Polarized photon beams provide a unique experimental tool for the study of various polarization dependent physical processes. Here, we report the experimental demonstration of full polarization control of an oscillator free-electron laser (FEL) using helical undulators of opposite helicities. Using two helical undulator magnets of opposite helicities and a buncher magnet in between, we have generated a linearly polarized FEL beam with any desirable polarization direction. With the development of a high-precision FEL polarimeter, we are able to optimize the highly polarized FEL beams in visible wavelengths and measure the polarization with high accuracy, demonstrating linear polarization $P_{\mathrm{\text{Lin}}}>0.99$ on the routine basis, and with the maximum polarization reaching $P_{\mathrm{\text{Lin}}}=0.998$. In this paper, we describe the FEL configuration, experimental setup, and related beam diagnostics, including the newly developed high-precision FEL polarimeter. We report our experimental approaches to generate, tune up and characterize the polarization controllable FEL beams, and share a new insight into how high-degree polarization is realized based upon our investigation of the temporal structure of the FEL beam. This FEL polarization control technique has been used successfully to generate a polarization controllable Compton $\gamma$-ray beam for nuclear physics experiments.

中文翻译：

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使用螺旋度相反的螺旋波荡器控制自由电子激光振荡器

偏振光子束为研究各种偏振相关的物理过程提供了独特的实验工具。在这里，我们报告了使用反向螺旋波状起伏器对振荡器自由电子激光器（FEL）进行全偏振控制的实验演示。使用两个螺旋度相反的螺旋波状起伏磁体和一个在两者之间的聚束磁体，我们生成了具有任何所需偏振方向的线性偏振FEL光束。随着高精度FEL旋光仪的发展，我们能够优化可见光波长下的高偏振FEL光束，并以高精度测量偏振，从而证明了线性偏振$P_{\mathrm{\text{林}}}>0.99$ 并在常规情况下达到最大极化 $P_{\mathrm{\text{林}}}=0.998$。在本文中，我们描述了FEL配置，实验装置以及相关的光束诊断，包括新开发的高精度FEL旋光仪。我们报告了我们的实验方法，以生成，调整和表征可控偏振的FEL光束，并基于对FEL光束的时间结构的研究，对如何实现高偏振实现了新的见解。这种FEL偏振控制技术已成功用于产生偏振可控的康普顿$\gamma$射线束用于核物理实验。