Double phase-retarder set-up at beamline P09 at PETRA III

Beamline P09 at PETRA III, DESY, is designed for general diffraction and resonant X-ray scattering experiments at low temperatures and high magnetic fields. The dependence of the X-ray cross-sections (Thomson, non-resonant magnetic, resonant exchange scattering, ATS) on the polarization state of the incident X-rays is an important property that one might want to capitalize on in a diffraction experiment. To that purpose, P09 is equipped with a double phase-retarder and diamond phase-plates making for the production of linearly and circularly polarized X-rays in the energy range between 3.5 and 8.5 keV as yet. Here we describe the double phase-retarder setup at P09, its principles of operation and its performances with respect to the generation of linearly polarized incident X-rays rotated by a variable angle η around the X-ray beam using two quarter-wave plates in series or a single half-wave plate.