A bunch-by-bunch polarization switching scheme for high-repetition-rate hard X-ray FELs

摘要: Over the past two decades, rapid advancement of free electron laser (FEL) technology has effectively addressed extensive research requirements. Currently, nearly all FEL facilities generate high-quality radiation with controllable polarization via devices like APPLE-type undulators or DELTA undulators. However, elliptically polarized undulators alone cannot achieve hertz-level or above polarization switching, which complicates experiments such as time-resolved circular dichroism spectroscopy. To solve this, based on the FEL-II beamline of the Shanghai High Repetition Rate X-ray FEL and Extreme Light Facility, this study proposes integrating a custom-designed bunch-by-bunch phase shifter into APPLE-type undulator arrays (following crossed-planar undulator design logic). We analyzed factors influencing post-superposition radiation polarization under ideal Gaussian mode propagation, determined optimal parameters for both fast polarization switching performance and fixed-polarization radiation quality, conducted validation via start-to-end simulations, and presented the phase shifter’s detailed design. Results show high-frequency undulator-section polarization switching is achievable at short wavelengths with a little polarization purity trade-off, and the scheme is universally applicable to FEL facilities with high-energy electron beams. Longer wavelengths may demand electron beams with stirct phase spaces.