First Lasing and Stable Operation of a Direct-Amplification Enabled Harmonic Generation Free-Electron Laser

Seeded free-electron lasers (FELs) capable of operating at repetition rates up to the MHz level are in high demand for advanced time-resolved spectroscopies, which require both full longitudinal coherence and high average photon flux in the extreme ultraviolet (EUV) and x-ray regimes. However, conventional external-seeding methods require ultraviolet seed lasers with peak powers on the order of 100 MW, constraining them to lower repetition rates. Here, we report the first lasing and stable operation of a direct-amplification enabled harmonic generation FEL driven by a weak seed laser with only MW-level peak power. Beginning with an ultraviolet seed laser with only 0.75  μJ pulse energy, we demonstrate its direct amplification to over 10  μJ within an 8-m-long modulator. We observe coherent harmonic generation up to the 12th harmonic of the seed and achieve saturation of the 7th harmonic in the radiator. These results represent a crucial milestone toward the realization of MHz-class, fully coherent EUV and x-ray light sources.