Design and Initial Characterisation of X-ray Beam Diagnostic Imagers for the European XFEL

The European X-Ray Free-Electron-Laser facility requires diagnostics of its x-ray photon beam. Besides other diagnostic components, imaging stations will be employed for the characterisation of beam properties like position, profile, and pointing, before and after different types of mirrors, slits and monochromators. In combination with soft x-ray grating monochromators or other dispersive devices, imagers can also deliver spectral information. The imagers will usually absorb the beam (invasive devices), however, for some applications they will be partially transmissive to allow for beam pointing monitoring together with a second imaging unit further downstream. For the first commissioning 25 diagnostic imagers are planned at various positions in the photon beam tunnels. Further similar devices are under development for monitoring the beam properties at the experimental stations. The design of theses imaging stations will be described. Initial testing has started and the optimization of some components will be reported. The main components of these imaging stations are: retractable scintillators for conversion of x-rays to visible light, mirrors, optics and CCD / CMOS cameras for image recording, an ultra-high vacuum (UHV) chamber, and the associated control electronics and software. Scintillators and mirrors will be the only components in an ultra-high vacuum chamber. Performance characteristics are addressed, especially mechanical stability, spatial resolution, signal-to-noise properties, and radiation hardness. The challenge in the design is to deal with a wide range of beam properties: photon energies from 0.26 – 25 keV, beam sizes from several 100 μm to several mm, large beam position shifts of up to 120 mm, pulse durations of 10 fs and pulse energies up to 10 mJ which may destroy materials by a single pulse.