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
@article{arxiv.2505.05954,
title = {Demonstration of Direct-amplification Enabled Harmonic Generation in an Ultraviolet Free-Electron Laser},
author = {Hao Sun and Jitao Sun and Li Zeng and Yifan Liang and Lingjun Tu and Huaiqian Yi and Qinming Li and Xiaofan Wang and Yong Yu and Jiayue Yang and Zhigang He and Yuhuan Tian and Likai Wang and Zequn Wang and Guorong Wu and Weiqing Zhang and Xueming Yang},
journal= {arXiv preprint arXiv:2505.05954},
year = {2025}
}