1,721,022 research outputs found
Analysis, simulation and testing of advanced electrodynamic systems for space propulsion
Get PDFHelicon plasma sources are an attractive means of plasma production in advanced applications of space propulsion thanks to their high ionization efficiency. By means of a compact helicon source, it is possible to enhance the performances of a small propulsive apparatus from a range of almost negligible values (few micro-Newton of thrust and few tens of seconds of specific impulse) to a range useful for many applications (thrust of milli-Newton and 1000s seconds of specific impulse). However, the design of such a system is complicated by the great inter-correlations between the involved parameters, and by the complex physical mechanisms involved. Moreover, thruster operation involve physical regimes which are not common in industrial plasma sources. In this research program, helicon physics has been deeply analyzed in a innovative way, not limiting the study to helicon wave analysis with dielctric tensors, but recognizing that the electromagnetic wave propagation must be closely related to macroscopic transport. An equilibrium theory of an helicon discharge has been derived, where both the plasma-wave local coupling of the RF antenna field with the plasma, and the transport of plasma species at the macroscopic level, are considered. The theory has been practically implemented on a code and validated with experiments in Laboratory. The theory has allowed to close the correlations between the design parameters and to identify preliminary design configurations of helicon thrusters.Grazie alla loro elevata efficienza, le sorgenti al plasma di tipo helicon sono un attraente mezzo di produzione di plasma per applicazioni avanzate di propulsione spaziale. Per mezzo di una compatta sorgente helicon, è possibile incrementare le prestazioni di un piccolo apparato propulsivo, da un range di valori pressoché trascurabili (pochi micro-Newton di spinta e qualche decina di secondi di impulso specifico) ad un range utile per molte applicazioni (spinta del milli-Newton e migliaia di secondi di impulso specifico). Tuttavia il design di un tale sistema è complicato dalla grande inter-correlazione tra i parametri coinvolti, e dai complessi meccanismi fisici coinvolti. Inoltre il regime operativo del thruster avviene in regimi fisici non comuni per plasmi di sorgenti industriali. In questo programma di ricerca la fisica fondamentale degli helicon è stata attentamente studiata, non limitando le analisi al problema di accoppiamento d'onda con tensore dielettrico, ma riconoscendo che il problema elettromagnetico è strettamente connesso al traporto macroscopico. E' stata sviluppata una teoria dell'equilibrio di una scarica helicon, dove sono tenuti in considerazione sia l'accoppiamento locale plasma-onda del campo RF (radiofrequenza) dell'antenna con il plasma, che il trasporto delle specie di plasma a livello macroscopico. La teoria è stata implementata in un codice e validata con esperimenti in Laboratorio. La teoria ha permesso di chiudere le correlazioni fra i parametri di design e di identificare delle configurazioni preliminari di propulsori helicon
Stability of Electrodynamic Tethers in a Three-Body System
No full textAbstract - The dynamics of an electrodynamic tether orbiting in a three-body gravitational field near the equilibrium
positions of the system is analyzed. First, the perturbed classical three-body problem is derived in general terms;
then, the in-plane force perpendicular to the local vertical is analyzed in more detail because of its relevance to
electrodynamic tethers. Because of the presence of electrodynamic forces, the locations of equilibrium points are
modified from their classical Lagrangian positions (that are valid for a null electrodynamic force) and finally
disrupted as the electrodynamic force increases.Alinear variational analysis is carried out to characterize the motion
of the tethered system around the perturbed equilibrium locations and to compute the variation of the
eigenfrequencies versus the intensity of the electrodynamic force. The study of small-amplitude motion about the
perturbed equilateral positions has shown the existence of in-plane orbits (Lyapunov) and out-of-plane orbits
(Lissajous and halo types) around those points. Large-amplitude orbits, which include nonlinear effects, have been
studied using numerical integration of the equations of motion. Numerical tests have proven that when the higher of
the two eigenfrequencies is excited, the trajectory is stable and converges toward the equilibrium point. The analysis
is of immediate use to the orbit design of motion around perturbed Lagrangian points of electrodynamic tethers
SPIREs: A Finite-Difference Frequency-Domain electromagnetic solver for inhomogeneous magnetized plasma cylinders
No full textWe present SPIREs (plaSma Padova Inhomogeneous Radial Electromagnetic solver), a Finite-Difference Frequency-Domain (FDFD) electromagnetic solver in one dimension for the rapid calculation of the electromagnetic fields and the deposited power of a large variety of cylindrical plasma problems. The two Maxwell wave equations have been discretized using a staggered Yee mesh along the radial direction of the cylinder, and Fourier transformed along the other two dimensions and in time. By means of this kind of discretization, we have found that mode-coupling of fast and slow branches can be fully resolved without singularity issues that flawed other well-established methods in the past. Fields are forced by an antenna placed at a given distance from the plasma. The plasma can be inhomogeneous, finite-temperature, collisional, magnetized and multi-species. Finite-temperature Maxwellian effects, comprising Landau and cyclotron damping, have been included by means of the plasma Z dispersion function. Finite Larmor radius effects have been neglected. Radial variations of the plasma parameters are taken into account, thus extending the range of applications to a large variety of inhomogeneous plasma systems. The method proved to be fast and reliable, with accuracy depending on the spatial grid size. Two physical examples are reported: fields in a forced vacuum waveguide with the antenna inside, and forced plasma oscillations in the helicon radiofrequency range
Exploration of the Galilean Moons using electrodynamic tethers for propellantless maneuvers and self-powering
Get PDFAbstract. Recent studies have demonstrated the benefits of using electrodynamic tethers (EDT) for the exploration of the inner region of the Jovian system. Intense planetary magnetic field and reasonable environmental plasma density make the electrodynamic interaction of the
conductive tether with the plasmasphere strong. The interaction is responsible for a Lorentz
force that can be conveniently used for propellantless maneuvers and extraction of electrical
power for on board use. Jupiter and the four Galilean Moons represent an exceptional gravitational
environment for the study of the orbital dynamics of an EDT. The dynamics of such a system was analyzed using a 3-body model, consisting of the planet plus one of its moons
(Io in this work) and the EDT itself. New and interesting features appear, like for example the
possibility to place the tether in equilibrium with respect to a frame co-rotating with the moon at points that do not coincide with the classical Lagrangian points for non-null electrodynamic forces
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
- …
