  This paper discusses the potential for enhancing the capabilities of the
European FEL in the soft X-ray regime. A high longitudinal coherence will be
the key to such performance upgrade. In order to reach this goal we study a
very compact soft X-ray self-seeding scheme originally designed at SLAC [1,2].
The scheme is based on a grating monochromator, and can be straightforwardly
installed in the SASE3 undulator beamline at the European XFEL. For the
European XFEL fully-coherent soft X-ray pulses are particularly valuable since
they naturally support the extraction of more FEL power than at saturation by
exploiting tapering in the tunable-gap SASE3 undulator. Tapering consists of a
stepwise change of the undulator gap from segment to segment. Based on
start-to-end simulations we show that soft X-ray FEL power reaches about 800
GW, that is about an order of magnitude higher than the SASE level at
saturation (100 GW). The self-seeding setup studied in this work is extremely
compact (about 5 m long), and cost-effective. This last characteristic may
justify to consider it as a possible addition to the European XFEL capabilities
from the very beginning of the operation phase.
  [1] Y. Feng, J. Hastings, P. Heimann, M. Rowen, J. Krzywinski, and J. Wu,
"X-ray Optics for soft X-ray self-seeding the LCLS-II", proceedings of 2010 FEL
conference, Malmo, Sweden, (2010).
  [2] Y. Feng, P. Heimann, J. Wu, J. Krzywinski, M. Rowen, and J. Hastings,
"Compact Grating Monochromator Design for LCLS-I Soft X-ray Self-Seeding",
https://slacportal.slac.stanford.edu/sites/lcls_public/lcls_ii/Lists/LCLS_II_Calendar/Physics_Meetings.aspx,
May 2011 and
https://sites.google.com/a/lbl.gov/realizing-the-potential-of-seeded-fels-in-the-soft-x-ray-regime-workshop/talks,
October 2011

Accelerator Physics