1,720,970 research outputs found
Noninvasive Measurements of Electron Bunch Current Profiles with Few-Femtosecond Resolution at MHz Repetition Rates
No full textEine neue Generation von Freien-Elektronenlasern im harten Röntgenbereich (XFEL) erzeugt Röntgenstrahlungspulse von unerreichter Brillianz mit Wiederholraten im Megahertzbereich, basierend auf supraleitenden Linearbeschleunigern. Der European XFEL, der erste XFEL dieser Generation, beschleunigt jede Zehntelsekunde einen Pulszug von bis zu 2700 Elektronenpaketen über eine 1.7 km lange Strecke auf maximal 17.5 GeV Strahlenergie. Die Elektronenpakete werden dabei mit bis zu 4.5 MHz einzeln auf drei FEL Strahllinien verteilt, in denen sie ultrakurze Strahlungspulse im harten und weichen Röntgenbereich erzeugen. Das Stromprofil ist dabei von fundamentaler Bedeutung für die Erzeugung dieser XFEL-Pulse und benötigt Spitzenströme im Kiloamperebereich. Allerdings gestaltet sich seine Diagnose, bei der Elektronenpaketdauern runter bis zu wenigen Femtosekunden mit MHz-Wiederholraten vermessen werden müssen, schwierig. Im Rahmen der vorliegenden Dissertation wurde das Einzelschussspektrometer CRISP am European XFEL installiert und in Betrieb genommen. Die Aufnahme des Spektrums kohärenter Diffraktionsstrahlung (CDR) ermöglicht zum ersten Mal Messungen des Formfaktors der einzelnen Elektronenpakete in zwei Frequenzbändern von 0.7 THz – 6.6 THz (438 μm – 45 μm) und 6.9 THz – 58.4 THz (44 μm – 5.1 μm), ohne die Elektronenpakete dabei für den FEL Betrieb untauglich zu machen. Die Detektorelektronik sowie Signalverarbeitung des Spektrometers wurden speziell für MHz-Wiederholraten angepasst und verfügen in ihrer Kombination über genügend Sensitivität für Paketladungen im Bereich von 50 pC bis 1 nC am European XFEL. Die Stromprofile werden mit einem weiterentwickeltem iterativem Verfahren zur Phasenrückgewinnung aus den gemessenen Formfaktoren rekonstruiert. Mittels einer transversal ablenkenden Struktur sowie eines Kompressionsmonitors wurden die Stromprofile durch Vergleichsmessungen etablierter Diagnostik bestätigt. Die in dieser Arbeit gezeigten Messungen decken eine Bunchdauer von 6 fs bis 100 fs (rms) sowie Spitzenströme bis zu 10 kA ab. Veränderungen der Paketdauer und des Spitzenstroms innerhalb eines Pulszuges wurden mit einer relativen Auflösung der Bunchdauer (rms) von weniger als 1 fs ausgemacht.The upcoming generation of hard X-ray free-electron lasers (XFEL) delivers X-ray pulses with unmatched brilliance at megahertz repetition rates based on superconducting linear accelerators. The European XFEL, the first XFEL of this generation, accelerates bursts of up to 2700 electron bunches every tenth of a second over a distance of 1.7 km to energies as high as 17.5 GeV. The bunches are distributed individually at a maximum of 4.5 MHz to three FEL beamlines, where they generate ultrashort radiation pulses in the soft and hard X-ray regime. The electron bunch current profile is of fundamental importance for the generation of these XFEL pulses and is required to have kilo-Ampere peak currents. However, diagnosis is challenging because bunch durations as short as a few femtoseconds have to be measured bunch-resolved at MHz repetition rates. In the scope of this thesis, the single-shot spectrometer CRISP has been installed and commissioned at European XFEL. Monitoring of coherent diffraction radiation (CDR) enables for the first time noninvasive and bunch-resolved form factor measurements in two frequency bands: 0.7 THz –
6.6 THz (438 μm – 45 μm) and 6.9 THz – 58.4 THz (44 μm – 5.1 μm). The detector electronics and signal processing of the CRISP spectrometer have been modified for MHz repetition rates and exhibit in combination sufficient sensitivity for the range of bunch charges at European XFEL from 50 pC to 1 nC. The current profiles are reconstructed from the measured form factors with a further developed iterative procedure. Using a transverse deflecting structure and a bunch compression monitor, the current profiles have been confirmed by established comparative diagnostic measurements. The measurements presented in this thesis cover rms bunch durations from 6 fs to 100 fs and peak currents up to 10 kA. Variations of bunch duration and peak current within the bursts of electron bunches have been identified with a relative rms bunch duration resolution of less than 1 fs
Noninvasive Measurements of Electron Bunch Current Profiles with Few-Femtosecond Resolution at MHz Repetition Rates
No full textThe upcoming generation of hard X-ray free-electron lasers (XFEL) delivers X-raypulses with unmatched brilliance at megahertz repetition rates based on supercon-ducting linear accelerators. The European XFEL, the first XFEL of this generation,accelerates bursts of up to 2700 electron bunches every tenth of a second over adistance of 1.7 km to energies as high as 17.5 GeV. The bunches are distributed in-dividually at a maximum of 4.5 MHz to three FEL beamlines, where they generateultrashort radiation pulses in the soft and hard X-ray regime. The electron bunchcurrent profile is of fundamental importance for the generation of these XFEL pulsesand is required to have kilo-Ampere peak currents. However, diagnosis is challeng-ing because bunch durations as short as a few femtoseconds have to be measuredbunch-resolved at MHz repetition rates. In the scope of this thesis, the single-shotspectrometer CRISP has been installed and commissioned at European XFEL. Mon-itoring of coherent diffraction radiation (CDR) enables for the first time noninvasiveand bunch-resolved form factor measurements in two frequency bands: 0.7 THz –6.6 THz (438 μm – 45 μm) and 6.9 THz – 58.4 THz (44 μm – 5.1 μm). The detectorelectronics and signal processing of the CRISP spectrometer have been modified forMHz repetition rates and exhibit in combination sufficient sensitivity for the rangeof bunch charges at European XFEL from 50 pC to 1 nC. The current profiles arereconstructed from the measured form factors with a further developed iterativeprocedure. Using a transverse deflecting structure and a bunch compression moni-tor, the current profiles have been confirmed by established comparative diagnosticmeasurements. The measurements presented in this thesis cover rms bunch dura-tions from 6 fs to 100 fs and peak currents up to 10 kA. Variations of bunch durationand peak current within the bursts of electron bunches have been identified with arelative rms bunch duration resolution of less than 1 fs
Longitudinal Distribution Function, Electrons
No full textEdited by internationally recognized authorities in the field, this expanded and updated new edition of the bestselling Handbook, containing many new articles, is aimed at the design and operation of modern particle accelerators. It is intended as a vade mecum for professional engineers and physicists engaged in these subjects. With a collection of more than 2000 equations, 300 illustrations and 500 graphs and tables, here one will find, in addition to common formulae of previous compilations, hard to find, specialized formulae, recipes and material data pooled from the lifetime experience of many of the world's most able practioners of the art and science of accelerators. The seven chapters include both theoretical and practical matters as well as an extensive glossary of accelerator types. Chapters on beam dynamics and electromagnetic and nuclear interactions deal with linear and nonlinear single particle and collective effects including spin motion, beam-environment, beam-beam, beam-electron, beam-ion and intrabeam interactions. The impedance concept and related calculations are dealt with at length as are the instabilities due to the various interactions mentioned. A chapter on operational considerations including discussions on the assessment and correction of orbit and optics errors, realtime feedbacks, generation of short photon pulses, bunch compression, phase-space exchange, tuning of normal and superconducting linacs, energy recovery linacs, free electron lasers, cryogenic vacuum systems, steady state microbuching cooling, space-charge compensation, brightness of light sources, collider luminosity optimization and collision schemes, machine learning, multiple frequency rf systems, FEL seeding. Chapters on mechanical and electrical considerations present material data and important aspects of component design including heat transfer and refrigeration. Hardware systems for particle sources, feedback systems, confinement, including undulators, and acceleration (both normal and superconducting) receive detailed treatment in a sub-systems chapter, beam measurement and apparatus being treated therein as well. A detailed name and subject index is provided together with reliable references to the literature where the most detailed information available on all subjects treated can be found
Bunch Compression Monitor Based on Coherent Diffraction Radiation at European XFEL and FLASH
No full textBunch compression monitors (BCMs) based on the detection of coherent diffraction radiation have been installed at the European XFEL for a beam-based stabilisation of the accelerating phases as well as monitoring of bunch lengths. The monitor systems comprise zero-bias Schottky and pyro-electric detectors in combination with low and high pass filters. The detector responses and filters are matched to the spectral ranges of the coherent part of the emitted diffraction radiation which is given by the particular beam energy and bunch lengths after each bunch compression stage. In this paper, we describe in detail the experimental setup of the BCMs. The last BCM has been calibrated with the help of a transverse deflecting structure to establish a (rms) bunch length monitor in the range of a few tens of femtoseconds, and results from compression scans are presented. To enable operation at megahertz repetition rates of the superconducting accelerator, a correction method for the signal pileup of the pyro-electric detectors has been applied. Installation of the same BCMs is foreseen at FLASH within the FLASH²⁰²⁰⁺ upgrade project
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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