  The spectroscopic techniques for time-resolved fine analysis of matter
require coherent X-ray radiation with femtosecond duration and high average
brightness. Seeded free-electron lasers (FELs), which use the frequency
up-conversion of an external seed laser to improve temporal coherence, are
ideal for providing fully coherent soft X-ray pulses. However, it is difficult
to operate seeded FELs at a high repetition rate due to the limitations of
present state-of-the-art laser systems. Here, we report the novel
self-modulation method for enhancing laser-induced energy modulation, thereby
significantly reducing the requirement of an external laser system. Driven by
this scheme, we experimentally realize high harmonic generation in a seeded FEL
using an unprecedentedly small energy modulation. An electron beam with a
laser-induced energy modulation as small as 1.8 times the slice energy spread
is used for lasing at the 7th harmonic of a 266-nm seed laser in a single-stage
high-gain harmonic generation (HGHG) setup and the 30th harmonic of the seed
laser in a two-stage HGHG setup. The results mark a major step towards a
high-repetition-rate, fully coherent X-ray FEL.
