  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.
