Demonstration of Direct-amplification Enabled Harmonic Generation in an Ultraviolet Free-Electron Laser

TL;DR

This paper experimentally demonstrates a novel ultraviolet free-electron laser technique that uses a versatile undulator configuration to amplify a seed laser and generate harmonics efficiently, reducing seed power requirements.

Contribution

It introduces a unified undulator setup enabling both seed amplification and harmonic generation, advancing high-repetition-rate, fully coherent FEL development.

Findings

Achieved over 100-fold seed energy gain.

Observed exponential growth at the second harmonic.

Demonstrated efficient harmonic bunching with a long modulator.

Abstract

We report the experimental demonstration of direct-amplification enabled harmonic generation in an ultraviolet free-electron laser (FEL) driven by a low-intensity seed laser. By employing a versatile undulator configuration that enables seed amplification and harmonic generation within a unified setup, we achieved over 100-fold energy gain of the seed and observed exponential growth at the second harmonic. The results demonstrate that a sufficiently long modulator can not only amplify a weak seed but also induce strong energy modulation of the electron beam, enabling efficient harmonic bunching. This method markedly relaxes the power requirements on external seed lasers and presents a viable route toward high-repetition-rate, fully coherent FELs