1,720,964 research outputs found
MAGNETIC RECONNECTION IN ELECTRON MAGNETOHYDRODYNAMICS
No full textThe magnetic field dynamics and reconnection processes in a highly conducting plasma are investigated in the regimes where Ohm's law is dominated by the Hall term, using a single (electron) fluid description (electron magnetohydrodynamics). In these regimes, which correspond to the frequency range of the so-called whistler (helicon) mode, the electromagnetic field is nearly force free: (j X B)/c + en(e)E = 0. The evolution of the magnetic field in the vicinity of an X line is discussed in the linear and nonlinear regimes. The propagation of whistler waves results in the steepening of their wave front and in the increase of the electric current density in the neighborhood of the magnetic separatrix surfaces. Small-scale magnetic reconnection occurs near surfaces where k.B = 0, with k the mode wave number, and tearing-type modes can be unstable due to the effect of electron inertia. A class of exact self-similar solutions is obtained. These describe, within the scope of a local approximation, the nonlinear time development (magnetic collapse) of the singularities that occur in three-dimensional magnetic configurations. Flat electric current sheets are formed during this collapse. Finally, the rate of reconnection in the electron-magnetohydrodynamic frequency range is estimated in the framework of a steady-state approximation
Transverse-wake wave breaking
No full textA finite-width laser pulse of high intensity propagating in an underdense plasma excites a transversely inhomogeneous, finite amplitude wakefield. This wake wave undergoes a transverse wave breaking due to the increase of the wake front curvature, followed by the self-intersection of electron trajectories. Transverse break occurs at much lower wave amplitudes than the conventional one-dimensional wave break. The resulting structures have generic forms that can be described by modified curves parallel to a parabola. Simulations with the particle-in-cell electromagnetic relativistic code VLPLZD show such structures appearing
Charged Particle and Photon Acceleration by Wake Field Plasma Waves in Nonuniform Plasmas
No full textThe acceleration of charged particles and the upshift of the frequency of short wave packets of laser radiation are discussed. The acceleration and the upshift are caused by plasma waves excited by a strong laser pulse in a nonuniform plasma. It is proved that, by a proper choice of the plasma density profile, it is possible to increase the efficiency of the particle acceleration and of the upshift of the wave packet frequency. It is also shown that unlimited acceleration of charged particles is possible for specific spatial dependences of the plasma density. In this unlimited acceleration regime, particles have a fixed phase relationship with respect to the plasma wave, as their energy increases with time. When the wave-breaking limit is approached and surpassed, the efficiency of the acceleration of the charged particles and of the frequency upshift of the photons can be increased significantly. This is due both to the excitation, in the nonstationary wake behind the laser pulse, of an electric field larger than that produced by a steady plasma wave, and to the rapid electron density variations that occur when the wave breaks
Formation of electromagnetic postsolitons in plasmas
No full textWith particle-in-cell simulations we show that electromagnetic relativistically strong solitons, formed in the wake of the laser pulse, during the interaction of a high-intensity ultrashort laser pulse with a collisionless plasma, evolve asymptotically into postsolitons. A postsoliton is a slowly expanding cavity in the ion and electron densities which traps electromagnetic energy. Fast ions are accelerated during the postsoliton formation. Postsolitons are elementary components of the relativistic electromagnetic turbulence in laser-irradiated plasmas
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
Laser acceleration of charged particles in inhomogeneous plasmas II: Particle injection into the acceleration phase due to nonlinear wake wave-breaking
No full textResults from analytical studies and computer simulations of the nonlinear evolution of wake waves in inhomogeneous plasmas are presented. It is shown that the wake wave-breaking that appears due to the inhomogeneity of the Langmuir frequency can be used to inject electrons into the acceleration phase of the wave. In the wake wave excited behind a finite-width laser pulse, the wave-breaking mechanism involves the increase in the curvature of the wake front with the distance from the pulse, followed by the self-intersection of the electron trajectories
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
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