Harmonic Lasing in X-ray Fels

Contrary to nonlinear harmonic generation, harmonic lasing in a high-gain FEL can provide much more intense, stable,and narrow-band FEL beam which is easier to handle if the fundamental is suppressed. We performed thorough study of the problem within framework of 3D model taking into account all essential effects. We found that harmonic lasing is much more robust than usually thought, and can be widely used in the existing or planned X-ray FEL facilities. LCLS after a minor modification can lase at the 3rd harmonic up to the photon energy of 25-30 keV providing multi-gigawatt power level. At the European XFEL the harmonic lasing would allow to extend operating range ultimately up to 100 keV, to reduce bandwidth and improve brilliance, to provide short-wavelength operation after CW upgrade etc.