Fast switching source for polarization modulation spectroscopy

A method for rapidly switching the polarization of soft X-rays from an insertion device on a synchrotron with minimal perturbation of the magnetic field is proposed. It is based on sequentially shifting the harmonics from two undulators into and out of the band pass of a monochromator. The chromatic shifting of the harmonics requires only small change of the magnetic field. This small perturbation of the magnetic fields should permit higher modulation frequencies and improved signal-to-noise ratios for polarization modulation spectroscopy.