This paper presents a method that uses a gap scan of the phase shifter to optimize the intensity of a free-electron laser (FEL) by matching its phase with that of the electron beam. Phase shifters are essential instruments, especially for a long undulator line, which is segmented by drift sections. The phase-matched (in-phase) and the 180° offset (out-of-phase) conditions are investigated using linear theory, FEL simulations, and experiments to understand how the phase shifter affects FEL amplification. We show that the FEL intensity is dominantly reduced by phase mismatch in the saturation region, where the microbunched electron beam is sufficiently developed, and that the difference of FEL intensity between the in-phase and out-of-phase conditions is an effect of evolution of the bunching factor. At the Pohang Accelerator Laboratory x-ray Free-Electron Laser (PAL-XFEL), the gap scan of the phase shifter at 9.7 keV increased FEL intensity by 4 times compared to the calculated gap of the phase shifter. This intensity increase was obtained dominantly in the saturation region.

中文翻译：

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利用移相器优化X射线自由电子激光器的强度

本文提出了一种使用相移器的间隙扫描来通过使自由电子激光器（FEL）的相位与电子束的相位匹配来优化其强度的方法。移相器是必不可少的工具，尤其是对于长的波动器线路（由漂移部分分段）而言。使用线性理论，FEL仿真和实验来研究相匹配（同相）和180°偏移（异相）条件，以了解移相器如何影响FEL放大。我们表明，FEL强度主要由饱和区域中的相位失配所降低，在该区域中，微束电子束已充分展开，并且同相和异相条件之间的FEL强度差是演化的影响聚集因子。在浦项加速器实验室X射线自由电子激光（PAL-XFEL）上，相移器的间隙扫描在9.7 keV时，与计算出的相移器间隙相比，FEL强度提高了4倍。该强度增加主要在饱和区域获得。