Polarization control of storage ring FELs using cross polarized helical undulators

For more than two decades, accelerator researchers have been working to gain control of polarization of synchrotron radiation and FELs using non-optical means. In 2005, using mixed linear and helical undulators, the first experimental demonstration of polarization control of an FEL beam was realized with the Duke storage ring FEL. With the recent upgrade of the undulator system, the Duke FEL can be operated with up to four helical undulators simultaneously. Using two sets of helical undulators with opposite helicities, for the first time, we have demonstrated full control of the polarization of a storage ring FEL, including helicity switching and rotat-able linear polarization. The helicity switching of the FEL beam has been realized with good lasing up to a few Hertz. The generation of a linearly polarized FEL beam using a set of cross polarized helical undulators has been demonstrated with a high degree of polarization (P_lin > 0.95). The FEL polarization direction can be fully controlled using a buncher magnet. Furthermore, the use of non-optical means to control the FEL polarization allows us to extend polarization control to γ-ray beams via Compton scattering. For the first time, we have produced linearly polarized Compton γ-ray beams with the rotatable polarization direction using helical undulators.