Unlike synchrotrons with multiple beamlines, free-electron lasers (FELs) are single-beam facilities, which nevertheless have a number of endstations. The latter requires development of complex radiation transport line, which should be efficient enough to avoid scaling down the FEL capabilities. Keeping the beam shape and radiation power level along the beamline is a challenge because the total length of the FEL radiation transport line can exceed a hundred meters. The FELs around the world differ from each other in both radiation parameters and endstations’ layout requiring individual design of their radiation transport lines. In this work, we describe the 120-m beamline for transporting the radiation of the Novosibirsk FEL facility, consisting of three FELs of the terahertz (THz), far-infrared, and mid-infrared ranges. The radiation of these three FELs is directed to the same beam transport channel, which is able to deliver the radiation to any of 14 endstations already commissioned. To compare the expected beam parameters with the actual ones, the radiation intensity distribution was measured in a number of places of the beamline that are THz radiation outputs for some endstations. Possible causes of the parameters’ mismatching observed for distant endstations are discussed. The problem of radiation absorption by water vapor is considered in detail.

中文翻译：

---

在太赫兹、远红外和中红外范围内运行的新西伯利亚自由电子激光装置的辐射光束线

与具有多个光束线的同步加速器不同，自由电子激光器 (FEL) 是单光束设施，但仍具有多个终端站。后者需要开发复杂的辐射传输线，其效率应足以避免缩小 FEL 能力。保持光束形状和沿光束线的辐射功率水平是一项挑战，因为 FEL 辐射传输线的总长度可能超过一百米。世界各地的 FEL 在辐射参数和终端站布局方面彼此不同，需要对其辐射传输线进行单独设计。在这项工作中，我们描述了用于传输新西伯利亚 FEL 设施辐射的 120 米光束线，由太赫兹 (THz)、远红外和中红外范围的三个 FEL 组成。这三个 FEL 的辐射被引导到同一个波束传输通道，该通道能够将辐射传送到已投入使用的 14 个终端站中的任何一个。为了将预期的光束参数与实际参数进行比较，在光束线的多个位置测量了辐射强度分布，这些位置是某些终端站的太赫兹辐射输出。讨论了远程终端站观察到的参数不匹配的可能原因。详细考虑了水蒸气的辐射吸收问题。在光束线的多个位置测量了辐射强度分布，这些位置是某些终端站的太赫兹辐射输出。讨论了远程终端站观察到的参数不匹配的可能原因。详细考虑了水蒸气的辐射吸收问题。在光束线的多个位置测量了辐射强度分布，这些位置是某些终端站的太赫兹辐射输出。讨论了远程终端站观察到的参数不匹配的可能原因。详细考虑了水蒸气的辐射吸收问题。