Construction and commissioning of an APPLE-III afterburner undulator as a prototype for radiators in the new seeding line of FLASH2020+

Abstract An APPLE-III undulator with 17.5 mm period length operating at a minimum magnetic gap of 8 mm was installed downstream of the FLASH2 SASE undulators. This device serves as an afterburner and was designed to provide arbitrarily (also circularly) polarized light at the 3rd harmonic of the FLASH2 radiation with a wavelength of 1.33 nm to 1.77 nm (890-700eV), covering the absorption L -edges of the magnetic 3d elements. After characterization of the different polarization modes at different wavelengths in a first commissioning phase, first user experiments have been conducted. Here, we report on our experiences with the mechanical and magnetic assembly of the APPLE-III UE17 as well as on magnetic measurements and tuning of trajectories and phase errors at the different polarization states.