61 research outputs found
Experimental evidence of intrabeam scattering in a free-electron laser driver
Abstract
The effect of multiple small-angle Coulomb scattering, or intrabeam scattering (IBS) is routinely observed in electron storage rings over the typical damping time scale of milliseconds. So far, IBS has not been observed in single pass electron accelerators because charge density orders of magnitude higher than in storage rings would be needed. We show that such density is now available at high brightness electron linacs for free-electron lasers (FELs). We report measurements of the beam energy spread in the FERMI linac in the presence of the microbunching instability, which are consistent with a revisited IBS model for single pass systems. We also show that neglecting the hereby demonstrated effect of IBS in the parameter range typical of seeded VUV and soft x-ray FELs, results in too conservative a facility design, or failure to realise the accessible potential performance. As an example, an optimization of the FERMI parameters driven by an experimentally benchmarked model, opens the door to the extension of stable single spectral line emission to the water window (2.3–4.4 nm), with far-reaching implications for experiments in a variety of disciplines, ranging from physics and chemistry to biology and material sciences, and including nonlinear x-ray optics based on the four-wave-mixing approach.</jats:p
Microbunching instability characterization via temporally modulated laser pulses
High-brightness electron bunches, such as those generated and accelerated in free-electron lasers (FELs), can develop small-scale structure in the longitudinal phase space. This causes variations in the slice energy spread and current profile of the bunch which then undergo amplification, in an effect known as the microbunching instability. By imposing energy spread modulations on the bunch in the low-energy section of an accelerator, using an undulator and a modulated laser pulse in the center of a dispersive chicane, it is possible to manipulate the bunch longitudinal phase space. This allows for the control and study of the instability in unprecedented detail. We report measurements and analysis of such modulated electron bunches in the 2D spectrotemporal domain at the Fermi FEL, for three different bunch compression schemes. We also perform corresponding simulations of these experiments and show that the codes are indeed able to reproduce the measurements across a wide spectral range. This detailed experimental verification of the ability of codes to capture the essential beam dynamics of the microbunching instability will benefit the design and performance of future FELs
Choosing project risk management techniques. A theoretical framework
The pressure for increasing quality while reducing time and costs places particular emphasis on managing risk in projects. To this end, several models and techniques have been developed in literature and applied in practice, so that there is a strong need for clarifying when and how each of them should be used. At the same time, knowledge about risk management is becoming of paramount importance to effectively deal with the complexity of projects. However, communication and knowledge creation are not easy tasks, especially when dealing with uncertainty, because decision-making is often fragmented and a comprehensive perspective on the goals, opportunities, and threats of a project is missing. With the purpose of providing guidelines for the selection of risk techniques taking into account the most relevant aspects characterising the managerial and operational scenario of a project, a theoretical framework to classify these techniques is proposed. Based on a literature review of the criteria to categorise risk techniques, three dimensions are defined: the phase of the risk management process, the phase of the project life cycle, and the corporate maturity towards risk. The taxonomy is then applied to a wide selection of risk techniques according to their documented applications. This work helps to integrate the risk management and the knowledge management processes. Future research efforts will be directed towards refining the framework and testing it in multiple industrie
The Impact of Alcohol on L1 versus L2
Alcohol intoxication is known to affect many aspects of human behavior and cognition; one of such affected systems is articulation during speech production. Although much research has revealed that alcohol negatively impacts pronunciation in a first language (L1), there is only initial evidence suggesting a potential beneficial effect of inebriation on articulation in a non-native language (L2). The aim of this study was thus to compare the effect of alcohol consumption on pronunciation in an L1 and an L2. Participants who had ingested different amounts of alcohol provided speech samples in their L1 (Dutch) and L2 (English), and native speakers of each language subsequently rated the pronunciation of these samples on their intelligibility (for the L1) and accent nativelikeness (for the L2). These data were analyzed with generalized additive mixed modeling. Participants’ blood alcohol concentration indeed negatively affected pronunciation in L1, but it produced no significant effect on the L2 accent ratings. The expected negative impact of alcohol on L1 articulation can be explained by reduction in fine motor control. We present two hypotheses to account for the absence of any effects of intoxication on L2 pronunciation: (1) there may be a reduction in L1 interference on L2 speech due to decreased motor control or (2) alcohol may produce a differential effect on each of the two linguistic subsystems
Microbunching instability characterization via temporally modulated laser pulses
High-brightness electron bunches, such as those generated and accelerated in
free-electron lasers (FELs), can develop small-scale structure in the
longitudinal phase space. This causes variations in the slice energy spread and
current profile of the bunch which then undergo amplification, in an effect
known as the microbunching instability. By imposing energy spread modulations
on the bunch in the low-energy section of an accelerator, using an undulator
and a modulated laser pulse in the centre of a dispersive chicane, it is
possible tomanipulate the bunch longitudinal phase space. This allows for the
control and study of the instability in unprecedented detail. We report
measurements and analysis of such modulated electron bunches in the
2Dspectro-temporal domain at the FERMI FEL, for three different bunch
compression schemes. We also perform corresponding simulations of these
experiments and show that the codes are indeed able to reproduce the
measurements across a wide spectral range. This detailed experimental
verification of the ability of codes to capture the essential beam dynamics of
the microbunching instability will benefit the design and performance of future
FELs
A new method for measuring angle-resolved phases in photoemission
Quantum mechanically, photoionization can be fully described by the complex
photoionization amplitudes that describe the transition between the ground
state and the continuum state. Knowledge of the value of the phase of these
amplitudes has been a central interest in photoionization studies and newly
developing attosecond science, since the phase can reveal important information
about phenomena such as electron correlation. We present a new
attosecond-precision interferometric method of angle-resolved measurement for
the phase of the photoionization amplitudes, using two phase-locked Extreme
Ultraviolet pulses of frequency and , from a Free-Electron
Laser. Phase differences between one- and two-photon
ionization channels, averaged over multiple wave packets, are extracted for
neon electrons as a function of emission angle at photoelectron energies
7.9, 10.2, and 16.6 eV. is nearly constant for emission
parallel to the electric vector but increases at 10.2 eV for emission
perpendicular to the electric vector. We model our observations with both
perturbation and \textit{ab initio} theory, and find excellent agreement. In
the existing method for attosecond measurement, Reconstruction of Attosecond
Beating By Interference of Two-photon Transitions (RABBITT), a phase difference
between two-photon pathways involving absorption and emission of an infrared
photon is extracted. Our method can be used for extraction of a phase
difference between single-photon and two-photon pathways and provides a new
tool for attosecond science, which is complementary to RABBITT
Opportunities for Gas-Phase Science at Short-Wavelength Free-Electron Lasers with Undulator-Based Polarization Control
Free-electron lasers (FELs) are the world's most brilliant light sources with
rapidly evolving technological capabilities in terms of ultrabright and
ultrashort pulses over a large range of accessible photon energies. Their
revolutionary and innovative developments have opened new fields of science
regarding nonlinear light-matter interaction, the investigation of ultrafast
processes from specific observer sites, and approaches to imaging matter with
atomic resolution. A core aspect of FEL science is the study of isolated and
prototypical systems in the gas phase with the possibility of addressing
well-defined electronic transitions or particular atomic sites in molecules.
Notably for polarization-controlled short-wavelength FELs, the gas phase offers
new avenues for investigations of nonlinear and ultrafast phenomena in spin
orientated systems, for decoding the function of the chiral building blocks of
life as well as steering reactions and particle emission dynamics in otherwise
inaccessible ways. This roadmap comprises descriptions of technological
capabilities of facilities worldwide, innovative diagnostics and
instrumentation, as well as recent scientific highlights, novel methodology and
mathematical modeling. The experimental and theoretical landscape of using
polarization controllable FELs for dichroic light-matter interaction in the gas
phase will be discussed and comprehensively outlined to stimulate and
strengthen global collaborative efforts of all disciplines
Coherent control with a short-wavelength free-electron laser
Extreme ultraviolet and X-ray free-electron lasers (FELs) produce short-wavelength pulses with high intensity, ultrashort duration, well-defined polarization and transverse coherence, and have been utilized for many experiments previously possible only at long wavelengths: multiphoton ionization, pumping an atomic laser and four-wave mixing spectroscopy. However one important optical technique, coherent control, has not yet been demonstrated, because self-amplified spontaneous emission FELs have limited longitudinal coherence. Single-colour pulses from the FERMI seeded FEL are longitudinally coherent, and two-colour emission is predicted to be coherent. Here, we demonstrate the phase correlation of two colours, and manipulate it to control an experiment. Light of wavelengths 63.0 and 31.5nm ionized neon, and we controlled the asymmetry of the photoelectron angular distribution by adjusting the phase, with a temporal resolution of 3as. This opens the door to new short-wavelength coherent control experiments with ultrahigh time resolution and chemical sensitivity
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