Commissioning and operation of the Apple-X undulator at the European XFEL

After the successful commissioning of the SASE3 planar undulator system at the European XFEL, one of the requests from the soft X-ray user community was to extend the radiation properties and create the possibility of obtaining variable polarization modes. As a result, it was decided to build a helical afterburner in collaboration with PSI. The first commissioning of a system of four APPLE-X undulators was carried out in 2022. However, shortly after receiving the first lasing, the undulator control components were damaged. The damage was caused by increased radiation, which was reduced later by improving the design of the vacuum system. The re-installation of the undulators in the tunnel after the replacement of the damaged components took place at the beginning of 2024. Since then, a series of activities have been carried out to commission the helical afterburner. Using reverse tapering of SASE3 planar undulators it is now possible to generate pulse energies above 1 mJ at a contrast of up to 90 with four APPLE-X undulators. The methods and results of the optimization to achieve the expected parameters of the helical radiation as well as the operation experience are presented.