Design of an X-ray Phase-Plate Analyzer to Measure the Circular Polarization Rate of a Helical Undulator Source

A quarter-wave plate made of a ca 16 mum-thick silicon single-crystal was used at energies as low as 2.8 keV to convert circularly polarized photons into linearly polarized photons. Coupled to a linear polarimeter, this quarter-wave plate enabled the characterization of the circular polarization rate of the radiation emitted by one of the ESRF helical undulators, Helios-I. The measured value (ca 97%) is in good agreement with theoretical predictions. Special attention was paid to the alignment procedures of all relevant optical components of the beamline.