Pulse-front tilt caused by the use of a grating monochromator and self-seeding of soft X-ray FELs

Source: arXiv


Self-seeding is a promising approach to significantly narrow the SASE
bandwidth of XFELs to produce nearly transform-limited pulses. The development
of such schemes in the soft X-ray wavelength range necessarily involves
gratings as dispersive elements. These introduce, in general, a pulse-front
tilt, which is directly proportional to the angular dispersion. Pulse-front
tilt may easily lead to a seed signal decrease by a factor two or more.
Suggestions on how to minimize the pulse-front tilt effect in the self-seeding
setup are given.

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    • "The physical meaning of this distortion is that the beam spot size is independent of time, but the beam central position changes as the pulse evolves in time. First we will show in a simple manner that, based on the only use of the Bragg law, we can directly arrive to an explanation of spatiotemporal coupling phenomena in the dynamical theory of diffraction [28]. "
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    ABSTRACT: We discuss the use of self-seeding schemes with wake monochromators to produce TW power, fully coherent pulses for applications at the dedicated bio-imaging bealine at the European X-ray FEL, a concept for an upgrade of the facility beyond the baseline previously proposed by the authors. We exploit the asymmetric and symmetric Bragg and Laue reflections (sigma polarization) in diamond crystal. Optimization of the bio-imaging beamline is performed with extensive start-to-end simulations, which also take into account effects such as the spatio-temporal coupling caused by the wake monochromator. The spatial shift is maximal in the range for small Bragg angles. A geometry with Bragg angles close to pi/2 would be a more advantageous option from this viewpoint, albeit with decrease of the spectral tunability. We show that it will be possible to cover the photon energy range from 3 keV to 13 keV by using four different planes of the same crystal with one rotational degree of freedom.
    Full-text · Article · Jan 2013
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    • "The physical meaning of this distortion is that the beam spot size is independent of time, but the beam central position changes as the pulse evolves in time. Here we show in a simple manner that, based on the use only Bragg law, we may arrive directly to explanation of spatiotemporal coupling phenomena in the dynamical theory of X-ray diffraction [45]. "
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    ABSTRACT: We recently proposed a basic concept for design and layout of the undulator source for a dedicated bio-imaging beamline at the European XFEL. The goal of the optimized scheme proposed here is to enable experimental simplification and performance improvement. The core of the scheme is composed by soft and hard X-ray self-seeding setups. Based on the use of an improved design for both monochromators it is possible to increase the design electron energy up to 17.5 GeV in photon energy range between 2 keV and 13 keV, which is the most preferable for life science experiments. An advantage of operating at such high electron energy is the increase of the X-ray output peak power. Another advantage is that 17.5 GeV is the preferred operation energy for SASE1 and SASE2 beamline users. Since it will be necessary to run all the XFEL lines at the same electron energy, this choice will reduce the interference with other undulator lines and increase the total amount of scheduled beam time. In this work we also propose a study of the performance of the self-seeding scheme accounting for spatiotemporal coupling caused by the use of a single crystal monochromator. Our analysis indicates that this distortion is easily suppressed by the right choice of diamond crystal planes and that the proposed undulator source yields about the same performance as in the case for a X-ray seed pulse with no coupling. Simulations show that the FEL power reaches 2 TW in the 3 keV - 5 keV photon energy range, which is the most preferable for single biomolecule imaging.
    Preview · Article · Sep 2012
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    ABSTRACT: The spatiotemporal response of crystals in x-ray Bragg diffraction resulting from excitation by an ultra-short, laterally confined x-ray pulse is studied theoretically. The theory presents an extension of the analysis in symmetric reflection geometry [1] to the generic case, which includes Bragg diffraction both in reflection (Bragg) and transmission (Laue) asymmetric scattering geometries. The spatiotemporal response is presented as a product of a crystal-intrinsic plane wave spatiotemporal response function and an envelope function defined by the crystal-independent transverse profile of the incident beam and the scattering geometry. The diffracted wavefields exhibit amplitude modulation perpendicular to the propagation direction due to both angular dispersion and the dispersion due to Bragg's law. The characteristic measure of the spatiotemporal response is expressed in terms of a few parameters: the extinction length, crystal thickness, Bragg angle, asymmetry angle, and the speed of light. Applications to self-seeding of hard x-ray free electron lasers are discussed, with particular emphasis on the relative advantages of using either the Bragg or Laue scattering geometries. Intensity front inclination in asymmetric diffraction can be used to make snapshots of ultra-fast processes with femtosecond resolution.
    Full-text · Article · Jul 2012 · Review of Modern Physics