Proposal of kHz-level polarization switching of undulator radiation with magnetic field modulation at Hefei Advanced...

A magnetic field modulation-based scheme for fast polarization switching of undulator radiation has been proposed for Hefei Advanced Light Facility (HALF), a fourth generation soft X-ray light source based on a diffraction-limited storage ring. In this scheme, two undulators with opposite helicities are employed, each equipped with electromagnetic coils placed on the upper and lower surfaces of the vacuum chamber. Through fast switching the power of the coils, the radiation frequencies of two undulators can be rapidly shifted into and out of the band pass of a monochromator; polarization switching at the kHz level in the photon energy range 330-1000 eV for user beamlines can be expected with a stainless steel vacuum chamber. Crucially, a vacuum chamber designed for HALF is selected to be used for the simulations, representing the first demonstration of achieving such switching frequency under realistic vacuum chamber conditions. The limitations imposed by chamber-induced eddy currents have been studied. Benefiting from the low current and compact coil size, polarization switching at approximately 100 Hz can still be achieved even with an aluminium or copper vacuum chamber. Numerical simulations confirm the feasibility of the scheme, demonstrating high polarization degree and maintained high photon flux. Furthermore, these coils are designed to correct the first and second integrals of magnetic field themselves. This method can also be easily adopted for other light sources requiring fast polarization control.