Pulse-front tilt caused by the use of a grating monochromator and   self-seeding of soft X-ray FELs
  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. 
Gianluca Geloni, Vitali Kocharyan and Evgeni Saldin
physics.optics physics.acc-ph