Self-enhanced coherent harmonic amplification in seeded free-electron lasers

TL;DR

This paper introduces a novel self-enhanced scheme for seeded free-electron lasers that significantly amplifies coherent X-ray pulses at high harmonics, enabling ultrashort, high-brightness radiation suitable for advanced spectroscopy.

Contribution

The paper proposes a new self-modulation and superradiance-based scheme for high-harmonic generation in seeded FELs, demonstrated through simulations and experiments.

Findings

Achieved ultrashort coherent pulses of around 2 GW within the water window.

Generated coherent signals up to the 15th harmonic of a 266-nm seed laser.

Demonstrated high-repetition-rate operation potential for seeded FELs.

Abstract

High-intensity, ultrashort, fully coherent X-ray pulses hold great potential for advancing spectroscopic techniques to unprecedented levels. Here, we propose a novel scheme for generating high-brightness and femtosecond-scale soft X-ray radiation within a seeded free-electron laser (FEL) operating at an MHz repetition rate. This scheme relies on the principles of self-modulation and superradiance. A relatively weak energy modulation of the pre-bunched electron beam is significantly amplified by the coherent radiation emitted in the self-modulator. Consequently, a coherent signal at ultra-high harmonics of the seed is achieved, and this signal is further amplified in the subsequent radiator through the fresh bunch and superradiant processes. Based on the parameters of the Shanghai soft X-ray FEL facility, three-dimensional simulations have been performed. The simulation results demonstrate that an electron beam with a laser-induced energy modulation as small as 2.3 times the slice energy spread can generate ultrashort coherent radiation pulses of around 2 GW within the water window spectral range. Moreover, the experimental results demonstrate that self-enhanced coherent energy modulation enables the production of coherent signals up to the 15th harmonic of a 266-nm seed laser. These findings indicate that the proposed scheme can facilitate the generation of high-repetition-rate seeded FEL.