Storage rings and free electron lasers use undulators to produce high-brilliant X-ray photon beams. In order to increase brilliance and photon energy tunability it is necessary to enhance the undulator magnetic peak field on axis by reducing its period without decreasing the electron beam stay clear. Undulator technologies aiming to reach this goal are presented.

In high-gain free-electron lasers (FELs) with planar undulators it is possible (in the linear regime) to independently amplify at the fundamental and at odd harmonics, a process referred to as Harmonic Lasing (HL). For the HL process preservation of the quality of the incoming high-brightness electron beam is essential. This requires suppression of the lasing at the fundamental, which can be achieved using several methods such as special phase shifter set points and attenuation of the fundamental radiation using intra-undulator optical high-pass filters. The European XFEL variable-gap undulator beamline SASE2 features two intra-undulator stations combining a magnetic chicane and the possibility to insert a thin diamond crystal onto the optical axis of the beamline. While installed for the operation in hard x-ray self seeding (HXRSS) mode, this hardware is well-suited for HL experiments at a low electron beam energy corresponding to a fundamental photon energy of about 2keV. In this contribution we present numerical simulations of third-harmonic lasing at this working point.

The SASE3 Variable Polarization project is intended to offer polarization control of the X-ray FEL pulses at the European XFEL. The project was completed in early 2022. During the winter shutdown 2021-2022, all four APPLE-X helical undulators were placed in the tunnel and first lasing was achieved in April 2022. Unfortunately, further use of the helical afterburner proved impossible, as the encoders used to position the magnetic structures of the undulator were damaged by radiation. To carry out repairs, all undulators were removed from the tunnel in the summer 2022, and investigations were carried out to determine the cause of the radiation damage. This article presents measures taken to minimize further radiation damage in order to ensure the continued operation of the helical afterburner.

In advanced light sources such as 4th generation synchrotrons and Free Electron Lasers (FELs), undulators are important devices to produce photons with high brilliance. This necessitates to reach highest possible magnetic fields. For a given magnetic gap and period length this demand can only be accomplished by using the superconducting undulator (SCU) technology. At the Institute for Beam Physics and Technology (IBPT) of the Karlsruhe Institute of Technology (KIT) there is an ongoing R&D collaboration on SCUs together with Bilfinger Noell GmbH (BNG). Within the latest project a SCU mock-up was designed and manufactured by BNG. This device is suitable for testing applications in liquid helium and conduction cooled environments at the IBPT measurement setups. Additionally, it aims for higher field applications as needed for implementation e.g., at the European XFEL. In this contribution we describe the general layout of a ~400 mm long mock-up coil package with 18 mm period length and present result of first cryogenic tests in liquid helium.