Photon Diagnostic for FLASH2020+: Analysis of Spontaneous Radiation for Undulator Alignment

Within the FLASH2020+ program, DESY is aiming to significantly improve the flexibility and availability of its pioneering FEL FLASH for users. Increasing the electron beam energy to 1.35GeV will make wavelengths available up to the oxygen K-edge on the fundamental harmonic. Exchanging the current fixed gap undulators in the FLASH1 branch for variable gap devices will allow for fully independent operation of both branches in the future. The 11 new Apple III type variable polarization undulator segments will have a period length of 33mm and a length of 2.5m each. This paper presents the development of a photon diagnostic station for FLASH2020+, an additional tool for alignment and commissioning of the undulators. The diagnostic station will be located about 50m downstream from the last undulator. It will allow for the measurement and correction of the electron beam orbit as well as for the measurement of the undulator K-values. In addition, the system can be used to investigate the phasing of the undulators in two adjacent cells by analysing their spontaneous radiation.