phenomenon is related to the effect that neighboring thrusters have on the plasma potential map and, in turn, on the expansion of the ions.Published versio
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
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