Polarization is a fundamental property of light used in experiments to probe various properties of matter such as the chirality of molecules and crystal structures. There is increasing interest in generating bespoke radiation pulses for experiments with increasingly complex structures of polarization. At short wavelengths, free electron lasers offer an avenue to control the polarization structure at the point where the radiation is emitted through manipulation of the electron beam, removing the requirement for polarizing optics not readily available at x-ray wavelengths. This talk discusses a method for manipulating the polarization of FEL generated light based on temporal intensity modulation of radiation emitted in orthogonally polarized undulators. Simulations demonstrate the method can produce radiation that switches between orthogonal polarization states at attosecond timescales. Implementation of this ultra-fast polarization switching would provide a valuable new tool to the scientific community.

Cavity-based X-ray Free Electron Lasers (FELs) such as the X-ray regenerative amplifier FEL (XRAFEL) [1] and the X-ray FEL oscillator [2] have drawn great interest as a means of producing high-brightness, fully coherent and stable hard x-ray pulses for high-repetition rate FELs [3]. However, high efficiency outcoupling of the stored cavity x-ray radiation remains challenging. Here we present a novel XRAFEL design to achieve efficient cavity outcoupling or Q-switching by introducing energy chirp in the electron beam while leaving the high-quality X-ray optics intact. During the FEL interaction, electron beam with an linear energy chirp can be slightly compressed or decompressed by the undulator, which leads to a gradual shift of radiation frequency outside the bandwidth of the Bragg crystal for efficient outcoupling. Our simulation results show that substantial power can be outcoupled from the X-ray cavity driven by chirped electron beams at 100 kHz repetition rate. We also discuss parameter tradeoff in such an XRAFEL scheme and a practical way to achieve the desired fast chirp control by a small, normal-conducting RF station in the LCLS-II [4]. [1] Z. Huang and R. D. Ruth. PRL96, 144801 (2006). [2] K.-J. Kim, Y. Shvyd'ko, S. Reiche, PRL100 244802 (2008). [3] G. Marcus, et al., PRL125, 254801 (2020). [4] M. Nasr, et al., in proceedings of IPAC'16 (Busan, Korea,2016).