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A major challenge in single-linac - multiple undulator setups like EuXFEL is the generation of individual shaped photon pulses, in particular, when working in a mode where a single pulse train, or cw stream, feeds all undulator lines. This work presents the experimental verification of a flexible delivery scheme producing photon pulses for each of the three undulator lines with their electron bunches individually shaped in charge, compression and optics on a single RF pulse burst.

A new generation of high-gradient normal conducting 1.3 GHz RF gun with 1% duty factor was developed to provide a high-quality electron source for superconducting linac driven free-electron lasers like FLASH and European XFEL. Compared to the Gun4 series, Gun5 aims for a ~50% longer RF pulse length (RF pulse duration of up to 1 ms at 10 Hz repetition rate) combined with high gradients (up to ~60 MV/m at the cathode). In addition to the improved cell geometry and cooling concept, the new cavity is equipped with an RF probe to measure and control the amplitude and phase of the RF field inside the gun. The first characterization of Gun5.1 included measurements of RF amplitude and phase stability (pulse-to-pulse and along 1 ms RF pulse). The dark current was measured at various peak power levels. The results of this characterization will be reported.

The photoinjectors of FLASH at DESY (Hamburg, Germany) and the European XFEL are operated by laser driven RF-guns. In both facilities cesium telluride photocathodes are successfully used since several years. We present recent data on the lifetime, quantum efficiency (QE), and dark current of the photocathodes currently in operation. In addition we present recent design changes in the photocathode transfer systems in order to further improve the cathode handling.

Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK 31st Linear Accelerator Conference, LINAC22, Liverpool, United Kingdom, 28 Aug 2022 - 2 Sep 2022; JACoW Publishing, Geneva, Switzerland 699-702 (2022). doi:10.18429/JACoW-LINAC2022-THPOJO03, A new generation of normal conducting 1.3GHz RF gun was developed to provide a high-quality electron source for superconducting linac driven free-electron lasers like FLASH and European XFEL. Compared to the Gun4 series, Gun5 aims for a 50% increase of the duration of the RF pulse (up to 1 ms at 10 Hz repetition rate) combined with high gradients (up to ~60 MV/m at the cathode). In addition to the improved impedance, the new cavity is equipped with an RF probe to measure and control the amplitude and phase of the RF field inside the gun. The first prototype of the new RF gun was manufactured at DESY and installed at the Photo Injector Test facility at DESY in Zeuthen (PITZ) in October 2021. In mid-October 2021 the RF conditioning began, aiming for achieving the aforementioned RF parameters. The conditioning procedure involves a slow gradual increase in repetition rate, RF pulse duration and peak power while carefully monitoring vacuum conditions and signals from interlock sensors. The results of RF conditioning will be reported., Published by JACoW Publishing, Geneva, Switzerland

Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand 13th International Particle Accelerator Conference, IPAC`22, Bangkok, Thailand, 12 Jun 2022 - 17 Jun 2022; JACoW Publishing, Geneva, Switzerland 1018-1021 (2022). doi:10.18429/JACOW-IPAC2022-TUPOPT010, Growing interests in ultra-hardX-rays are pushing forward the frontier of commissioning the European X-ray Free-Electron Laser (XFEL) for routine operation towards the sub-ångström regime, where a photon energy of 25 keV (0.5 ångström) and above is desired. Such X-rays allow for larger penetration depths and enable the investigation of materials in highly absorbing environments. Delivering the requested X-rays to user experiments is of crucial importance for the XFEL development. Unique capabilities of the European XFEL are formed by combining a high energy linac and the long variable-gap undulator systems for generating intense X-rays at 25 keV and pushing the limit even further to 30 keV. However, the FEL performance relies on achievable electron bunch qualities. Low-emittance electron bunch production, and the associated start-to-end modelling of beam physics thus becomes a prerequisite to dig into the XFEL potentials. Here, we present the obtained simulation results from a virtual commissioning of the XFEL for the user experiments at 25 keV and beyond, including the optimized electron bunch qualities and corresponding FEL lasing performance. Experimental results at 30 keV from the first test run are presented., Published by JACoW Publishing, Geneva, Switzerland

Applied Sciences 11(22), 10768 (2021). doi:10.3390/app112210768, Sub-��ngstr��m working regime refers to a working state of free-electron lasers which allows the generation of hard X-rays at a photon wavelength of 1 ��ngstr��m and below, that is, a photon energy of 12.5 keV and above. It is demonstrated that the accelerators of the European X-ray Free-Electron Laser can provide highly energetic electron beams of up to 17.5 GeV. Along with long variable-gap undulators, the facility offers superior conditions for exploring self-amplified spontaneous emission (SASE) in the sub-��ngstr��m regime. However, the overall FEL performance relies quantitatively on achievable electron beam qualities through a kilometers-long accelerator beamline. Low-emittance electron beam production and the associated start-to-end beam physics thus becomes a prerequisite to dig in the potentials of SASE performance towards higher photon energies. In this article, we present the obtained results on electron beam qualities produced with different accelerating gradients of 40 MV/m���56 MV/m at the cathode, as well as the final beam qualities in front of the undulators via start-to-end simulations considering realistic conditions. SASE studies in the sub-��ngstr��m regime, using optimized electron beams, are carried out at varied energy levels according to the present state of the facility, that is, a pulsed mode operating with a 10 Hz-repetition 0.65 ms-long bunch train energized to 14 GeV and 17.5 GeV. Millijoule-level SASE intensity is obtained at a photon energy of 25 keV at 14 GeV electron beam energy using a gain length of about 7 m. At 17.5 GeV, half-millijoule lasing is achieved at 40 keV. Lasing at up to 50 keV is demonstrated with pulse energies in the range of a few hundreds and tens of microjoules with existing undulators and currently achievable electron beam qualities., Published by MDPI, Basel

12th International Particle Accelerator Conference, IPAC`21, Campinas, Brazil, 24 May 2021 - 28 May 2021; 3953-3955 (2021). doi:doi:10.18429/JACoW-IPAC2021-THPAB080, Hard X-ray free-electron lasers (XFELs) generate intense coherent X-ray beams by passing electrons through undulators, i.e. very long periodic magnet structures, which extend over hundreds of meters. A crucial condition for the lasing process is the spatial overlap of the electrons with the electromagnetic field. Well-established electron beam-based procedures allow finding a straight trajectory for the electrons defined by the beam position monitors (BPM) between the undulators. A bending of the trajectory in between the BPMs cannot be seen by these methods. A general field offset inside the undulators has the effect that the synchrotron radiation is emitted at a different angle at the beginning and the end of the undulator which can result in a degradation of the FEL-gain especially for very short wavelengths. We report on how the spectral and spatial characteristics of the monochromatized radiation of a single undulator can be used to minimize the field offset in situ with the help of correction coils.

Physical review accelerators and beams 24(3), 030701 (1-12) (2021). doi:10.1103/PhysRevAccelBeams.24.030701, Harmonic lasing provides an opportunity to extend the photon energy range of existing and planned X-ray FEL user facilities. Contrary to nonlinear harmonic generation, harmonic lasing can generate a much more intense, stable, and narrow-band FEL beam. Another interesting application is Harmonic Lasing Self-Seeding (HLSS) that allows to improve the longitudinal coherence and spectral power of a Self-Amplified Spontaneous Emission (SASE) FEL. This concept was tested at FLASH in the range of 4.5 - 15 nm and at PAL XFEL at 1 nm. In this paper we present recent results from the European XFEL where we successfully demonstrated harmonic lasing at 5.9 Angstrom and 2.8 Angstrom. In the latter case we obtained both 3rd and 5th harmonic lasing and, for the first time, operated a harmonic lasing cascade (5th-3rd-1st harmonics of the undulator). These results pave the way for reaching very high photon energies, up to 100 keV., Published by American Physical Society, College Park, MD

We consider the collective beam dynamics at the injector section of the European XFEL. The results of the measurements for the longitudinal phase space (LPS) of the electron beam are compared with those obtained from numerical modeling. A new approach is proposed for the analysis of the self-field effects in the LPS measurements. It allows to determine accurately the synchronous RF phase in an accelerating module and to subtract properly the RF curvature imprinted in the LPS. A further incorporation with the simulation made it possible to separate the collective effects originating from different sources and thus to quantify, individually, the impact of these effects on the beam dynamics. This includes the space-charge dominated beam dynamics in the RF gun as well as the collective effects dominated by the wake fields after the gun up to the end of the injector section. A new analytical model is also proposed for the short-range wake function of a finite chain of RF cavities and verified by both numerical analysis and experimental results.

The photoinjectors of FLASH and the European XFEL at DESY (Hamburg, Germany) are operated by laser driven RF-guns. In both facilities Cs2Te photocathodes are successfully used. In this paper we give an update on the lifetime, quantum efficiency (QE) and dark current of the photocathodes used over the last years. At FLASH cathode #73.3 was operated for a record lifetime of 1413 days and was replaced December 2018 by cathode #10⁵.2. At the European XFEL cathode #680.1 is in operation since December 2015, for 1356 days up to now., Proceedings of the 39th Free Electron Laser Conference, FEL2019, Hamburg, Germany

Harmonic lasing in XFELs is an opportunity to extend the photon energy range of existing and planned X-ray FEL user facilities. Contrary to nonlinear harmonic generation, harmonic lasing can provide much more intense, stable, and narrow-band FEL beam. Another interesting application is Harmonic Lasing Self-Seeding (HLSS) that allows to improve longitudinal coherence and spectral power of a Self-Amplified Spontaneous Emission (SASE) FEL. This concept was successfully tested at FLASH2 in the range of 4.5 - 15 nm and at PAL XFEL at 1 nm. In this contribution we present recent results from the European XFEL where we successfully demonstrated operation of HLSS FEL at 5.9 A, thus pushing harmonic lasing for the first time into the Angstrom regime., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

39th International Free-Electron Laser Conference, FEL19, Hamburg, Germany, 26 Aug 2019 - 30 Aug 2019; JACoW Publishing, Geneva, Switzerland 25-28 (2019). doi:10.18429/JACOW-FEL2019-TUA01, At the European XFEL a hard X-Ray SASE FEL (SA-SE1) and a soft X-Ray SASE FEL (SASE3) share in series the same electron beamline. This configuration couples the operation conditions for both undulators and their subsequent user experiments in terms of SASE in-tensity and background. We report on our experience in parallel operation and discuss the solutions that enable the operation of both undulators as independently as possible., Published by JACoW Publishing, Geneva, Switzerland

39th International Free-Electron Laser Conference, FEL19, Hamburg, Germany, 26 Aug 2019 - 30 Aug 2019; JACoW Publishing, Geneva, Switzerland 599-602 (2019). doi:10.18429/JACOW-FEL2019-THP007, We model and characterize a transverse kick which results from the coaxial RF coupler in the L-band RF gun at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). The RF pulse is typically 600 µs long and used to produce a train of up to 2700 electron bunches. The kick is transient and found to be dependent on the detuning of the resonance frequency of the gun cavity. The frequency detuning within the RF macro-pulse results in a variation in the kick strength along the pulse. This leads to a downstream orbit and size change of individual bunches within the train. Using 3D RF field distributions calculated at detuned frequencies of the cavity, particle tracking simulations are performed to simulate the transient kick onto the bunch train. Given a drift distance, the orbit and size change along a train of fixed length is estimated. Systematic measurements of the kick have meanwhile been carried out. The temperature of the cooling water for the gun is tuned allowing detailed characterization of the frequency detuning within the RF pulse, and thereby measurements of the kick under conditions of practical interest. Experimental findings and simulation results will be presented., Published by JACoW Publishing, Geneva, Switzerland

Beam halo collimation is of great importance for the high repetition rate operation at the European XFEL and for the future CW machines. At the European XFEL several different types of collimators are installed at different locations of the beam line, which include the gun collimators, the bunch compressor collimators, and the main and supplementary collimators in the collimation section. Beam halo measurements have been performed using the wire scanners downstream of the main linac, which show that large part of beam halo is collimated by the gun collimator. Remaining losses in the collimation section are mainly due to misalignment. Alignment using orbit bumps in the collimation section is performed and presented in this paper., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

The European XFEL uses a unique beam distribution scheme to direct electron bunches to its three undulator lines. The accelerator delivers up to 600 microsecond long bunch trains, out of which parts or individual bunches can be selected for photon production in any of the FELs. This contribution gives a brief overview of the kicker-septum scheme facilitating this and highlights how even complex bunch patterns can easily be configured via the timing system., Proceedings of the 39th Free Electron Laser Conference, FEL2019, Hamburg, Germany

North branch of the European XFEL, SASE1, produced first light on May 3rd, 2017, and XFEL operation has been gradually improved then. First characterization of the photon beam has been performed in July / August 2017, just before an official starting date of user experiments (September 1st, 2017). Energy of the electron beam was 14 GeV, bunch charge was 500 pC, photon energy was 9.3 keV. With photon diagnostics available at that time (X-ray gas monitor (XGM) and FEL imager) we measured the gain curve and traced evolution of the FEL radiation mode along the undulator. An important conclusion is that experimental results demonstrate reasonable agreement with baseline parameters. Developed techniques of the photon beam characterization also provided solid base for identification of the problems and means for improving SASE FEL tuning and operation., Proceedings of the 39th Free Electron Laser Conference, FEL2019, Hamburg, Germany

PITZ Collaboration meeting May 2018, Zeuthen, DESY, Germany, 29 May 2018 - 30 May 2018

[Ebook] FEL 2017 : 38th International Free Electron Laser Conference : 20th-25th August, 2017, Santa Fe Convention Center, Santa Fe, NM, USA / editors: Kip Bishofberger (LANL), Bruce Carlsten (LANL), Volker RW Schaa (GSI , [Genf] : [CERN], [2018], 38th International Free Electron Laser Conference, FEL2017, Santa Fe, NM, USA, 20 Aug 2017 - 25 Aug 2017; Genf : JACoW 450-451 (2018). doi:10.18429/JACoW-FEL2017-WEP018, The commissioning of the European XFEL is ongoing. To reduce unwanted longitudinal micro-bunching effects, a laser heater is implemented. Here we present the first heating steps and commissioning of the laser heater at the injector section., Published by JACoW, Genf

8th EPS-QEOD EUROPHOTON CONFERENCE "Solid State, Fibre, and Waveguide Coherent Light Sources", Barcelona, Spain, 2 Sep 2018 - 7 Sep 2018, We present first results of deep ultraviolet spatial flat-top laser beam shaping for photocathode lasers of X-ray Free Electron Lasers using diffractive optical elements including 22 m beam transport. A significantly higher transmission and equal electron beam quality compared with the conventional hard aperture clipping technique was achieved.

The OLYMPUS collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, $R_{2\gamma}$, a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01~GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of $\approx 20\degree$ to $80\degree$. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved GEM and MWPC detectors at $12\degree$, as well as symmetric M{\o}ller/Bhabha calorimeters at $1.29\degree$. A total integrated luminosity of 4.5~fb$^{-1}$ was collected. In the extraction of $R_{2\gamma}$, radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of $R_{2\gamma}$, presented here for a wide range of virtual photon polarization $0.456, Comment: 5 pages, 3 figures, 2 tables

The European XFEL is a 3.4 km long free-electron laser (FEL) which will deliver radiation in the wavelength regime of 0.05 to 4.7 nm. To avoid problems with longitudinal microbunching instabilities a laser heater is implemented. It heats up the electron bunches which will improve the overall brightness level of the FEL. I report the commissioning steps undertaken and the first recorded heating outputs observed in the injector section., Proceedings of the 5th Int. Beam Instrumentation Conf., IBIC2016, Barcelona, Spain

PITZ Collaboration Meeting, DESY, Zeuthen

The Laser Heater of the European XFEL has been installed and commissioning is in progress. We discuss the setup and the results of the first electron beam heating in the injector section., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

7th International Particle Accelerator Conference, IPAC 2016, Busan, Korea, 8 May 2016 - 13 May 2016 ; (2016)., The European XFEL Injector consists of an L-Band RF photoinjector, a TESLA type 1.3 GHz module, a 3rd har-monic RF section, a laser heater and an extensive diag-nostic section to determine projected and slice properties of the beam. The commissioning of the complete system has been started in December 2015 after several years of construction. We will report on commissioning results and perspectives for the later XFEL operation.

4(2016). doi:10.3204/PUBDB-2016-05585, The OLYMPUS collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, $\it R_{2 \gamma}$, a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01 GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of $\approx$ 20° to 80°. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved GEM and MWPC detectors at 12°, as well as symmetric Møller/Bhabha calorimeters at 1:29°. A total integrated luminosity of $4.5 fb^{-1}$ was collected. In the extraction of $\it R_{2\gamma}$, radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of $\it R_{2\gamma}$, presented here for a wide range of virtual photon polarization 0:456 < $\epsilon$< 0:978, are smaller than hadronic two-photon exchange calculations predict, but are consistent with phenomenological models., Published by APS, College Park, Md.

The injector of the European XFEL is in operation since December 2015. It includes, beside the gun and the accelerating section, containing 1.3 and a 3.9 GHz accelerating module, a variety of standard diagnostics systems specially designed for this facility. With very few exceptions, all types of diagnostics systems are installed in the injector. Therefore the operation of the injector is served to validate and prove the diagnostics characteristics for the complete European XFEL. Most of the standard diagnostics has been available for the start of beam operation and showed the evidence of first beam along the beam line. In the following months the diagnostics has been optimized and used for improvements of beam quality. First operational experiences and results from the standard beam diagnostics in the injector of the European XFEL will be reported in this contribution., Proceedings of the 5th Int. Beam Instrumentation Conf., IBIC2016, Barcelona, Spain

The 11th International Workshop on Personal Computers and Particle Accelerator Controls (PCaPAC), PCaPAC 2016, Campinas, Brazil, 25 Oct 2016 - 28 Oct 2016 ; 1 - 3(2016)., PETRA-III is a 3rd generation synchrotron light source dedicated to users at 14 beam lines with 30 instruments in the Max von Laue Hall and 10 beamlines in newly constructed Extensions in North and East which are under installations. PETRA-III is operated with several filling modes such as 60, 480 and 960 bunches with 100 mA or 40 bunches with 90 mA at an electron beam energy of 6 GeV. Transverse orbits are corrected to a reference orbit, which is based on BBA measurements taking into account requirements of the beamlines and results of the dispersion correction. Histograms from 244 BPMs are displayed by means of this Java Client OrbitKorrektur. The orbits may be corrected using the Effective Correction Method with a few correctors or can be corrected using the SVD Method with proper singular Eigen values using theoretical response matrices from an Optic Server of used Optics. In this application additional features are implemented for better observation and analysis of orbits. Furthermore it can also be used to do many additional jobs, such as showing corrector set currents, loading golden and reference orbits, local bumps, displaying the first turn data during commissioning etc.

3 pp. (2004)., During the HERA luminosity upgrade the new super conducting mini beta quadrupoles have been placed inside the experiments for final focussing and separation of the lepton and proton beams. The synchrotron radiation of about 12 kW produced in these magnets passes through the detector and is absorbed behind the experiments. In order to avoid background events from synchrotron radiation it is a mandatory to adjust precisely the beam trajectory before and inside the detector. A procedure has been developed to calculate the trajectory in the interaction regions. With a beam-based alignment (bba) the offsets of the beam with respect to the quadrupoles is measured. From this measurement the offsets of the quadrupoles and of the beam position monitors are fitted. With the knowledge of these offsets the trajectory of the beam is calculated with high precision. The display of the trajectory is online available as an operational tool for beam steering and background optimization.

The 8th European Particle Accelerator Conference, EPAC2002, Paris, France, 3 Jun 2002 - 7 Jun 2002; 365-367 (2002)., In order to maximize the achievable luminosity of HERA the final focusing triplets of the lepton beam are installed close to the interaction point (IP). The dipole component of the magnets produces the necessary separation angle between the lepton and the proton beam. The synchrotron radiation generated in the magnets has to be passed through the detector beam pipe with extremely small losses for background reasons. Shape and power of the synchrotron radiation fan depend critically on the precise alignment of those magnets. We present measurements aimed at the determination of the beam vs. magnet offsets, as well as the identification of magnet position errors. We discuss systematic and random errors and difficulties that arise from the non-zero design offsets of the magnets. In order to increase the reliability and the speed of the measurements,automated procedures were developed and incorporated into the HERA control system.

DORIS ist konzipiert worden als Speicherring für Hochenergiephysik Experimente und wird seit 1973 betrieben. Erst 1993 wurde DORIS dedizierte Synchrotronstrahlungsquelle, ohne das die Magnetoptik der Maschine auf die speziellen Belange dieser Nutzerklientel ausgerichtet werden konnte. Es gibt nun Überlegungen, wie sich die Qualität der Synchrotronstrahlung deutlich verbessern lässt. Um den Aufwand gering zu halten, soll die HF-Gerade komplett erhalten bleiben. Ebenso sollen die Wiggler Positionen unverändert bleiben, um einen Umbau der HASYLAB Beamlines zu vermeiden.

Recently three RF guns were prepared at the Photo Injector Test Facility at DESY, location Zeuthen (PITZ) for their subsequent operation at FLASH and the European XFEL. The gun 3.1 is a previous cavity design and is currently installed and operated at FLASH, the other two guns 4.3 and 4.4 were of the current cavity design and are dedicated to serve for the start-up of the European XFEL photo-injector. All three cavities had been dry-ice-cleaned prior their conditioning and hence showed low dark current levels. The lowest dark current level – as low as 60μA at 65MV/m field amplitude – has been observed for the gun 3.1. This paper reports in details about the conditioning process of the most recent gun 4.4. It informs about experience gained at PITZ during establishing of the RF conditioning procedure and provides a comparison with the other gun cavities in terms of the dark currents. It also summarizes the major setup upgrades, which have affected the conditioning processes of the cavities., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany