1,721,251 research outputs found
Bunch length characterization at the TTF VUV-FEL
No full textThe characterization of the longitudinal density profile or bunch shape of picosecond and sub-picosecond relativistic particle bunches is a fundamental requirement in many particle accelerator facilities, since knowledge of the characteristics of the accelerated beams is of utmost importance for the successful development of the next generation light sources and linear colliders. The development of suitable beam diagnostics, non-invasive and non-intercepting, is thus necessary to produce and control such beams. First experimental evidences of the non-intercepting and non-destructive nature of DR diagnostics are presented. The precise longitudinal bunch distributions downstream of the second bunch compressor (DBC3) of the DESY TTF VUV-FEL have been reconstructed using a frequency-domain technique based on the autocorrelation of coherent diffraction radiation (CDR). A Kramers-Kronig analysis has then been applied to derive the minimal phase. The autocorrelation of diffraction radiation coming from a thin conducting foil placed in the vacuum chamber has been obtained using a MartinPuplett interferometer and sub-millimeter radiation. The measurement and the reconstruction of the longitudinal bunch distribution are discussed in detail showing also the strong dependence of the bunch length on the accelerating module phase. Due to the low and high frequency suppression, introduced by the experimental apparatus, only a portion of the CDR spectrum participates to the reconstruction of the longitudinal bunch profile. The knowledge of the system frequency response, in particular in the millimeter and sub-millimeter range is then crucial in order to correct the results and extrapolate a bunch shape as close as possible to the real one. At this purpose, a precise characterization over three different frequency regions of the Golay cell detector used for the measurement is also discussed, thus extending the cell calibration into the long wavelengths region
Phase control effects in optical diffraction radiation from a slit
No full textOptical Diffraction Radiation (ODR) from a slit is becoming a real promising instrument for measuring
the transverse beam size, or even the emittance of the very high brightness linacs driving SASE FELs or
the future ILC which require non-intercepting diagnostic devices. But to correctly explain experimental
data it is necessary to further develop the theory of this radiation emission. In this paper we discuss in
particular the effects of a non-perfect coplanarity of the two halves of a slit, that introduces a phase
difference in the field produced by the two half-planes. Such non-coplanarity can result from
mechanical stress for instance
Characterisation of Pb thin films prepared by the nanosecond pulsed laser deposition technique for photocathode application
No full textCharacterisation of Pb thin films prepared by the nanosecond pulsed laser deposition technique for photocathode application
No full textAbstract Pb thin films were prepared by the nanosecond pulsed laser deposition technique on Si (100) and polycrystalline Nb substrates for photocathode application. As the photoemission performances of a cathode are strongly affected by its surface characteristics, the Pb films were grown at different substrate temperatures with the aim of modifying the morphology and structure of thin films. An evident morphological modification in the deposited films with the formation of spherical grains at higher temperatures has been observed. X-ray diffraction measurements showed that a preferred orientation of Pb (111) normal to the substrate was achieved at 30 C while the Pb (200) plane became strongly pronounced with the increase in the substrate temperature. Finally, a Pb thin film deposited on Nb substrate at 30 C and tested as the photocathode showed interesting results for the application of such a device in superconducting radio frequency guns
Diffraction Radiation as ultra-high intensity electron beams non-intercepting diagnostics
Full text linkDiffraction Radiation is receiving ever more consideration as a nonintercepting
diagnostic tool. On the superconducting linac at FLASH diagnostic
instruments based on DR are routinely used, and new developments are continuously
experimented. The analysis of the coherent part of DR spectrum allows
the measurement of the longitudinal charge distribution in the bunch, while the
incoherent emission at optical wavelengths can give information on the transverse
beam size
Zemax simulations describing collective effects in transition and diffraction radiation
No full textTransition and diffraction radiation from charged particles is commonly used for
diagnostics purposes in accelerator facilities as well as THz sources for spectroscopy applications.
Therefore, an accurate analysis of the emission process and the transport optics is crucial to
properly characterize the source and precisely retrieve beam parameters. In this regard, we have
developed a new algorithm, based on Zemax, to simulate both transition and diffraction radiation
as generated by relativistic electron bunches, therefore considering collective effects. In particular,
unlike other previous works, we take into account electron beam physical size and transverse
momentum, reproducing some effects visible on the produced radiation, not observable in a single
electron analysis. The simulation results have been compared with two experiments showing an
excellent agreement
Effects of Transverse Electron Beam Size on Transition Radiation Angular Distribution.
No full textIn this paper weconsider the effect of the transverse electron beam size on the Optical Transition Radiation (OTR) angular distribution in case of both incoherent and coherent emission.Our results
confute the theoretical argumentations presented first in Optics Communications 211, 109(2002),
which predicts a dependence of the incoherent OTR angular distribution on the beam size and emission wavelength. We present here theoretical and experimental data not only to validate the well-
established Ginzburg–Frank theory, but also to show the impact of the transverse beam size in case
of coherent emission
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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