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Abstract
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 ${P}_{\mathrm{lin}}g0.99$ on the routine basis and with the maximum polarization reaching ${P}_{\mathrm{lin}}=0.998$. 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 $\ensuremath{\gamma}$-ray beam for nuclear physics experiments.
Highlights
The free-electron laser (FEL), driven by either a storage ring or a linac, is widely used to produce wavelengthtunable, high-intensity coherent radiation [1,2,3,4,5]
Using two helical undulators with opposite helicities, we have experimentally demonstrated for the first time the generation of a fully controllable linearly polarized FEL beam using a storage ring FEL oscillator
This paper summarizes our strategies and techniques for producing an FEL beam with the unprecedented level of linear polarization: (i) exploring advantages of an FEL oscillator, (ii) configuring two mechanically identical helical undulators in the middle of the FEL cavity to operate with opposite helicities, (iii) developing a new FEL polarimeter which allows for a real-time optimization of the FEL beam polarization and for highly accurate measurements of the degree of linear polarization, and (iv) tuning multiple operational parameters such as the undulator field strengths, buncher setting, FEL optical axis, and FEL detuning
Summary
The free-electron laser (FEL), driven by either a storage ring or a linac, is widely used to produce wavelengthtunable, high-intensity coherent radiation [1,2,3,4,5]. With such a crossed undulator configuration, photon beam polarization can be controlled by properly mixing two orthogonal beam components with a variable phase shift This idea was first proposed in the 1980s [33] for direct spontaneous radiation using undulators and experimentally explored at the storage ring BESSY in the early 1990s by using two crossed planar undulators and a monochromator to produce circularly polarized radiation [34]. In a seeded FEL with much improved longitudinal coherence, the low degree of polarization is mainly attributed to either an imperfect transverse superposition or an optical phase mismatch of the radiation from the two different undulators along the electron beam path [17,18]. We will share a new insight into how high-degree polarization is realized based upon our investigation of the temporal structure of the FEL beam
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