Seeded free-electron lasers (FELs), which use frequency up-conversion of an external seed laser to improve temporal coherence, are regarded ideal for supplying stable, fully coherent, soft X-ray pulses. However, the requirement for an external seed laser with sufficient peak power to modulate the electron beam can hardly be met by the present state-of-the-art laser systems, it remains challenging for seeded FELs to operate at high repetition rate, e.g., MHz repetition rate.

Motivated by such a challenge, the FEL teams at Shanghai Advanced Research Institute and Shanghai Institute of Applied Physics of the Chinese Academy of Sciences collaborated and reported a novel self-modulation method for enhancing laser-induced energy modulation, thereby significantly reducing the requirement of an external laser system. The research results were published in Physical Review Letters entitled "Self-Amplification of Coherent Energy Modulation in Seeded Free-Electron Lasers."

Based on the Shanghai soft x-ray FEL test facility, the self-amplification of coherent energy modulation in a seeded FEL is experimentally verified. The peak power requirement of an external seed laser is demonstrated to be relaxed by a factor of 10 to 25 when utilizing the proposed scheme.  

Moreover, the high harmonic generation in a seeded FEL is realized by using an unprecedentedly small energy modulation. A 795 MeV electron beam with a laser-induced energy modulation amplitude as small as 1.8 times the slice energy spread is used for lasing at the 7^th harmonic of a 266-nm seed laser in a single-stage high-gain harmonic generation (HGHG) and the 30^th harmonic of the seed laser in a two-stage HGHG.