The FEL performance strongly correlates with the undulator field quality. The definition of mechanical tolerances for the undulator magnets allows us to achieve the wished field quality. These mechanical tolerances should be defined both on short and long-range errors. With long-range errors, we address problems like deformations of the yoke caused by the support structures or unwanted tapering, which can arise in the positioning procedure of the ideally parallel undulator coils. In this contribution, we quantify the effect and set tolerances of a few types of long-range errors on the FEL radiation generated specifically from superconducting undulator coils.

Superconducting undulators constitute an important element of the European XFEL facility development program. The application of SCU technology at European XFEL paves the way to potentially enable lasing above 50 keV, opening new frontiers in scientific research and offering unprecedented capabilities in high-energy applications. A total of six modules are foreseen. The Superconducting undulator PRE-SerieS mOdule (S-PRESSO) is the preseries module for the free-electron laser superconducting undulator afterburner. This article presents a comprehensive assessment of the S-PRESSO superconducting undulator mock-up's magnetic performance conducted at the Institute for Beam Physics and Technology of the Karlsruhe Institute of Technology. After a description of the experimental setup and the coil, the results of training and of the magnetic field measurements are presented. The results demonstrate a robust design of the mock-up, which achieved the short sample limit, a long lasting stability at nominal current tested for 8 h, and a straight trajectory below European XFEL requirements. The measured quality of the magnetic field is studied and compared with simulations using the geometric values measured at room temperature with a coordinate measuring machine. This analysis suggests areas for potential improvement in the manufacturing process.

For more than 5 years, superconducting undulators (SCUs) have been successfully delivering X-rays in storage rings. The European X-Ray Free-Electron Laser Facility (XFEL) plans to demonstrate the operation of SCUs in X-ray free-electron lasers (FELs). For the same geometry, SCUs can reach a higher peak field on the axis with respect to all other available technologies, offering a larger photon energy tunability range. The application of short-period SCUs in a high electron beam energy FEL > 11 GeV will enable lasing at very hard X-rays > 40 keV. The large tunability range of SCUs will allow covering the complete photon energy range of the soft X-ray experiments at the European XFEL without changing electron beam energy, as currently needed with the installed permanent magnet undulators. For a possible continuous-wave (CW) upgrade under discussion at the European XFEL with a lower electron beam energy of approximately 7–8 GeV, SCUs can provide the same photon energy range as available at present with the permanent magnet undulators and electron energies. This paper will describe the potential of SCUs for X-ray FELs. In particular, it will focus on the different activities ongoing at the European XFEL and in collaboration with DESY to allow the implementation of SCUs in the European XFEL in the upcoming years.

At the European XFEL, the undulator systems group has started in 2020 an R&D project dedicated to the development of innovative superconducting undulator (SCU) coils. SCUs are of great interest for storage rings and free electron lasers (FEL) facilities as they enable to widen the tunability range of the generated photon energy for the same electron beam energy compared with the established technology of the permanent magnet undulators. In this contribution, we present the characterization study and the achievable performance of HybriSCU a graded SCU coil combining two different superconductors: NbTi and high temperature superconductor (HTS) based on Rare-Earth Barium Copper Oxide (ReBCO) tape.

Superconducting Undulators (SCUs) can produce higher photon flux and cover a wider photon energy range compared to permanent magnet undulators (PMUs) with the same vacuum gap and period length. To build the know-how to implement superconducting undulators for future upgrades of the European XFEL facility, the test stand SUNDAE1 for the characterization of SCU is being developed. The purpose of SUNDAE1 is the training, tuning and development of new SCU coils by means of precise magnetic field measurements. The experimental setup will allow the characterization of magnets up to 2m in length. These magnets will be immersed in a Helium bath at 4K or 2K temperature. In this article, we describe the experimental setup and highlight its expected performances.

European XFEL is investing in the development of superconducting undulators (SCUs) for future upgrade of its beamlines. SCUs made of NbTi, working at 2 K, with a period length of 15 mm and a vacuum gap of 5 mm allow covering a range between 54 keV and 100 keV. The effect of mechanical errors in the distribution of the undulator parameter K along the undulators is more relevant for working points at lower photon energy, which are obtained using a higher magnetic field in the undulator. In this article we investigate the effect of error distribution in the K-parameter for a working point at 50 keV photon energy obtained injecting an electron beam with 16.5 GeV energy from the XFEL linear accelerator in a undulator line composed by SCUs with 1.6 T peak magnetic field.

At the European XFEL, a superconducting afterburner is planned for the SASE2 hard X-ray beamline. It will consist of 5 undulator modules plus a pre-series module called S-PRESSO. Within each module, two superconducting undulators (SCU) 2 m long are present. Such an afterburner will enable photon energies above 30 keV. The magnetic field of the SCU determines the quality of the electron beam trajectory and the free-electron laser (FEL) radiation. The mechanical accuracy of the SCU determines its magnetic field quality. In this contribution, we present the mechanical errors and an analytical study to determine the tolerances for our SCUs.

We propose to develop, characterize and operate a superconducting undulator (SCU) afterburner consisting of 5 undulator modules (1 module = 2 SCU coils of 2 m length and 1 phase shifter) plus a pre-series prototype at the SASE2 hard X-ray beamline of European XFEL. This afterburner will produce an output in the order of 1010 ph/pulse at photon energies above 30 keV. The project is divided into the production of a pre-series prototype module and a small-series production of 5 modules. Central goals of this R&D activity are: the demonstration of the functionality of SCUs at an X-ray FEL, the set up of the needed infrastructure to characterize and operate SCUs, the industrialization of such undulators, and the reduction of the price per module. In this contribution, the main parameters and specifications of the pre-series prototype module are described.

At the European XFEL, we are designing a superconducting afterburner for the SASE2 hard X-ray beamline. It will consist of a series of five undulator modules plus a pre-series one called S-PRESSO. One module corresponds to two superconducting undulator (SCU) coils of 2m length plus one phase shifter. Such an afterburner will enable photon energies above 30 keV. We foresee to install superconducting (SC) phase shifters in each undulator module to keep the correct phase delay between the electron beam and photon beam. In this contribution, we present the required SC phase shifter parameters to enable operation in the electron beam energy range 11.5-17.5 GeV. We also analyze different magnetic designs satisfying the calculated specifications.

We propose to develop, characterize and operate a superconducting undulator (SCU) afterburner consisting of five undulator modules (1 module = 2 times SCU coil of 2 m length and 1 phase shifter) at the SASE2 hard X-ray beamline of European XFEL. This afterburner has the potential to produce an output of more than 1010 ph/pulse at photon energies above 30 keV. The project is divided into the production of a pre-series prototype module and a small-series production of five modules. Central goals of this R&D activity are: the demonstration of the functionality of SCUs at a X-ray FEL, the set up of the needed infrastructure to characterize and operate SCUs, the industrialization of such undulators and the reduction of the price per module. In this contribution the main parameters and specifications of the pre-series prototype module (S-PRESSO) are described. INTRODUCTION AND MOTIVATION The European XFEL (EuXFEL) plans to develop the technology of superconducting undulators (SCUs) as part of its facility development program. Superconducting undulator technology enables, for the same period length and vacuum gap, about a three-times stronger magnetic field, in comparison to the permanent-magnet undulators (PMUs) currently used at the EuXFEL facility. The use of SCUs will allow to improve the performance and flexibility of the EuXFEL FEL sources, both in terms of reach towards higher photon energies and in terms of tuning range of an individual FEL undulator. The benefits of SCUs R&D for the EuXFEL strategic plans are manifold: 1) Enabling lasing at very high photon energies towards 100 keV, fully exploiting the capability of the FEL linac with the highest electron beam energy worldwide [1]. FEL lasing at such photon energies will enable new types of experiments and thereby open the access to new scientific applications of FEL radiation, especially in the area of material sciences with a focus on new energy technologies. 2) Enhancing the tunability range up to factor of ten for future soft X-ray SASE (self-amplified spontaneous emission) lines, allowing to cover the complete photon energy range offered by the present soft X-ray experiments at EuXFEL with the same electron beam energy. 3) The continuous-wave (CW) operation mode upgrade under consideration at the EuXFEL would limit the electron beam energy to 7-8 GeV. In this case a SASE SCU line would allow to cover the same ∗ sara.casalbuoni@xfel.eu photon energy range as provided now by the installed PMUs with higher electron beam energies (up to 17.5 GeV). An SCU afterburner consisting of five undulator modules, as sketched in Fig. 1, is proposed for SASE2. Each module will contain two 2 m long undulator coils, horizontal and vertical correctors at the exit of the first set of SCU coils and at the entrance of the second set of SCU coils, as well as a phase shifter (see inset of Fig. 1). Each module is 5 m long, as the presently installed PMUs. This allows to use the same room temperature intersections with focusing quadrupoles, phase shifters and electron beam diagnostics, as in the present undulator lines. Two horizontal and vertical correction coils, placed at the beginning and at the end of the intersection, will also be employed. Figure 1: Sketch of the SCU afterburner after SASE2 (bottom) and of one SCU module (top). Before the small-series production of five modules, a preseries prototype module (S-PRESSO) will be produced and installed in the EuXFEL. Aims of S-PRESSO are to test the alignment of the two 2 m long SCU coils in the 5 m long cryostat, the mechanical tolerances necessary for the FEL process, and the implementation of the module in the accelerator. While for the afterburner modules a 2 K cryoplant is under consideration, in order to test this first device with electron beam a cooling scheme based on cryocoolers is planned. S-PRESSO will be used to amplify the fundamental produced by the PMUs of SASE2 in the hardest X-ray part of the spectrum which they can generate. In this configuration it will be possible to measure the contribution of the SCUs to the FEL amplification process at specific photon energies. Moreover, harmonic configuration tests at larger photon energies are planned. The projected photon performance of the five modules of the SCU afterburner is presented in Fig. 2. The SCUs 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB132 MC2: Photon Sources and Electron Accelerators T15 Undulators and Wigglers WEPAB132 2921 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I

The free-electron laser (FEL) community is interested in taking full advantage of the high-repetition-rates of FELs run by superconducting machines while maintaining the spectral properties achieved with external seeding techniques. Since the feasibility of seed lasers operating at a repetition-rate of MHz and with sufficient energy in a useful wavelength range, such as the ultraviolet (UV) range is challenging, a seeded oscillator-amplifier scheme is proposed instead for generation of fully coherent and high-repetition-rate radiation. The process is triggered by an external seed laser while an optical feedback system feeds the radiation back to the entrance of the modulator where it overlaps with the next electron bunch. Downstream from the feedback system, the electron bunches are then used for harmonic generation. We discuss the optimization of dedicated simulations and we investigate the stability of this scheme with numerical simulations. As a result, we address the control of the reflectivity of the resonator as a key parameter to achieve a stable HGHG seeded radiation. Finally, we show the impact of the power fluctuations in the oscillator on the bunching amplitude with analytical and simulated results. The output FEL radiation wavelengths considered are 4.167 nm and 60 nm.

European XFEL is investing in the development of superconducting undulators (SCUs) for future upgrade of its beamlines. SCUs made of NbTi, working at 2 K, with a period length of 15 mm and a vacuum gap of 5 mm allow covering a range between 54 keV and 100 keV for 17.5 GeV electron energy. The effect of mechanical errors in the distribution of the undulator parameter K along the undulators is more relevant for working points at lower photon energy, which are obtained using a higher magnetic field in the undulator. In this article we investigate the effect of error distribution in the K-parameter for a working point at 50 keV photon energy obtained injecting an electron beam with 16.5 GeV energy from the XFEL linear accelerator in a undulator line composed by SCUs with 1.58 T peak magnetic field.

For more than a decade free-electron lasers (FELs) have been in operation, providing scientists from many disciplines with the benefits of ultrashort, nearly transversely coherent radiation pulses with wavelengths down to the \AA{}ngstrom range. If no further techniques are applied, the FEL will only amplify radiation from the stochastic distributed electron density in the electron bunch. Contemporary developments aim at producing stable and single-mode radiation by preparing an electron bunch with favorable longitudinal electron density distributions using magnets and conventional laser pulses (seed), hence the name ``seeding.'' In recent years, short wavelength FELs at high electron beam energies and high repetition rates were proposed and built. At those repetition rates, an external seed with sufficient power to manipulate the electron beam cannot be provided by present state-of-the-art laser systems, thus no external seeding scheme could be applied yet. In this paper, we present ways to seed FELs to generate short wavelength radiation at high repetition rates, making use of tested electron beam manipulation schemes. For our parameter study, we used the parameters of FLASH, the free-electron laser in Hamburg. First simulations are presented, showing the feasibility of the method proposed.

Longitudinal space-charge effects can act as a driver for short wavelength radiation production in a longitudinal space-charge amplifier (LSCA) *. A single cascade of an LSCA was tested using the hardware of the sFLASH experiment installed at the FEL user facility FLASH (at DESY, Hamburg). Scans of the longitudinal dispersion of the chicane were performed with the tightly focused electron beam for different compression settings, while recording the intensity of the emission from a few-period undulator. We present experimental results and estimates on electron beam properties.

We report on the successful operation of a Free Electron Laser (FEL) in the Echo Enabled Harmonic Generation (EEHG) scheme at the FERMI facility at Sincrotrone Trieste. The experiment required a modification of the FEL-2 undulator line which, in normal operation, uses two stages of high-gain harmonic generation separated by a delay line. In addition to a new seed laser, the dispersion in the delay-line was increased, the second stage modulator changed and a new manipulator installed in the delay-line chicane hosting additional diagnostic components. With this modified setup we have demonstrated the first evidence of strong exponential gain in a free electron laser operated in EEHG mode at wavelengths as short as 5 nm.

Echo-enabled harmonic generation (EEHG) is a promising technique for seeded free electron lasers (FELs) not only to go down to wavelengths of 4 nm but also to simplify the schemes that are currently used to achieve a similar wavelength range (double cascade HGHG). Thus a study optimizing the EEHG performance in the wavelength range from 60 to 4nm has been performed. The more critical working point, at 4 nm, is here analyzed in terms of seed laser stability for two different seed laser frequencies: visible and UV.

In recent years, there is interest of the Free-Electron Laser (FEL) community in external-seeding techniques such as the Echo-Enabled Harmonic Generation (EEHG) and the HighGain Harmonic Generation (HGHG). With these techniques, pulses of an improved temporal coherence are generated, but at the same time, they are limited by the repetition rates that seed lasers can currently offer with the required pulse energies. A big challenge is to combine the advantages of seeding schemes with high repetition rates. For this purpose, we study a combination of an oscillator-amplifier. The modulator in the oscillator is used at a long wavelength to modulate the electron beam and an amplifier is operated to extract the FEL radiation of the desired harmonic. This way we can use a seed laser of 10 Hz in a burst mode and a resonator to feedback the radiation at repetition rates of superconducting accelerators instead of using an external seed at these high-repetition rates. In this contribution, we present simulation results of a seeded oscillator-amplifier FEL in an HGHG scheme.

The spectral and temporal properties of Free-Electron Lasers (FEL) operating on the basis of self-amplified spontaneous emission (SASE) suffer from the stochastic behavior of the start-up process that fluctuates on a bunch-to-bunch basis. Several so-called ”seeding”-techniques using external radiation fields to overcome this limitation have been proposed and demonstrated. The external seed is usually generated by high-power laser systems, which are not yet available with a sufficient laser pulse energy at the high repetition rates of superconducting FEL facilities. In this contribution we discuss several seeding schemes that lower the requirements for the used laser systems, enabling seeded operation at high repetition rates by the means of a resonatoramplifier setup.

Free-electron lasers (FELs) based on the self-amplified spontaneous emission (SASE) principle generate photon pulses with typically poor longitudinal coherence. FEL seeding techniques greatly improve longitudinal coherence by initiating FEL amplification in a controlled way using coherent light pulses. The sFLASH experiment installed at the FEL user facility FLASH at DESY in Hamburg is dedicated to the study of external seeding techniques. In this paper, the layout of the sFLASH seeding experiment is presented and an overview of recent developments is given.