  Fine time-resolved analysis of matter - i.e. spectroscopy and photon
scattering - in the linear response regime requires a fs-scale pulsed, high
repetition rate, fully coherent X-ray source. A seeded Free-Electron Laser
(FEL) driven by a Super-Conducting Linac, generating $10^{8}$-$10^{10}$
coherent photons at 2-5 keV with abou 0.5 MHz of repetition rate, can address
this need. The seeding scheme proposed is the Echo-Enabled Harmonic Generation,
alimented by a FEL Oscillator working at 13.6 nm with a cavity based on Mo-Si
mirrors. The whole chain of the X-ray generation is here described by means of
start-to-end simulations. Comparisons with the Self Amplified Spontaneus
Emission and a fresh-bunch harmonic cascade performed with similar electron
beams show the validity of this scheme.
