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

TL;DR

This paper reports the first stable operation of a harmonic generation free-electron laser driven by a weak seed laser, achieving high harmonic output and paving the way for MHz-class coherent EUV and x-ray sources.

Contribution

It demonstrates a direct-amplification-enabled FEL with harmonic generation driven by a low-energy seed laser, enabling high repetition rate EUV and x-ray light sources.

Findings

Achieved stable lasing at the 7th harmonic.

Amplified seed laser from 0.75 μJ to over 10 μJ.

Demonstrated harmonic generation up to the 12th harmonic.

Abstract

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-seed laser systems cannot sustain MHz operation with sufficient hundreds of megawatts peak power requirement due to their limited total power. Here, we report the first lasing and stable operation of a direct-amplification-enabled harmonic generation FEL driven by a weak seed laser with MW-level peak power. Beginning with an ultraviolet seed laser with only 0.75 {\mu}J pulse energy, we demonstrate its direct amplification to over 10 {\mu}J within an 8-meter-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.