1,721,102 research outputs found
RF compression and microbunching
No full textThe production of ultrashort electron bunches represents a fundamental process for the realization of short wavelength free electron lasers (FEL). Due to space charge effects at low energy, a short electron bunch with a high charge will degrade its emittance and will be lengthened within a few centimeters downstream the cathode. As a conse- quence, bunch compression is necessary to reach the required length for high peak currents. A common device used so far for this goal is the magnetic com- pressor, in which a bunch with a time-energy corre- lation (or chirp) is driven along an energy-dependent path by a dispersive, non-isochronous beam trans- port section, consisting of four dipoles placed in a chicane configuration. The magnetic compression may often degrade the beam quality, however, due to the so called micro-bunching instability caused by coherent synchrotron radiation effects in bends, that limits the performances of high intensities electron accelerators. In recent years, a new method, named velocity bunching, has been developed and it is able to com- press the bunch using rectilinear trajectories at rel- atively low energy. In this process the electrons on the tail of the bunch are faster than electrons in the bunch head, thus producing, at the exit of a trav- elling rf structure, a compressed beam. The possi- bility to achieve RF compression without emittance degradation has been recently demonstrated at the SPARC accelerator, that is the prototype of the in- jector of the SPARX FEL facility in which a hybrid (RF plus magnetic chicane) compression scheme is going to be employed. In this work we present an analytical model for studying the evolution of the longitudinal phase space modulation of a particle beam through an RF compressor and we compare the results with numer- ical simulations based on a macro-particle code. Collective effects have been considered all along the schematic case of a drift plus an RF compressor, downstream the photocathode emission
SIMPLE CHARACTERIZATION METHOD OF SMALL HIGH GRADIENT PERMANENT MAGNET QUADRUPOLES
No full textThe application of quadrupoles with high or ultra-high
gradient and small apertures requires a precise control
over harmonic components of the field. A simple, fast,
low cost measurement method on small size PMQs
(Permanent Magnet Quadrupoles) is described. It is based
on the same principle of the familiar "rotating coil
technique", but in this case, profiting of the small
dimensions of the PMQ, it consists in rotating the PMQ
itself instead of the coil. In such way a gain on accuracy
and measure time is obtained. It has been applied to
characterize a set of commercial PMQs with a gradient
around 200 T/m and an internal radius of 3.5 mm to be
mounted in a SCDTL (Side Coupled Drift Tube Linac)
structure for the acceleration of a proton beam from 7 to
12 MeV. This structure has been developed in the
framework of the Italian TOP-IMPLART (Intensity
Modulated Proton Linear Accelerator for Radiotherapy)
Project
Hybrid schemes for the post-acceleration of laser generated protons
No full textProtons generated by the irradiation of a thin metal foil by a high-intensity short-pulse laser have shown to posses interesting characteristics in terms of energy, emittance, current and pulse duration. They might therefore become in the next future a competitive source to conventional proton sources. Previous theoretical and numerical studies already demonstrated the possibility of an efficient coupling between laser-plasma acceleration of protons with traditional RF based beam-line accelerator techniques. This hybrid proton accelerator would therefore benefit from the good properties of the laser-based source and from the flexibility and know-how of beam handling as given from RF based accelerator structure. The proton beam parameters of the source have been obtained from published laser interaction experimental results and are given as input to the numerical study by conventional accelerator design tools. In this paper we discuss recent results in the optimization and design of the such hybrid schemes in the context of proton accelerators for medical treatments
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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