Harmonic lasing provides an opportunity to extend the photon energy range of existing and planned x-ray free electron laser (FEL) user facilities. Contrary to nonlinear harmonic generation, harmonic lasing can generate a much more intense, stable, and narrow-band FEL beam. Another interesting application is harmonic lasing self-seeding that allows to improve the longitudinal coherence and spectral power of a self-amplified spontaneous emission FEL. This concept was tested at the soft x-ray FEL user facility FLASH in the range of 4.5–15 nm and at Pohang accelerator laboratory X-ray Free Electron Laser (XFEL) at 1 nm. In this paper we present recent results from the European XFEL where we successfully demonstrated harmonic lasing at 5.9 Angstrom and 2.8 Angstrom. In the latter case we obtained both third and fifth harmonic lasing and, for the first time, operated a harmonic lasing cascade (fifth-third-first harmonics of the undulator). These results pave the way for reaching very high photon energies, up to 100 keV.

中文翻译：

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欧洲X射线自由电子激光在埃范围内的谐波激射观察

谐波激射提供了扩展现有和计划中的X射线自由电子激光（FEL）用户设施的光子能量范围的机会。与非线性谐波产生相反，谐波激射可以产生更强烈，稳定和窄带的FEL光束。另一个有趣的应用是谐波激射自种，它可以改善自放大自发发射FEL的纵向相干性和频谱功率。在软X射线FEL用户设施FLASH的4.5-15 nm范围和浦项加速器实验室的X射线自由电子激光（XFEL）的1 nm处对该概念进行了测试。在本文中，我们介绍了欧洲XFEL的最新结果，我们成功地证明了5.9埃和2.8埃的谐波激射。在后一种情况下，我们获得了三次谐波和第五次谐波激射，并且 首次操作了谐波激射级联（波荡器的五分之一至三分之一谐波）。这些结果为达到高达100 keV的非常高的光子能量铺平了道路。