Angle resolved photo electron spectrometer for hard x-ray photon diagnostics at the European XFEL

We have developed and commissioned an angle-resolved photoelectron spectrometer, based on the electron time-of-flight concept, for hard x-ray photon diagnostics at the European Free-Electron Laser. The instrument provides users and operators with pulse-resolved, non-invasive spectral distribution diagnostics, which in the hard x-ray regime is a challenge due to the poor cross-section and high kinetic energy of photoelectrons for the available target gases. We report on the performance of this instrument as obtained using hard x-rays at the PETRA III synchrotron at DESY and the SASE1 beamline at the European XFEL. We demonstrate a resolving power of 10 eV at incident photon energies up to 20 keV.