Yan Jiawei, Gao Zhangfeng, Qi Zheng, Zhang Kaiqing, Zhou Kaishang, Liu Tao, Chen Si, Feng Chao, Li Chunlei, Feng Lie, Lan Taihe, Zhang Wenyan, Wang Xingtao, Li Xuan, Jiang Zenggong, Wang Baoliang, Wang Zhen, Gu Duan, Zhang Meng, Deng Haixiao, Gu Qiang, Leng Yongbin, Yin Lixin, Liu Bo, Wang Dong, Zhao Zhentang
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Phys Rev Lett. 2021 Feb 26;126(8):084801. doi: 10.1103/PhysRevLett.126.084801.
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 a 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 external laser-induced 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 seventh 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 toward a high-repetition-rate, fully coherent x-ray FEL.
用于物质时间分辨精细分析的光谱技术需要持续时间为飞秒且平均亮度高的相干X射线辐射。种子自由电子激光器（FEL）利用外部种子激光的频率上转换来提高时间相干性，是提供完全相干软X射线脉冲的理想选择。然而，由于当前最先进激光系统的限制，难以使种子FEL以高重复率运行。在此，我们报告一种用于增强激光诱导能量调制的新型自调制方法，从而显著降低对外部激光系统的要求。受此方案驱动，我们通过前所未有的小外部激光诱导能量调制，在种子FEL中通过实验实现了高次谐波产生。在单级高增益谐波产生（HGHG）装置中，使用激光诱导能量调制小至切片能量展宽1.8倍的电子束，在266纳米种子激光的第七谐波处产生激光；在两级HGHG装置中，在种子激光的第30谐波处产生激光。这些结果标志着朝着高重复率、完全相干的X射线FEL迈出了重要一步。
