The layout of the Variable Polarization XUV Beamline P04 at PETRA III is described with emphasis on selected examples of optics, mirrors and gratings. A precise characterization of the optics, their performance inside the holder and of the surrounding mechanics is presented. This also includes a detailed characterization of the different beamline mechanics as a whole (grating unit, exit slit unit, re-focusing unit) including the environment.

Abstract.The photon-ion merged-beams technique has been employed at the new PhotonIon spectrometer at PETRA III (PIPE) for measuring multiple photoionization of Xe Absolute cross sections for 3d photoionization of Xe q+ ions (1 ≤ q ≤ 5) 2

X-ray Free-Electron Lasers (XFELs) are unique sources of high brightness coherent radiation. However, existing XFELs supply only linearly polarized light, precluding studies of chiral dynamics. A device called the Delta undulator has been installed at the Linac Coherent Light Source (LCLS) to provide tunable polarization. With a reverse tapered planar undulator line to pre-microbunch the beam and the novel technique of beam diverting, hundreds of microjoules of circularly polarized X-ray pulses are produced at 500-1200 eV. These X-ray pulses are tens of femtoseconds long, have a degree of circular polarization of 0.98 +0.02 −0.04 at 707 eV, and may be scanned in energy. We also present a new twocolor X-ray pump X-ray probe operating mode for LCLS. Energy differences of ΔE/E = 2.4% are supported, and the second pulse can be adjusted to any elliptical polarization. In this mode the pointing, timing, intensity, and wavelength of the two pulses can be modified.

International audienceThe two single-pass, externally seeded free-electron lasers (FELs) of the FERMI user facility are designed around Apple-II-type undulators that can operate at arbitrary polarization in the vacuum ultraviolet-to-soft x-ray spectral range. Furthermore, within each FEL tuning range, any output wavelength and polarization can be set in less than a minute of routine operations. We report the first demonstration of the full output polarization capabilities of FERMI FEL-1 in a campaign of experiments where the wavelength and nominal polarization are set to a series of representative values, and the polarization of the emitted intense pulses is thoroughly characterized by three independent instruments and methods, expressly developed for the task. The measured radiation polarization is consistently >90% and is not significantly spoiled by the transport optics; differing, relative transport losses for horizontal and vertical polarization become more prominent at longer wavelengths and lead to a non-negligible ellipticity for an originally circularly polarized state. The results from the different polarimeter setups validate each other, allow a cross-calibration of the instruments, and constitute a benchmark for user experiments