High-repetition rate and coherent free-electron laserin the tender x rays based on the echo-enabledharmonic generation of an ultraviolet oscillator pulse

Fine time-resolved analysis of matter—i.e., spectroscopy and photon scattering—in the linear responseregime requires a fs-scale pulsed, high repetition rate, fully coherent x-ray source. A seeded free-electronlaser (FEL) driven by a super-conducting linac, generating108–1010coherent photons at 2–5 keV withabout 0.5 MHz of repetition rate, can address this need. The seeding scheme proposed is the echo-enabledharmonic generation, alimented by a FEL oscillator working at 13.6 nm with a cavity based on Mo-Simirrors. The whole chain of the x-ray generation is here described by means of start-to-end simulations.Comparisons with the self-amplified spontaneous emission and a fresh-bunch harmonic cascade performedwith similar electron beams show the validity of this scheme.