  Various scientific and industrial researches such as spectroscopy and
advanced lithography have been demanding high flux and coherent EUV and X-ray
radiation. These demands can be commendably satisfied with a MHz-level
repetition-rate seeded free-electron laser (FEL). Dictated by the seed laser
system, seeded FELs have faced obstacles for the realization of MHz repetition
rate. Reducing the required peak power of the seed laser is the most economic
way to increase the repetition frequency of a seeded FEL. A novel technique
named direct-amplification enabled harmonic generation is presented in this
paper, which holds the ability to reduce the peak power of the seed laser by
about three orders of magnitude while possessing consistent high coherence and
stability relative to conventional seeded FELs. The proposed technique paves
the way for the realization of fully coherent EUV and soft X-ray radiation with
a repetition frequency of MHz.
