1,721,186 research outputs found
Coupling between nonlinear Langmuir waves and electron holes in quantum plasmas
No full textThe nonlinear effects on a slow timescale, compared with the electron plasma frequency, are studied using the Wigner-Poisson system, in the plasma regimes characterized by the overlapping of the wavefunctions of individual electrons, by the presence of a large amplitude Langmuir pump wave, and whose temperature is higher than the Fermi temperature. It is shown that the electron trapping on closed orbits in phase space is strongly affected both by the classical nonlinear ponderomotive effects and by the quantum super-diffusion. A solution in the form of a quantum corrected electron hole is found in terms of a generalized energy in the Wigner equation that contains higher derivatives in velocity space. In the classical limit, the high-frequency pump hampers the electron trapping due to the unfavorable sign of the ponderomotive potential and due to deformation of their distribution function by the diffusion. Conversely, in the modulational regime the leading quantum effect is shown to be related with the effective super-diffusion in the velocity space associated with the quantum effects on the high-frequency pump, which facilitates the electron trapping and allows the creation of holes with smaller amplitudes. © 2006 Elsevier B.V. All rights reserved
Description of the Nonlinear and Collective Behavior of Charged-Particle Beams by means of a Nonlinear Schroedinger Equation
No full textCharged-particle beams are employed for a number of scientific and technological applications.
The conventional description of their collective behavior is usually given in terms of the Vlasov equation. In the last 15 years some alternative descriptions have been developed in terms of
a nonlinear Schrödinger equation governing the collective dynamics of the beam while interacting with the surrounding medium. This approach gives new insights, providing an alternative "key of reading" of the charged-particle beam dynamics, and have been applied to a number of physical
problems concerning conventional particle accelerating machines as well as plasma-based accelerator schemes. Remarkably, it is based on a mathematical formalism fully similar to those used for
the propagation of e.m. radiation beams in nonlinear media a well as the nonlinear dynamics of the Bose-Einsten condensates.
In this paper, a presentation of some significant nonlinear collective effects of a charged-particle beam in particle accelerators, that have been recently investigated in the framework of the above Schrödinger-like descriptions, is given
A self-consistent picture for hyper-velocity metal dust in FTU
No full textAbstract:
Numerical simulations of the propagation of plasma blobs in the scrape-off layer (SOL) of the Frascati Tokamak Upgrade (FTU) are presented. The maximum distances that a plasma blob may travel in the radial direction are calculated under FTU physical conditions and geometry, taking into account the dissipation due to the presence of dust. It is found that a relatively small amount of nanometre iron dust particles can dissipate blobs before they reach the wall. The amount of dust required is of the same order as, or smaller than, that needed to produce hyper-velocity dust particles, so that a consistent interpretation is reached for experiments in FTU where evidence of the dust impact ionization was reported
Vortices in relativistic electron beams
No full textWe demonstrate that a relativistic electron beam is properly described in the moving frame by the electron-magnetohydrodynamic equations of plasma physics. For large beam currents, the accelerator magnetic field is expected to be unstable to the fast magnetic reconnection. We present a plausible saturated state of the fast reconnection, in the form of a complex vortex pattern. The nonlinear dispersion equations of the vortex are derived and the relationship between the vortex structure and the background magnetic field is discussed
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
Self-modulation of a long externally injected relativistic charged-particle beam in a laser wake field acceleration scheme. A preliminary quantum-like investigation
No full textRecent investigations indicate that sufficiently long beams of charged particles, travelling in a plasma, experience the phenomenon of self-modulation. The self-modulation is driven by the plasma wake field excitation due to the beam itself, and it may become unstable under certain conditions. A preliminary theoretical investigation of the self-modulation of a relativistic charged-particle beam in overdense plasma in the presence of a preformed plasma wave is carried out, within the quantum-like description of charged particle beams provided by the Thermal Wave Model. A simple physical model for the self-modulation is put forward, described by a nonlinear Schrodinger equation coupled with the Poisson-like equation for the plasma wake potential (so-called Fedele-Shukla equations). The physical mechanism is based on the interplay of three concomitant effects, the radial thermal dispersion (associated with the emittance epsilon), the radial ponderomotive effects of a preexisting plasma wave (which provides the guidance for the beam), and the self-interaction of the plasma wake field generated by the beam itself
Propagation of ultrastrong femtosecond laser pulses in PLASMON-X
No full textIn this work we present the derivation of the nonlinear equations
that describe the propagation of ultrashort laser pulses in a
plasma, in the PLASMON-X device, using a fully relativistic
hydrodynamic description for electrons. It is shown that for the
PLASMONX scheme used for the electron acceleration, it is
justified to use a stationary 1-D approximation in the electron
hydrodynamic equations, since the pulse width is sufficiently
bigger than the pulse length. Furthermore, with the laser power of TW and the initial spot size, the nonlinearity is sufficiently weak to allow for the power expansion in the nonlinear Poissons's equation, yielding a version of the nonlocal nonlinear Schr\"{o}dinger eqiation, with a periodic nonlocality. While in a one-dimensional limit the standard wakefield generation is obtained, our two-dimensional numerical studies, including the full nonlinear response, reveal the transverse collapse (or the self-focussing) of the pulse. Under
the typical operating conditions, the self-focussing is
sufficiently slow to allow the interaction between the laser pulse
and the accelerated electrons along an interaction length (in the
laboratory frame) that exceeds 1 m
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