Fully coherent x rays can be generated by self-seeding x-ray free electron lasers (XFELs). Self-seeding\r\nby a forward Bragg diffraction (FBD) monochromator has been recently proposed [G. Geloni, V.\r\nKocharyan, and E. Saldin, J. Mod. Opt. 58, 1391 (2011)] and demonstrated [J. Amann et al., Nat.\r\nPhotonics 6, 693 (2012)]. Characteristic time T 0 of FBD determines the power, spectral, and time\r\ncharacteristics of the FBD seed [Yu. Shvyd�ko and R. Lindberg, Phys. Rev. ST Accel. Beams 15, 100702\r\n(2012)]. Here we show that for a given electron bunch with duration e the spectral flux of the self-seeding\r\nXFEL can be maximized, and the spectral bandwidth can be respectively minimized by choosing T 0 \r\ne= and by optimizing the electron bunch delay e. The choices of T 0 and e are not unique. In all\r\ncases, the maximum value of the spectral flux and the minimum bandwidth are primarily determined\r\nby e. Two-color seeding takes place if T 0 e=. The studies are performed, for a Gaussian\r\nelectron bunch distribution with the parameters, close to those used in the short-bunch (e � 5 fs) and\r\nlong-bunch (e � 20 fs) operation modes of the Linac Coherent Light Source XFEL.
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