• J. Wu, P. Emma, J.B. Hastings
  SLAC, Menlo Park, California
• C. Pellegrini
  UCLA, Los Angeles, California

Success of the world's first x-ray (0.15-1.5 nm) free electron laser (FEL) - LCLS - at SLAC opens the gate for new science. In this paper, we study the FEL performance for a two-stage self-seeding scheme by introducing a photon monochromator and an electron by-pass in the undulator system. The FEL generated in the first part of the undulator system is purified in spectrum, recombines with the electron bunch, and is amplified in the second part of the undulator system to saturation. Such modifications will improve the FEL longitudinal coherence, reducing the FEL band-width by two-orders of magnitude, but with similar peak power; hence improving the peak brightness by two-orders of magnitude. Such a self-seeding scheme is studied for both soft x-ray (200 eV to 2 keV) and hard x-ray (800 eV to 8 keV) cases with single electron bunch. The photon monochromator system is configurated as variable line spacing gratings for soft x-ray and single crystal for hard x-ray. Harmonic Generation and Chirped FEL are also considered aiming at reaching even shorter wavelength x-ray photons and at generating FEL pulse with even shorter temporal duration, respectively.

• Y. Feng, J.B. Hastings, J. Krzywinski, M. Rowen, J. Wu
  SLAC, Menlo Park, California
• P.A. Heimann
  LBNL, Berkeley, California

A complete optical system including grating monochromator and mirrors was designed to provide self-seeding of the soft X-ray undulators to be possibly built as part of the LCLS-II project. The grating monochromator consisted of a cylindrical horizontally focusing mirror, a plane vertically deflecting pre-mirror, a variable-line-spacing plane vertically deflecting grating, a horizontal exit slits, and a spherical vertically collimating mirror. The grating monochromator was designed to operate in the fixed-focus mode and tuning of the energy was designed to be achieved by rotations of only the pre-mirror and the grating. Only one ruling of 2200 l/mm was needed to cover the energy range from 200 to 2000 eV with an almost constant resolving power of greater than 22700. The monochromator would produce fully transform-limited pulses of 12 fs (rms) long at 2000 eV or 120 fs (rms) long at 200 eV with sufficient power to allow seeding. The optical system produced a slightly energy-dependent time delay of about 10 ps. The transverse size of the input beam was preserved in the horizontal direction, but was reduced in the vertical direction depending on the tuning energy.