  External seeded free-electron lasers (FELs) are compelling tools for
generating fully coherent EUV and soft X-ray radiations. Echo-enabled harmonic
generation (EEHG), the most typical representative of external seeded FELs, has
witnessed a remarkable growth of fully coherent FELs in the last decade,
continuously evolving towards higher harmonic conversions and shorter
wavelengths. Ultra-high harmonic generation is imperative in the field of FELs.
This paper presents a novel method for generating FEL radiation with ultra-high
harmonic conversion, utilizing harmonic optical klystron in combination with
EEHG. This method can effectively increase the harmonic conversion order to
about 90. Theoretical analysis and numerical simulations show that intense and
almost fully coherent FEL pulses can be generated with a wavelength of 3 nm. At
the same time, the seed laser intensity required by this scheme is lower
compared to nominal EEHG, thus facilitating the generation of
high-repetition-rate seeded FELs.
