Demonstration of High-Gain Harmonic Lasing in a Terahertz Free-Electron Laser
Abstract
Compact Free-Electron Lasers (FELs) offering broad, continuous spectral tunability are traditionally constrained by fixed-parameter magnetic structures and the necessity for high-energy electron beams. High-gain Harmonic Lasing (HL) has long been proposed as a solution to overcome these limitations; however, a robust experimental verification of this principle has remained absent. Here, we report the first experimental demonstration of high-gain HL. By employing a frequency-tunable electron beam density modulation to dominate the fundamental instability, we achieved sustained FEL amplification at the 3rd and 5th harmonics of the wiggler. The HL mode generated output power comparable to conventional fundamental operation with enhanced stability and narrower spectral bandwidth. Notably, we demonstrate that HL extends the spectral coverage by a factor of two under fixed facility constraints, achieving pulse energies up to 540 {\mu}J. These results establish high-gain HL as a versatile mechanism for advancing compact, wavelength-flexible FEL facilities.
Keywords
Cite
@article{arxiv.2602.13743,
title = {Demonstration of High-Gain Harmonic Lasing in a Terahertz Free-Electron Laser},
author = {Yin Kang and Cheng Yu and Yue Wang and Weiyi Yin and Zhangfeng Gao and Hanghua Xu and Hang Luo and Jian Chen and Taihe Lan and Xiaoqing Liu and Jinguo Wang and Huan Zhao and Fei Gao and Liping Sun and YanYan Zhu and Yongmei Wen and Chengcheng Xiao and Yongfang Liu and Yixuan Liu and Xingtao Wang and Jiaqiang Xu and Zheng Qi and Tao Liu and Bin Li and Kaiqing Zhang and Zhen Wang and Chao Feng and Bo Liu and Zhentang Zhao},
journal= {arXiv preprint arXiv:2602.13743},
year = {2026}
}