1,721,002 research outputs found
Analysis of Femtosecond Electron Bunches at the SwissFEL Injector Test Facility
Full text linkThe SwissFEL (Swiss Free Electron Laser) is a free electron laser which is being built at the Paul Scherrer Institute. The SwissFEL will be a user facility to study processes on unexplored scales of space and time. In order to achieve these aims, X-ray pulses on the order of femtoseconds must be generated. For such short X-ray pulses, the electron bunch length must be comparable to the final X-ray pulse length. In order to achieve such a short electron bunch length, it must be monitored at every compression stage. This work proposes two novel methods to measure such electron bunches, namely a novel spectral method that analyses the frequency content of electron bunches that becomes easier with shorter bunch length and a transverse deflecting cavity method which can conceptually measure a zero bunch length. Because the SwissFEL was not built at the time of the PhD, all experiments were performed at the SwissFEL Injector Test Facility (SITF). The SITF is a 200 MeV test bed linear electron accelerator. Theoretically the bunch length is limited due to the uncorrelated energy spread and the higher order components of the bunch compressor. However, due to the limited electron energy in combination with the short electron bunches, space charge effects dominate the beam in the transverse phase space as well as in the longitudinal phase space. A transverse deflecting cavity (TDC) measurement was used to measure a reference bunch length to which the spectral fluctuations could be correlated. In a TDC the electric fields are oriented perpendicular to the beam direction and streaks the head and tail in opposite directions onto a screen. In a conventional TDC measurement, the projected beam size onto the temporal axis is the current profile, provided that the intrinsic beam size is small. However, in the SITF the intrinsic beam size was large due to the aforementioned space charge effects. This led to the development of a novel bunch length analysis method in a space charge dominated beam. In the proposed analysis method, a dispersive section in combination with a TDC was used to visualise the longitudinal phase space. The longitudinal phase space was sliced in the momentum domain to reveal the difference between the intrinsic and the streaked beam in a momentum slice. The bunch length could be extracted by recording the times for all slices. Both the simulations and experiments suggest the method enables the measurement of bunch length in a space charge dominated beam in which the transverse intrinsic beam size is large. The measured minimal bunch length from simulations and experiments are of the same order, 19.5 fs to 24.5 fs, respectively. Since the TDC at PSI has a specified resolution of 20 fs at 200 pC, this method may be used to reduce the resolution. The spectral content of femtosecond electron bunches contains electron bunch length information. As an electron beam collides with an optical transition radiation (OTR) foil, radiation is emitted. The radiation is collected by means of a mirror setup into an optical fibre and into an optical spectrometer. The spectral range of the spectrometer is 316 THz (950 nm) to 1364 THz (220 nm). From OTR theory a flat OTR spectrum was expected and measured. As the electron bunch length was reduced below 70 fs, periodic fluctuations started to appear on the spectra. These fluctuations broaden with decreasing bunch length and show a good agreement with the bunch length measured with the TDC.LPA
Femtosecond pulse length and arrival time measurement of hard X-Ray FELs
Full text linkThe ultra-bright short-pulsed radiation provided by the free electron lasers (FEL) is used for many new discoveries in different fields of science and industry. The advancement of the FEL technologies allows the generation of shorter photon pulses with higher photon energies or shorter radiation wavelengths that open new horizons for the new research. In order to better understand the measurements using the free electron laser pulses, it is important to know the properties of these pulses. Particularly, for the time-resolved experiments the temporal properties of the photon pulses such as their relative arrival times and the temporal durations, are of utmost importance. One of the techniques to measure these parameters of the FEL pulses is the THz streaking method. Thus far, this method has been used only for the photon pulses in ultraviolet and soft X-ray regions. This thesis provides a full characterization of the method and demonstrates its applicability in the hard X-ray photon energy range, measuring FEL pulses with photon energies of up to 10000 electronvolts. Measurement accuracies of sub-10 femtoseconds were achieved for both the arrival time and the pulse length measurements. Furthermore, it is shown here that the THz streaking method is able to simultaneously measure two FEL pulses of different energies. The results from some measurements were also compared to other independent measurement techniques. In addition to the performed measurements, simulation of the THz streaking effect was performed to better understand the measurement method. The simulations showed the ways of achieving higher accuracies with the THz streaking techniques. The results obtained from the experiments were consistent with the results provided by the simulations. The results obtained in this thesis provide new possibilities for the applications of the THz streaking method. They enable temporal diagnostics for photon pulses with a wide range of wavelengths and temporal durations. Such diagnostics can contribute largely to the success of the experiments performed at free electron laser sources.LPA
From SwissFEL wire scanners to the novel wire scanner on-a-chip: a resolution improvement
No full textOver the past few years, the accelerator science community has striven more and more to generate electron beams with extremely low transverse size and emittance.
This research stems mainly from the requirements inherent in X-ray free electron lasers technology and novel accelerator development. The challenge of designing extremely low emittance accelerators brings the need for transverse beam size diagnostics with sub-micrometer resolution.
However, the resolution limit of state-of-the-art beam profile monitors prevents the possibility of measuring sub-micrometer spot sizes. The ultimate goal of this thesis is to develop a novel beam profile monitor capable of sub-micrometer resolution.
Our assessment study identifies wire scanners as the transverse profile monitors that best lend themselves to a resolution improvement.
In this context, the commissioning of a SwissFEL wire scanner is presented as first contribution of the thesis. The challenges tackled during the commissioning include but are not limited to: evaluating the reliability of the WSC beam profile measurements at different scanning velocities; studying the performances of both wire materials; selecting the best suited beam loss monitor to detect the electromagnetic shower generated in the interaction between the wire and the beam.
This preparatory work enabled acquiring the experience necessary to create the new device, and proved the limits of the existing wire scanners and of the conventional manufacturing technique.
As a result, we realised that a completely new design was necessary.
The second contribution of this thesis is the design, fabrication and experimental test of a novel WSC on-a-chip with sub-micrometer resolution. SwissFEL WSCs feature a maximum spatial resolution of 1.25 μm, which is mainly determined by the wire diameter. The resolution of a wire scanner is indeed limited by the encoder readout, the wire diameter and vibrations. The wire diameter defines the so-called geometrical resolution. In SwissFEL, a geometrical resolution of 1.25 μm was reached with a 5 μm tungsten wire. The wire scanner motion system is provided of an encoder with resolution of 0.1 μm. The measured wire-vibrations are largely below the geometrical resolution. Therefore, the wire diameter is the bottleneck in the spatial resolution. A natural way to increase the resolution is to decrease this value. However, the strength of a wire reduces with its diameter. Therefore, the conventional manufacturing technique of stretching a wire onto a wire fork limits its width to a few micrometers. To overcome this limitation, this thesis proposes an innovative design featuring micrometer sized metallic stripes written on a chip. A novel nanofabrication technique has been exploited to produce a 1 μm wide gold stripe on a membrane by electron-beam lithography and electroplating. The proposed design ensures a (rms)geometrical resolution of 0.3 μm and enables the integration of a wire scanner on a chip.
The opportunity of measuring sub-micrometer electron beams is vital to test experimentally the WSC on-a-chip, and assess its performances in terms of resolution. Hence, the thesis describes the generation of a sub-micrometer electron beam at particle energy of 330 MeV, and bunch charge below 1 pC. We attained a measured vertical emittance of 53 nm with estimated errors of ∼ 10%, and an expected vertical beam size of 460 nm. The generated beam has been characterized through the fabricated prototype
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
I.10 — Synchrotron radiation
No full textElectrons circulating in a storage ring emit synchrotron radiation. The spectrum of this powerful radiation spans from the far infrared to the X-rays. Synchrotron radiation has evolved from being a mere byproduct of particle acceleration to a powerful tool leveraged in diverse scientific and engineering fields. Indeed, synchrotrons are the most brilliant X-ray sources on Earth, and they find use in a wide range of fields in research. In this chapter, we will look at the generation of radiation of charged particles in an accelerator, at the influence of this on the beam dynamics, and on the physics behind applications of synchrotron radiation for research
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
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