1,721,025 research outputs found
One-dimensional free-electron laser equations without the slowly varying envelope approximation
Full text linkA set of one-dimensional equations has been deduced in the time domain from the Maxwell-Lorentz system with the aim of describing the free-electron laser radiation without using the slowly varying envelope approximation (SVEA). These equations are valid even in the case of arbitrarily short electron bunches and of current distributions with ripples on the scale of or shorter than the wavelength. Numerical examples are presented, showing that for long homogeneous bunches the new set of equations gives results in agreement with the SVEA free-electron laser theory and that the use of short or prebunched electron beams leads to a decrease of the emission lethargy. Furthermore, we demonstrate that in all cases in which the backward low frequency wave has negligible effects, these equations can be reduced to a form similar to the usual 1D SVEA equations but with a different definition of the bunching term
Nonlinear effects in Thomson backscattering
No full textWe analyze the nonlinear classical effects of the X/γ radiation produced by Thomson/Compton sources. We confirm the development of spectral fringes of the radiation on axis, which comports broadening, shift, and deformation of the spectrum. For the nominal parameters of the SPARC-LAB Thomson scattering and of the European Proposal for the gamma source ELI-NP, however, the radiation, when collected in the suitable acceptance angle, does not reveal many differences from that predicted by the linear model and the nonlinear redshift is subdominant with respect to the quantum recoil. An experiment aimed to the study of the nonlinearities is proposed on the SPARC-LAB source
Maximizing the brightness of an electron beam by means of a genetic algorithm
No full textWe present the architecture and some applications of a genetic algorithm developed for solving problems in beam dynamics of high brightness beams, where highly non-linear behaviour of the beam characteristics are optimized through a multi-dimensional variation technique based on genetic evolution criteria
Collective effects in the back-scattering Thomson between a laser pulse and a relativistic electron beam
No full textCollective effects in the radiation emission via Thomson back-scattering of an intense optical laser pulse by high brightness electron beams are analyzed. The micro-bunching of the electron beam on the scale of the wavelength of the emitted radiation and the consequent free-electron-laser instability may significantly enhance the number of photons emitted. Scaling-laws of the radiation properties, both in the collective and incoherent spontaneous regimes versus laser and electron beam parameters are discussed in the framework of the one-dimensional model
Theoretical considerations for X-ray phase contrast mammography by Thomson source
No full textThe advent, in the near future, of compact X-ray sources like Thomson Back-Scattering (TBS) will allow the clinical application of advanced X-ray imaging techniques, such as phase contrast, with higher sensitivity and lower impact in terms of dose delivery. In this work, we theoretically investigated the possibility of using such sources for phase contrast imaging of micro-calcifications included in a breast tissue. In our study we analyzed the phase and amplitude distribution of the TBS source and we showed that this source can be used for phase contrast imaging since the source coherence at the sample position is sufficiently high for achieving good contrast and micrometer spatial resolution. Indeed the spatial coherence of a TBS source is closer to that of a synchrotron radiation source, and much better than that of a laboratory source. Moreover, we showed the advantages of phase imaging with respect to standard absorption imaging, in the specific case of micro-calcifications detection
Numerical treatment of retarded radiation effects from high brightness electron beams
No full textA form is given of the retarded electric field and magnetic induction generated by the motion of a charged particle that expresses these fields as integrals of the retarded charge density only, with kernels depending on the charge velocity and acceleration fields. In the case of a single pointlike charge, the usual Liénard-Wiechert fields follow very easily. The set of equations for the dynamics of particles in assigned electromagnetic fields with the self-consistent field is written and integrated. The code Retar for the dynamics of charged particles in external and self-consistent fields is described and a few examples of benchmark are proposed. As a physical application, the case of an electron beam moving in a bending magnetic dipole is examined, and the radiation produced analyzed, in order to characterize a therahertz radiation source
Transverse effects in the production of x rays with a free-electron laser based on an optical undulator
Full text linkThe interaction between high-brilliance electron beams and counterpropagating laser pulses produces x rays via Thomson backscattering. If the laser source is long and intense enough, the electrons of the beam can bunch on the scale of the emitted x-ray wavelength and a regime of collective effects can establish. In this case of dominating collective effects, the FEL instability can develop and the system behaves like a free-electron laser based on an optical undulator. Coherent x rays can be irradiated, with a bandwidth very much thinner than that of the corresponding incoherent emission. The emittance of the electron beam and the distribution nonuniformity of the laser energy are the principal quantities that limit the growth of the x-ray signal. In this work we analyze with a 3D code the transverse effects in the emission produced by a relativistic electron beam when it is under the action of an optical laser pulse and the x-ray spectra obtained. The scalings typical of the optical wiggler, characterized by very short gain lengths and overall time durations of the process, make possible considerable emission also in violation of the Pellegrini criterion for static wigglers. A generalized form of this criterion is validated on the basis of the numerical evidence
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
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