LAUSR

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.