A self‐amplified spontaneous emission free‐electron laser (FEL) is under construction at the Shanghai Soft X‐ray Free‐Electron Facility. Therefore, it is necessary to develop a suitable diagnostic tool capable of resolving the natural emission band of each FEL pulse. Thus, an online spectrometer with a plane mirror and plane variable‐line‐spacing grating at grazing incidence to monitor each single FEL pulse during the propagation of FEL radiation has been designed and is presented in this work. The method of ray tracing is used for monitoring incident radiation in order to understand spectral characteristics, and SHADOW, an X‐ray optics simulation tool, and SRW, an X‐ray optics wavefront tool, are applied to study the resolving power and focusing properties of the grating. The designed resolving power is ∼3 × 10⁴ at 620 eV. Meanwhile, the effect of the actual slope error of mirrors on the ray‐tracing results is also discussed. In order to provide further optimization for the choice of grating, a comparison of resolving powers between 2000 lines mm⁻¹ and 3000 lines mm⁻¹ gratings at different energies is analyzed in detail and radiation damage of mirrors as well as parameters such as the first‐order diffraction angle β, the exit‐arm length r₂, and the tilt angle gθ between the focal plane and the diffraction arm are studied and optimized. This work has provided comprehensive designing methods and detailed data for the design of diagnostic spectrometers in soft X‐ray FELs and will be favorable to the design of other similar instruments.

中文翻译：

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诊断自由电子激光的在线光谱仪的设计

上海软X射线自由电子工厂正在建造自放大自发自由电子激光器（FEL）。因此，有必要开发一种能够解决每个FEL脉冲的自然发射带的合适诊断工具。因此，设计并提出了一种在线光谱仪，该光谱仪在掠入射时具有平面镜和平面可变行距光栅，以监视FEL辐射传播期间的每个单个FEL脉冲。射线追踪法用于监视入射辐射以了解光谱特性，X射线光学仿真工具SHADOW和SRWX射线光学波前工具，用于研究光栅的分辨能力和聚焦特性。设计的分辨能力在620 eV时约为3×10 ⁴。同时，还讨论了反射镜的实际倾斜误差对光线追踪结果的影响。为了进一步优化光栅的选择，详细分析了2000毫米mm ^-1和3000毫米mm ^-1光栅在不同能量下的分辨能力，并分析了反射镜的辐射损伤以及诸如阶衍射角β，出射臂长度r ₂对焦平面和衍射臂之间的倾斜角gθ进行了研究和优化。这项工作为软X射线FEL中的诊断光谱仪的设计提供了全面的设计方法和详细的数据，将有利于其他类似仪器的设计。