In this contribution, we present the design of a resonant slow extraction based on the radio-frequency knockout (rf-KO) scheme, where we make use of the transverse resonance islands bucket (TRIB) optics. The generation of the TRIB optics is presented in two example lattices, which are considered for the potential upgrade of the current booster at DESY. The slow extraction studies are motivated to potentially supply high energy electron beams to the high energy physics beamline users. Simulations show an extraction efficiency in excess of 90% with a septum blade thickness of 100 μm. Published by the American Physical Society 2025

Methodical studies to improve the existing e-beam Longitudinal Phase Space (LPS) tomography were performed at the Photo Injector Test facility at DESY in Zeuthen. Proof-of-principle simulations were done to address some core concerns e.g. booster phase range, space charge effects and noisy artefacts in results. Phase advance analysis was done with the help of an analytical model that determined the booster phase range and step size. A slit was introduced before the booster to truncate the beam and reduce space charge forces. The reconstruction method adopted was image space reconstruction algorithm owing to its assurance of non-negative solution. An initial scientific presumption of LPS from low energy momentum measurements was established to reduce artefacts in the phase space. This paper will explain the proof-of-principle simulations highlighting the key aspects to obtain accurate results. Reconstructed LPS for different experimental cases will be presented to demonstrate the diagnostic capability.

Development of an accelerator-based tunable THz source prototype for pump-probe experiments at the European XFEL is ongoing at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The proof-of-principle experiments on the THz SASE FEL are performed utilizing the LCLS-I undulator (on loan from SLAC) installed in the PITZ beamline. The first lasing at a center wavelength of 100 mu m was observed in the summer of 2022. The lasing of the narrowband THz source was achieved using an electron beam with an energy of similar to 17 MeV and a bunch charge up to several nC. Optimization of beam transport and matching resulted in the measurement of THz radiation with a pulse energy of tens of mu J, measured with pyroelectric detectors. The THz FEL gain curves were measured by means of specially designed short coils along the undulator. The results of the first characterization of the THz source at PITZ will be presented.

We propose a new beam diagnostics method to reconstruct the phase space of charged particle bunches in 5 dimensions, which consist of the horizontal and vertical positions and divergences as well as the time axis. This is achieved by combining a quadrupole-based transverse phase-space tomography with the adjustable streaking angle of a polarizable X-band transverse deflection structure (PolariX TDS). We demonstrate with detailed simulations that the method is able to reconstruct various complex phase-space distributions and that the quality of the reconstruction depends on the number of input projections. This method allows for the identification and visualization of previously unnoticed detailed features in the phase-space distribution, and can thereby be used as a tool towards improving the performance of particle accelerators, or performing more accurate simulation studies.

Dielectric gratings are already used in Dielectric Laser Acceleration due to their high damage thresholds at high acceleration gradients. When an electron bunch passes close to one of these gratings, it emits radiation, and the features of this radiation will be dependent upon the beam position relative to the grating, the bunch charge, and the bunch length. A compact high-resolution diagnostics device will be developed that consists of multiple gratings with different periods; these types of devices are required for the accurate operation of future compact accelerators which are currently undergoing development and testing. ARES linac at DESY is able to provide sub-fs electron bunches and has a range of high-resolution diagnostic devices installed, such as the PolariX Transverse Deflecting Structure, which will allow for performance verification of a new diagnostic. The electron bunches can be altered, allowing for the measurement and analysis of the emitted radiation for different bunch lengths and charges. This work will present the current progress in this area, including the presentation and discussion of simulations, and a discussion of the planned experiments at ARES.

Advanced experiments using THz pump and X-ray probe pulses at modern free-electron lasers (FELs) like the European X-ray FEL require a frequency-tunable, high-power, narrow-band THz source maintaining the repetition rate and pulse structure of the X-ray pulses. This paper reports the first results from a THz source, that is based on a single-pass high-gain THz FEL operating with a central wavelength of 100 micrometers. The THz FEL prototype is currently in operation at the Photo Injector Test facility at DESY in Zeuthen (PITZ) and uses the same type of electron source as the European XFEL photo injector. A self-amplified spontaneous emission (SASE) FEL was envisioned as the main mechanism for generating the THz pulses. Although the THz FEL at PITZ is supposed to use the same mechanism as at X-ray facilities, it cannot be considered as a simple scaling of the radiation wavelength because there is a large difference in the number of electrons per radiation wavelength, which is five orders of magnitude higher for the THz case. The bunching factor arising from the electron beam current profile contributes strongly to the initial spontaneous emission starting the FEL process. Proof-of-principle experiments were done at PITZ using an LCLS-I undulator to generate the first high-power, high-repetition-rate single-pass THz FEL radiation. Electron bunches with a beam energy of  17 MeV and a bunch charge of up to several nC are used to generate THz pulses with a pulse energy of several tens of microjoules. For example, for an electron beam with a charge of  2.4 nC, more than 100 microjoules were generated at a central wavelength of 100 micrometers. The narrowband spectrum was also demonstrated by spectral measurements. These proof-of-principle experiments pave the way for a tunable, high-repetition-rate THz source providing pulses with energies in the millijoule range.

Currently, the Free electron laser user facility FLASH at DESY is undergoing a significant upgrade involving the complete transformation of one of its beamlines to allow external seeding. With the Echo-Enabled Harmonic Generation (EEHG) seeding method, we aim for the generation of fully coherent XUV and soft X-ray pulses at wavelengths down to 4 nm. The generated FEL radiation is sensitive to various electron beam properties, e.g., its energy profile imprinted either deliberately or by collective effects such as Coherent Synchrotron Radiation (CSR). In dedicated particle tracking simulations, one usually makes certain assumptions concerning the beam properties and the collective effects to simplify implementation and analysis. Here, we estimate the influence of some of the common assumptions made in EEHG simulations on the properties of the output FEL radiation, using the example of FLASH and its proposed seeding beamline. We conclude that the inherent properties of the FLASH1 beam, namely the negatively chirped energy profile, has dominant effect on the spectral intensity profile of the radiators output compare to that of the CSR induced chirp.

A superconducting radio-frequency (SRF) photo injector is in operation at the electron linac for beams with high brilliance and low emittance (ELBE) radiation center and generates continuous wave (CW) electron beams with high average current and high brightness for user operation since 2018. The speed of emittance measurement at the SRF gun beamline can be increased by improving the slit-scan system, thus the measurement time for one phase space mapping can be shortened from about 15 minutes to 90 seconds. A parallel algorithm and machine learning have been used to reduce the beamlet image noise. In order to estimate the uncertainty in the calculation of normalized emittance, we analyze the main error contributions such as slit position uncertainty, image noise, space charge effects and energy measurement inaccuracy.

Echo-Enabled Harmonic Generation (EEHG) is an external seeding technique for XUV and soft X-ray Free Electron Lasers (FEL). It has recently been experimentally demonstrated and currently many facilities worldwide intend to incorporate it in user operation. The EEHG process relies on very accurate and complex transformations of electron beam phase space by means of a series of undulators coupled to lasers and dispersive chicanes. As a result of the phase space manipulation, electrons are bunched at a high harmonic of the seed laser wavelength allowing coherent emission at few nm wavelength. Dispersion occurring in strong chicanes is imperative for implementation of this scheme and effective electron bunching generation. However, strong chicanes at the same time can be source of beam instability effects, such as Coherent Synchrotron Radiation (CSR), that can significantly grow in these conditions and suppress the bunching process. Therefore, there is a common need to investigate such effects in detail. Here, we discuss their treatment with simulation codes applied to a typical EEHG setup.

With the FLASH2020+ upgrade, one of the beamlines of the free-electron laser FLASH at DESY will be based on the Echo-Enabled Harmonic Generation (EEHG) seeding scheme and provide high-repetition-rate, coherent radiation down to 4 nm. To reach this wavelength, it is necessary to imprint intricate structures on the longitudinal phase space of the electron bunch at a very high harmonic of the seed laser wavelength, making the scheme potentially vulnerable to beam instabilities. Part of the beamline is a strong chicane, which is necessary to create the dispersion required by EEHG. Resulting effects such as Coherent Synchrotron Radiation (CSR) can be very detrimental for the bunching process and have to be taken into account already in the design of the beamline to ensure optimum FEL performance. We investigate and propose possible mitigation solutions to such instabilities in the FLASH2020+ parameter range. INTRODUCTION In the course of the FLASH2020+ upgrade [1] of the superconducting free-electron laser (FEL) user facility FLASH [2–4] in Hamburg, Germany, it is foreseen that one of the beamlines will be based on the Echo-Enabled Harmonic Generation (EEHG) seeding scheme [5]. EEHG provides a defined electron beam density distribution, which makes the startup process in the FEL not dependent on the stochastic nature of the shot noise and thus allows for shot-toshot reproducibility of fourier limited pulses [6,7]. However, beam instabilities arising within the EEHG section could be a limiting factor to achieve the necessary bunching [8]. Intrabeam Scattering (IBS) and Incoherent Synchrotron Radiation (ISR) have been studied based on analytical formulas [9] for the FLASH2020+ parameter space and result in no significant reduction of the bunching. Coherent Synchrotron Radiation (CSR) describes the phenomenon that electrons traveling through a dipole magnet can emit coherent radiation at wavelengths comparable to the bunch length. Due to the curved trajectory, the radiation can take a shortcut, which leads to a tail-head interaction. This finally results in an energy modulation along the bunch [10]. In the following, the effect of this energy modulation on the bunching is studied with the general-purpose accelerator simulation code ELEGANT [11] for EEHG at 4 nm. Two mitigation solutions, taking into account the duration of the seed laser and different EEHG working points, are examined. ∗ fabian.pannek@desy.de THEORY In the EEHG seeding scheme, the longitudinal phase space distribution of the electron bunch is manipulated in a beamline which consists of two undulators, so-called modulators, and two chicanes, as shown in Fig. 1. M 1 Q C 1 Q M 2 Q C 2 Q 3.3 m 6.6 m 3.3 m 3.3 m Figure 1: EEHG beamline used in the simulations with Modulators (M), Quadrupoles (Q) and Chicanes (C). In each modulator the electron bunch is modulated in energy by interacting with a seed laser, decribed by the energy modulation amplitudes A1,2 = ΔE1,2/σE, that is the energy modulation ΔE produced by the seed laser expressed as a multiple of the beam energy spread σE. The first chicane is used to create multiple energy bands in the longitudinal phase space and therefore requires a large dispersion R(1) 56 . The second dispersive section, described by R(2) 56 , compresses the energy modulated bands, creating a density modulation at the wavelength λE = λ1/aE. For two seed lasers operating at the same wavelength λ1 = λ2, the harmonic number is given by aE = n + m, where n and m are non-zero integers of opposite signs. The degree of bunching is decribed by the bunching factor |bn,m|. Its maximum value is approached for A1 ⪆ 3 and n = −1 and it scales approximately as |b−1,m| ≈ 0.39 ⋅ m−1/3 for m > 4 [12]. For optimized bunching at a specific harmonic, the ratio of the dispersions has to be close to R(1) 56 /R (2) 56 ≈ aE/|n|. Since the required strength of the second chicane is inversely proportinal to the energy modulation imposed in the second modulator, R(2) 56 ∝ 1/A2, a large A2 decreases the required dispersion of both chicanes. This is, however, accompanied by an increase in energy spread, resulting in a decreased FEL performance. CSR STUDY For this study, a σz = 100 μm Gaussian electron beam with an energy of E = 1.35 GeV, an energy spread of σE = 150 keV, a peak current of Ip = 500 A and a normalized emittance of εn = 0.6 mm mrad is used. Both seed lasers are set to λ1,2 = 300 nm. The chicane and modulator parameters used for the simulations are shown in Table 1. Each modulator and chicane is followed by quadrupoles to 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-FRXA06 FRXA06 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 4514 MC2: Photon Sources and Electron Accelerators A04 Circular Accelerators ensure proper matching. The energy modulation amplitudes are set to A1 = 3 and A2 = 5. The corresponding bunching factor |b| can be calculated analytically and is shown in Fig. 2 for different chicane configurations. Since maximum bunching can be achieved for n = −1, simulations with the FEL code GENESIS1.3, v4 [13,14] have been carried out for this case to optimize the dispersive strengths for power gain and spectral properties in the radiator beamline. For the upper bunching peak in Fig. 2 optimum values were found to be R(1) 56 = 7.05 mm and R (2) 56 = 81.25 μm. Table 1: Simulation Beamline Parameters Chicanes 1 2 Modulators length (m) 6.124 2.824 λu (mm) 82.6 Ldipole (m) 0.42 0.31 Periods 30 Ldrift (m) 2.00 0.57 K 9.97 75 80 85 90 95 100 105 110 115 R (2) 56 (μm) 2 3 4 5 6 7 8 9 10 R (1 ) 56 (m m ) n = −1

ARES is an electron linear accelerator at the SINBAD facility at DESY. It aims to deliver reliable high-brightness beams with an energy in the range of 100 to 150 MeV with fs to sub-fs bunch lengths. This is ideal for injection into novel high-gradient acceleration devices, such as dielectric laser accelerators and laser-plasma accelerators (LPAs), which feature fields with fs to ps period. Here, we report the conceptual design of a final focus system for injecting into an LPA experiment at ARES. The design includes permanent magnetic quadrupoles (PMQ), sufficient longitudinal space for collinear laser and electron transport, space for required diagnostics and an LPA setup. The performance of the design is evaluated by means of start-to-end simulations of the linac, focusing system, and injection into the LPA, including sensitivity studies to relevant error sources.

As Nb superconducting radio-frequency cavities are now approaching the theoretical limits of the material, a variety of different surface treatments have been developed to further improve their performance, although no fully understood theory is yet available. Small superconducting samples are studied to characterize their material properties and their evolution under different surface treatments. To study the RF properties of such samples under realistic SRF conditions at low temperatures, a test cavity called quadrupole resonator is currently being fabricated. In this work we report the status of the QPR at Universität Hamburg in collaboration with DESY. Our device is based on the QPRs operated at CERN [1] and at HZB [2] and its design will allow for testing samples under cavity-like conditions, i.e., at temperatures between 2 K and 8 K, under magnetic fields up to 120 mT and with operating frequencies of 433 MHz, 866 MHz and 1300 MHz. Fabrication tolerance studies on the electromagnetic field distributions and simulations of the static detuning of the device, together with a status report on the current manufacturing process, will be presented.

Emerging high-frequency accelerator technology in the terahertz regime is promising for the development of compact high-brightness accelerators and high resolution-power beam diagnostics. One resounding challenge when scaling to higher frequencies and to smaller structures is the proportional scaling of tolerances which can hinder the overall performance of the structure. Consequently, characterizing these structures is essential for nominal operation. Here, we present a novel and simple self-calibration technique to characterize the dispersion relation of integrated hollow THz-waveguides. The developed model is verified in simulation by extracting dispersion characteristics of a standard waveguide a priori known by theory. The extracted phase velocity does not deviate from the true value by more than $ 9 \times 10^{-5} ~\%$. In experiments the method demonstrates its ability to measure dispersion characteristics of non-standard waveguides embedded with their couplers with an accuracy below $ \approx 0.5~\% $ and precision of $ \approx 0.05~\% $. Equipped with dielectric lining the metallic waveguides act as slow wave structures, and the dispersion curves are compared without and with dielectric. A phase synchronous mode, suitable for transverse deflection, is found at $ 275~\text{GHz} $.

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.

In response to the 2013 Update of the European Strategy for Particle Physics (EPPSU), the Future Circular Collider (FCC) study was launched as a world-wide international collaboration hosted by CERN. The FCC study covered an energy-frontier hadron collider (FCC-hh), a highest-luminosity high-energy lepton collider (FCC-ee), the corresponding 100km tunnel infrastructure, as well as the physics opportunities of these two colliders, and a high-energy LHC, based on FCC-hh technology. This document constitutes the third volume of the FCC Conceptual Design Report, devoted to the hadron collider FCC-hh. It summarizes the FCC-hh physics discovery opportunities, presents the FCC-hh accelerator design, performance reach, and staged operation plan, discusses the underlying technologies, the civil engineering and technical infrastructure, and also sketches a possible implementation. Combining ingredients from the Large Hadron Collider (LHC), the high-luminosity LHC upgrade and adding novel technologies and approaches, the FCC-hh design aims at significantly extending the energy frontier to 100TeV. Its unprecedented centre of-mass collision energy will make the FCC-hh a unique instrument to explore physics beyond the Standard Model, offering great direct sensitivity to new physics and discoveries.

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.

To explore the fundamental properties of superconducting materials used in modern particle accelerators, high precision surface-resistance measurements in a dedicated testing equipment are of key importance. The quadrupole resonator, originally developed at CERN, and then successfully modified at the Helmholtz-Zentrum Berlin, is ideally suited for characterization of samples at temperatures of 1.8 K to more than 20 K, RF fields of up to 120 mT and frequencies of 433 MHz, 866 MHz and 1.3 GHz. In the past years, this set-up has been subject of intensive research on both its capabilities and limitations. Yet, one of the main challenges is the accuracy of the surface resistance measurement, which is determined by both the uncertainty in the RF measurement and manufacturing imperfections related to the production tolerances such as quenching and chemical polishing processes, etc. In this contribution, we focus on the influence of key geometrical parameters on three operating modes of the quadrupole resonator especially on the third mode since the surface-resistance measurements show some unexpected behavior for this frequency. INTRODUCTION In modern particle accelerators, superconducting radio frequency (SRF) cavities are applied to provide large accelerating gradients to high current beams while demanding moderate power requirements. Since power consumption and maximum accelerating gradient are mainly determined by the material properties, the surface resistance and the critical RF-field, the physical features of those materials used for building cavities are of key importance. Particularly, systematic research on superconductors requires conducting precision measurements of the RF properties as a function of an applied magnetic field, the operating temperature and the frequency, respectively. The quadrupole resonator (QPR), originally developed at CERN [1], and then successfully modified at the HelmholtzZentrum Berlin, is dedicated for the characterization of samples at temperatures of 1.8 K to more than 20 K, RF fields of up to 120 mT and frequencies of 433 MHz, 866 MHz and 1.3 GHz [2, 3]. This device is treated as a case study in our ∗ Work supported by the German Federal Ministry for Research and Education BMBF under contract 05H18HRRB1. † piotr.putek@uni-rostock.de research. Its cross sectional view is schematically shown in Fig. 1. The structure consists of a pillbox-like niobium cavity with four vertical niobium rods, collinearly arranged inside the resonator. The rods are welded to the top-plate of the cavity. At their lower end, they are bent into semiannular pole shoes and positioned a short distance over the sample surface. Furthermore, at the bottom of the cylinder, the calorimetry chamber is mounted as inner conductor of a coaxial structure, which is additionally thermally decoupled from the resonator. It is equipped with a resistive DC heater and sensors placed on the bottom of the disc-shaped sample. This design provides focusing of the RF magnetic field into the area of the sample. The resulting power dissipation can then be measured by temperature probes inside the calorimetry chamber. As a result, the surface resistance RS is studied by means of the so-called "RF-DC-compensation" method proposed in [1].

Being able to obtain BCS surface resistance and material properties from the same surface is necessary to gain a fundamental understanding of the evolution of SRF surfaces. A test resonator which will allow to obtain BCS properties from samples is currently under development at Universität Hamburg (UHH) and DESY and is based on the Quadrupole Resonators developed and operated at CERN [1] and HZB [2]. The current status of the necessary infrastructure, the procurement process and design considerations are shown. In addition, an outline of the planned R&D project with the Quadrupole Resonator will be presented and first RF measurements and surface analysis results of samples will be shown.