Self-seeding scheme for the soft X-ray line at the European XFEL
  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 
Gianluca Geloni, Vitaly Kocharyan and Evgeni Saldin
physics.acc-ph