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

Source: arXiv

ABSTRACT 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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    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.


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