Self-Amplification of Coherent Energy Modulation in Seeded Free-Electron Lasers

TL;DR

This paper introduces a self-amplification method for energy modulation in seeded free-electron lasers, enabling high harmonic generation with minimal external laser input, advancing high-repetition-rate coherent X-ray sources.

Contribution

The novel self-modulation technique significantly reduces external laser requirements, allowing high harmonic generation with very small energy modulation in seeded FELs.

Findings

Achieved high harmonic generation with only 1.8 times the slice energy spread.

Demonstrated lasing at the 7th and 30th harmonics in single- and two-stage HGHG setups.

Paved the way for high-repetition-rate, fully coherent X-ray FELs.

Abstract

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.