Search

The FLASH2020+ project has started to transform the FLASH facility to broaden the facility profile and meet demands of future user experiments. In a nine-month lasting shutdown until August 2022 the linear accelerator of the FLASH facility has, among others, been upgraded with a laser heater, new bunch compressors and new modules. The latter results in an energy upgrade to 1.35 GeV allowing to reach sub 4 nm wavelength. In the following 14-month lasting shutdown starting mid 2024 the FLASH1 FEL beamline will be completely rebuild. The design is based on external seeding at MHz repetition rate in burst mode allowing for coherent tuneable FEL radiation in wavelength and polarization by installation new APPLE-III undulators. Post compression of the beam downstream of the radiators will allow for high quality THz generation and together with the new experimental endstations and pump probe lasers provide a unique portfolio for next generation user experiments.

FLASH2020+ is an upgrade project for the FLASH facility at Hamburg. A main goal of the project is to generate fully coherent soft X-ray FEL radiation at a high repetition rate (MHz). The project will utilize two external laser seeding principles in order to produced Seeded FEL with tunable wavelength from 4-60 nm. In order to achieve this goal, both HGHG (High Gain Harmonic Generation) and EEHG (Echo-Enhanced Harmonic Generation) methods provide FEL emission at harmonics of a seed laser. For HGHG, a tunable UV laser system (297-317 nm) and for EEHG a combination of the tunable UV laser and fixed wavelength (343 nm) laser system would be used to cover the whole range of wavelengths between 4-60 nm. In this contribution, we will describe the requirements of the seed laser to initiate the seeding process and will explain the concept of seed laser beam transport and incoupling into the modulators for FEL radiation production. The first seed laser (Seed1) with fixed wavelength is transported about 28 meters from laser lab to the incoupling chicane. The second seed laser (Seed2) with a tunable UV wavelength is transported about 35 meters. Our concept uses a full relay imaging system and in vacuum components for the laser transport in addition to high repetition rate diagnostics to deliver, monitor and control the beam and pulse parameters at the interaction with electron beam. We investigate the technical and engineering limitations for the design and address those challenges to provide the demanding seed laser parameters for generating high repetition rate seeded FEL.

A tunable and multicolor light source with near Fourier-limited pulses, controlled delay, and fully coherent beam with precisely adjustable phase profiles enables state-of-the-art measurements and studies of femtosecond dynamic processes with high elemental sensitivity and contrast. The start-to-end simulations efforts aim to take advantage of the available global pool of software and past and present extensive efforts to provide realistic simulations, particularly for cases where precise and fine manipulation of the beam phase space is concerned. Since, for such cases, tracking of beams with billions of particles through magnetic structures and handover between multiple codes are required, extensive realistic studies for such cases are limited. Here we will describe a workflow that reduces the needed computational resources and share studies of the EEHG seed section for the FLASH2020+ [1] project.

The free-electron laser FLASH at DESY can produce SASE-FEL pulses in the extreme ultraviolet to the soft X-ray region. The flexibility of the variable gap undulators in the FLASH2 beamline opens a wide range of scientific opportunities. Different advanced lasing schemes have been tested in the past years, like the ``frequency doubler`` scheme, two-color lasing, and ``harmonic lasing self-seeded FEL (HLSS)``. A recent user experiment required parameters not yet provided: a similar power in the fundamental and the third harmonic. To fulfill these requirements, a new way of lasing had to be developed ad hoc. A combination of HLSS and two-color lasing has been identified as the appropriated scheme to deliver a tailored two-color beam to the user experiment. In this article we describe difficulties of the setup and discuss the results achieved.

In January 2021 two X-band (12 GHz) PolariX Transverse Deflecting Structures with variable streak polarization were installed into the FLASH2 beamline at FLASH. Since none of the RF components for the FLASH2-PolariX RF-distribution system nor the two PolariX structures could be pre-conditioned, RF-conditioning was and is quite tedious. Nevertheless, after 6 weeks of conditioning, we have already been able to streak the electron beam enough to start commissioning of the PolariX controls and the software. After 4 months of conditioning in parallel to FLASH2 user operation, we achieved a stable 5.5 MW flat top of 400 ns operation. Next step will be to include RF pulse compression to achieve the design power of 22 MW. Since then operational experience with the PolariX system has continuously evolved and it has quickly become a valuable if not indispensable tool for tuning FLASH2. Even with the reduced power, a measurement resolution of 12 fs could be reached. In this article we describe key aspects of the conditioning and commissioning process as well as the first experiences and first results of beam measurements both for SASE tuning and for dedicated micro-bunching studies.

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.

In the framework of the FLASH2020+ project, the FLASH1 beamline will be upgraded to deliver seeded FEL pulses for users. This upgrade will be achieved by combining high gain harmonic generation and echo-enabled harmonic generation with a wide-range wavelength-tunable seed laser, to efficiently cover the 60-4 nm wavelength range. The undulator chain will also be refurbished entirely using new radiators based on the APPLE-III design, allowing for polarization control of the generated light beams. With the superconducting linac of FLASH delivering electron beams at MHz repetition rate in burst mode, laser systems are being developed to seed at full repetition rates. In the contribution, we will report about the progress of the project.

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.

The FLASH facility houses a superconducting linac powering two FEL beamlines with MHz repetition rate in 10 Hz bursts. Within the FLASH2020+ project, which is taking care of facility development, one major aspect is the transformation of one of the two FEL beam lines to deliver externally seeded fully coherent FEL pulses to photon user experiments. At the same time the second beam line will use the SASE principle to provide photon pulses of different properties to users. Since the electron beam phase space conducive for SASE or seeded operation is drastically different, here a proof-of-principle experiment using the existing experimental seeding hardware has been performed demonstrating the possibility of simultaneous operation. In this contribution we will describe the setup of the experiment and accelerator, and discuss the chances and limitations of the experimental seeding hardware. Finally, we will discuss the results and their implications also for the FLASH2020+ project.

The free-electron laser FLASH at DESY can produce SASE-FEL pulses in the extreme ultraviolet to the soft X-ray regime. A superconducting linear accelerator drives two undulator lines (FLASH1 and FLASH2). The FLASH1 undulator beam line contains six fixed gap undulator which implies that the SASE wavelength can only be changed via the electron beam energy, while FLASH2 contains twelve variable gap undulators. Preparing different charges and compression schemes to the two parts of the bunch trains for the two undulator beamlines allows to adjust the phase space in wide range and meeting the various requirements of photon pulse trains properties. In order to improve the SASE performance reference files for standard energies and standard charges are regularly prepared. In the FLASH2 undulator beamline beam-based alignment and phase shifter scans have been applied to improve SASE operations and FEL beam quality. Improving set-up and tuning procedures allow to decrease setup times and optimize performance and stability. Procedures and optimization of FEL parameters towards a reliable SASE-FEL operation as well as the achieved results are discussed.

FLASH, the free-electron laser user facility at DESY, delivers XUV and soft X-ray radiation for photon experiments since 2005. It is driven by a superconducting linear accelerator, and has two undulator lines (FLASH1 and FLASH2). A third electron beam line hosts the plasma wakefield experiment FLASHForward. Presently, the FLASH facility is undergoing an extensive refurbishment and a substantial upgrade (FLASH2020+). In this paper we summarize the FLASH operation in 2019 - 2021, and report on the main upgrades realized in a long installation shutdown from November 2021 to summer 2022.

Scientific reports 11(1), 3560 (2021). doi:10.1038/s41598-021-82687-2, The PolariX TDS (Polarizable X‑Band Transverse Deflection Structure) is an innovative TDS‑design operating in the X‑band frequency‑range. The design gives full control of the streaking plane, which can be tuned in order to characterize the projections of the beam distribution onto arbitrary transverse axes. This novel feature opens up new opportunities for detailed characterization of the electron beam. In this paper we present first measurements of the Polarix TDS at the FLASHForward beamline at DESY, including three‑dimensional reconstruction of the charge‑density distribution of the bunch and slice emittance measurements in both transverse directions. The experimental results open the path toward novel and more extensive beam characterization in the direction of multi‑dimensional‑beam‑phase‑space reconstruction., Published by Macmillan Publishers Limited, part of Springer Nature, [London]

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 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil, 24 May 2021 - 28 May 2021; JACoW Publishing, Geneva, Switzerland 4 pp. (2021). doi:10.18429/JACOW-IPAC2021-TUPAB115, The free-electron laser in Hamburg (FLASH) is a high brilliance XUV and soft X-ray SASE FEL user-facility at DESY. FLASH���s superconducting linac can accelerate several thousand electron bunches per second in 10 Hz bursts of up to 800 ��s length. The long bunch trains can be split in two parts and shared between two undulator beamlines. During 2020, FLASH supplied, in standard operation, up to 500 bunches at 10 Hz in two bunch trains with independent fill patterns and compression schemes. The FLASH2 undulator beamline comprises variable gap undulators that allow different novel lasing schemes. A third beamline accommodates the FLASHForward plasma wakefield acceleration experiment. We report on the FLASH operation in 2019 - 2021 and present a few highlights., Published by JACoW Publishing, Geneva, Switzerland

Applied Sciences 11(20), 9729 (2021). doi:10.3390/app11209729 special issue: "Latest Trends in Free Electron Lasers", The successful realization of high gain free-electron lasers has opened new possibilities to X-ray scientists for investigating matter in different states. The availability of unprecedented photon properties stimulated the development of new experimental techniques capable of taking full advantage of these options and has started a virtuous collaboration between machine experts and photon users to improve further and optimize the generated X-ray pulses. Over the recent years, this has led to the development of several advanced free-electron laser (FEL) schemes to tailor the photon properties to specific experimental needs. Nowadays, tunable wavelength X-ray pulses with extremely high brilliance and short pulse characteristics are a few of the many options available at FELs. Few facilities can offer options such as narrowband or extremely short pulses below one~fs duration and simultaneous pulses of multiple colors enabling resonant X-ray pump - X-ray probe experiments with sub fs resolution. Fully coherent X-ray radiation (both spatial and temporal) can also be provided and has stimulated the application of coherent control techniques to the X-ray world, allowing for experiments with few attoseconds resolution. FELs often operate at a relatively low repetition rate, typically on the order of tens of Hz. At FLASH and the European XFEL, however, the superconducting accelerators allow generating thousands of pulses per second.With the implementation of a new seeded FEL line and with an upgrade at FLASH linac, all the new features will become available in the soft-X-ray spectral range down to the oxygen K edge with unprecedented average photon flux due to the high repetition rate of pulses., Published by MDPI, Basel

Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil 12th International Particle Accelerator Conference, IPAC���21, Campinas,SP, Brazil, 24 May 2021 - 28 May 2021; 4 pp. (2021). doi:10.18429/JACOW-IPAC2021-TUPAB086, With FLASH2020+, a major upgrade of the FLASH facility has started to meet the new requirements of the growing soft-x ray user community. The design of the FEL beamlines aims at photon properties suitable to the needs of future user experiments with high repetition rate XUV and soft X-ray radiation. By the end of the project, both existing FEL lines at FLASH will be equipped with fully tunable undulators capable of delivering photon pulses with variable polarization. The use of the external seeding at 1 MHz in burst mode is part of the design of the new FLASH1 beamline, while FLASH2 will exploit novel lasing concepts based on different undulator configurations. The new FLASH2020+ will rely on an electron beam energy of 1.35 GeV that will extend the accessible wavelength range to the oxygen K-edge with variable polarization. The facility will be completed with new laser sources for pump and probe experiment and new experimental stations.

Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil 12th International Particle Accelerator Conference, IPAC'21, Campinas, SP, Brazil, 24 May 2021 - 28 May 2021; [Geneva] : JACoW Publishing, Geneva, Switzerland 4 pp. (2021). doi:10.18429/JACOW-IPAC2021-TUPAB104, FLASH2 is one of the two SASE (Self-Amplified Spontaneous Emission) undulator beamlines lines comprising variable gap undulators to produce radiation in the XUV and soft X-ray regime at FLASH. Downstream of the SASE undulators the beamline is currently undergoing a major redesign. During shutdowns in summer 2020 and winter 2021 two PolariX TDSs (Polarizable X-band Transverse Deflecting Structure) were installed, as well as additional diagnostics, to monitor the longitudinal phase space density of the electron bunches. Additionally, an afterburner undulator will be integrated in the next shutdown to produce circularly polarized light with wavelengths down to 1.39 nm. In this paper, we will present the modifications that were and will be made to the electron beamline in the course of this redesign., Published by JACoW Publishing, Geneva, Switzerland, [Geneva]

PITZ Collaboration Meeting November 2021, Zeuthen, DESY, Germany, 23 Nov 2021 - 24 Nov 2021

An accurate transmission measurement of thin foils (usually made of elemental metals and/or semiconductors), which routinely are used as attenuators in soft x-ray beamlines, end-stations and instruments, represents a long standing problem over the wide experimentation field with photon beams, see for example [1-4]. Such foils are also frequently utilized for blocking long wavelength emission, i.e., UV-Vis-IR radiation, from plasma and high order harmonic sources, whilst soft x-rays emitted from the source pass through the foil with only a slight attenuation. Despite the enormous amount of data available in the literature, e.g., Henke’s tables [5], measurements made on real foils often provide surprising results. In this study, a procedure based on the ablation imprints method is utilized for determination of soft x-ray filter transmission, namely the f-scan technique [6,7]. This technique combines the GMD (Gas Monitor Detector) pulse energy measurement and attenuation of the beam by foils (made of different metallic/semiconducting elements of varying thickness) with areas of ablation imprints created on a suitable target, e.g. PMMA – Poly(methyl methacrylate). The results show only a partial overlap with transmission values found in Henke’s tables. Nevertheless, a good agreement with transmission values determined by conventional radiometry techniques at synchrotron radiation beamlines has been found. Such a difference between the experimentally obtained values and transmissions calculated for a pure element is usually explained by spontaneous formation of oxidized layers on the filter surface and in the near-surface layer and their possible alteration by intense FEL radiation. The first results obtained with Al, Nb, Zr and Si filters at FLASH/FLASH2 (Free-electron LASer in Hamburg tuned to 13.5 nm) facilities will be shown and discussed in this presentation.

39th International Free-Electron Laser Conference, FEL19, Hamburg, Germany, 26 Aug 2019 - 30 Aug 2019; 0-3 (2019).

Free-electron laser simulation codes employ either the Slowly-Varying Envelope Approximation (SVEA) or a Particle-in-Cell (PiC) formulation. Maxwell’s equations are averaged over the fast time scale in the SVEA so that there is no need to resolve the wave period. In contrast, the fast oscillation is retained in PiC codes. As a result, the SVEA codes are much less computationally intensive and are used more frequently than PiC codes. While the orbit dynamics in PiC codes and some SVEA Codes (MEDUSA and MINERVA) use the full unaveraged Lorentz force equations, some SVEA codes use the Kroll-Morton-Rosenbluth (KMR) approximation (GENESIS, GINGER, FAST, and TDA3D). Steady-state simulation comparisons [1] have appeared in the literature between different codes using the averaged and unaveraged particle dynamics. Recently, a comparison between three KMR SVEA codes (GENESIS, GINGER, and FAST) and the PUFFIN PiC code in the time-dependent regime has been reported [2]. In this paper, we present a comparison between the unaveraged PiC code PUFFIN, the unaveraged SVEA code MINERVA for the time-dependent simulation of SASE free-electron lasers with the experimental measurements from SPARC SASE FEL at ENEA Frascati., Proceedings of the 39th Free Electron Laser Conference, FEL2019, Hamburg, Germany

FLASH, the Free-Electron Laser at DESY in Hamburg was the first FEL user facility in the XUV and soft X-ray range. The superconducting RF technology allows to produce several thousand SASE pulses per second with a high peak and average brilliance. It developed to a user facility with a 1.25 GeV linear accelerator, two undulator beamlines running in parallel, and a third electron beamline containing the FLASHForward plasma wakefield experiment. Actual user operation and FEL research are discussed. New concepts and a redesign of the facility are developed to ensure that also in future FLASH will allow cutting-edge research. Upgrade plans are discussed in the contribution., Proceedings of the 39th Free Electron Laser Conference, FEL2019, Hamburg, Germany

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

FLASH, the free electron laser user facility at DESY (Hamburg, Germany), delivers high brilliance XUV and soft x-ray FEL radiation to photon experiments with different parameters at two undulator beamlines simultaneously. FLASH’s superconducting linac can produce bunch trains of up to 800 bunches within a 0.8 ms RF flat top at a repetition rate of 10 Hz. In standard operation during 2018, FLASH supplied up to 500 bunches in two bunch trains with independent fill patterns and compression schemes to each of the two beamlines. In 2018 first successful plasma accelerating experiments could be reported by the FLASHForward plasma wakefield acceleration experiment situated in a third beamline. We report on the highlights of FLASH operation in 2018/2019., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

FLASH, the free-electron laser (FEL) at DESY (Hamburg) started user operation in summer 2005. It delivers high peak and average brilliance XUV and soft X-ray FEL radiation to photon experiments. Nowadays, FLASH has a 1.25 GeV superconducting linac, and two undulator beamlines, which are operated simultaneously. This paper provides an overview of its evolution from a test facility for superconducting accelerator technology to a full-scale FEL user facility., Proceedings of the 39th Free Electron Laser Conference, FEL2019, Hamburg, Germany

39th International Free-Electron Laser Conference, FEL19, Hamburg, Germany, 26 Aug 2019 - 30 Aug 2019; JACoW Publishing, Geneva, Switzerland 430-433 (2019). doi:10.18429/JACOW-FEL2019-WEP048, The free-electron laser facility FLASH at DESY (Hamburg, Germany) operates two undulator beamlines simultaneously for FEL operation and a third for plasma acceleration experiments (FLASHForward). The L-band superconducting technology allows accelerating fields of up to 0.8 ms in length at a repetition rate of 10 Hz (burst mode). A fast kicker-septum system picks one part of the 1 MHz electron bunch train and kicks it to the second beamline such that two beamlines are operated simultaneously with the full repetition rate of 10 Hz. The photoinjector operates three laser systems. They have different pulse durations and transverse shapes and are chosen to serve best for the given user experiment in terms of electron bunch charge, bunch compression, and bunch pattern. It is also possible to operate the laser systems on the same beamline to provide specific double pulses for certain type of experiments., Published by JACoW Publishing, Geneva, Switzerland

10th International Particle Accelerator Conference, IPAC'19, Melbourne, Australia, 19 May 2019 - 24 May 2019; JACoW Publishing, Geneva, Switzerland 1736 - 1739 (2019). doi:10.18429/JACOW-IPAC2019-TUPRB024, FLASH is the first soft X-ray FEL user facility, routinely providing brilliant photon beams for users since 2005. There are plans to upgrade both existing undulator lines of this facility, FLASH1 and FLASH2. FLASH1 will mainly operate in XUV range in seeding and SASE modes, while FLASH2 will use the standard SASE regime as well as new lasing concepts aiming at production of brilliant photon beams on the fundamental and harmonics down to 1nm. In this paper we present a concept for FLASH2 upgrade, and discuss different advanced options., Published by JACoW Publishing, Geneva, Switzerland

10th International Particle Accelerator Conference, IPAC'19, Melbourne, Australia, 24 May 2019 - 24 May 2019; Hamburg 1909-1911 (2019)., FLASH, the free electron laser user facility at DESY (Hamburg, Germany), delivers high brilliance XUV and soft X-ray FEL radiation to photon experiments with different parameters at two undulator beamlines simultaneously. FLASH’s superconducting linac can produce bunch trains of up to 800 bunches within a 0.8ms RF flat top at a repetition rate of 10Hz. In standard operation during 2018, FLASH supplied up to 500 bunches in two bunch trains with independent fill patterns and compression schemes to each of the two beamlines. In 2018 first successful plasma accelerating experiments could be reported by the FLASHForward plasma wakefield acceleration experiment situated in a third beamline. We report on the highlights of FLASH operation in 2018/2019., Published by Hamburg

FLASH is a unique superconducting soft X-ray FEL capable of producing up to 8000 photon pulses per second. A substantial upgrade is planned to keep FLASH attractive and competitive. Several upgrade scenarios are discussed., 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 396-399 (2019). doi:10.18429/JACOW-FEL2019-WEP036, A collaboration between DESY, PSI and CERN has been established to develop and build an advanced modular X- band transverse deflection structure (TDS) system with the new feature of providing variable polarization of the deflecting force. The prototype of the novel X-band TDS, the Polarizable X-band (PolariX) TDS, was fabricated at PSI following the high-precision tuning-free production process developed for the C-band Linac of the SwissFEL project. Bead-pull RF measurements were also performed at PSI to verify, in particular, that the polarization of the dipole fields does not have any rotation along the structure. The high-power test was performed at CERN and now the TDS is at DESY and has been installed in FLASHForward, where the first streaking experience with beam will be accomplished. We summarize in this paper the status of the project, the results of the bead-pull measurements and the high power test., Published by JACoW Publishing, Geneva, Switzerland

FLASH, the free-electron laser (FEL) user facility at DESY, has delivered high brilliance VUV and soft x-ray FEL radiation for photon experiments since summer 2005. In 2014 and 2015 a second beamline, FLASH2, has been commissioned in parallel to user operation at FLASH1. FLASH's superconducting linac can produce bunch trains of up to 800 bunches within a 0.8 ms RF flat top at a repetition rate of 10 Hz. In standard operation during 2017 FLASH supplied up to 500 bunches in two bunch trains with independent fill patterns and compression schemes. Since mid 2017 initial commissioning of a third experimental beamline, accommodating the FLASHForward plasma wakefield acceleration experiment, has started. We report on the highlights of the FLASH operation in 2017/2018., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

9th International Particle Accelerator Conference, IPAC'18, Vancouver, Canada, 29 Apr 2018 - 4 May 2018; Geneva, Switzerland : JACoW Publishing 3808-3811 (2018). doi:10.18429/JACOW-IPAC2018-THPAL068, A collaboration between DESY, PSI and CERN has been established to develop and build an advanced modular X-band transverse deflection structure (TDS) system with the new feature of providing variable polarization of the deflecting force. This innovative CERN design requires very high manufacturing precision to guarantee highest azimuthal symmetry of the structure to avoid the deterioration of the polarization of the streaking field. Therefore, the high-precision tuning-free production process developed at PSI for the C-band and X-band accelerating structures will be used for the manufacturing. We summarize in this paper the status of the production of the prototype and the waveguide networks foreseen in the different facilities., Published by JACoW Publishing, Geneva, Switzerland

29th Linear Accelerator Conference, Beijing, China, 11 Sep 2018 - 21 Sep 2018; JACoW 3 pp. (2018)., Linear accelerator facilities driving a free-electron laser require femtosecond precision synchronization between external laser systems and the electron beam. Such high precision is required for pump-probe experiments and also for example for the electron bunch injection into a plasma bubble for laser plasma acceleration. An upgrade of the fast intra-train beam-based feedback system is planned at the Free-Electron Laser FLASH in Hamburg, Germany. This linear accelerator is based on superconducting (SRF) technology operating with pulse trains of maximum 1 MHz bunch repetition rate. Arrival time fluctuations of the electron beam are correctable by introducing small energy modulations prior to the magnetic bunch compressor. This contribution focuses on the design and the characterization of a normal-conducting RF (NRF) cavity with large bandwidth, mandatory to correct fast arrival time fluctuations. The cavity has recently been installed in the FLASH beamline. First measurements with the new cavity will be presented., Published by JACoW

International Particle Accelerator Conference 2018, IPAC18, Vancouver, Kanada, 29 Apr 2018 - 4 May 2018

Photondiag 2017, ; Journal of synchrotron radiation 25, 39-43 (2018). doi:10.1107/S1600577517015442 special issue: "Photondiag2017 workshop", FLASH, the X-ray free-electron laser in Hamburg, Germany, employs a narrowband high-field accelerator THz source for unique THz pump X-ray probe experiments. However, the large difference in optical paths of the THz and X-ray beamlines prevents utilization of the machine’s full potential (e.g. extreme pulse energies in the soft X-ray range). To solve this issue, lasing of double electron bunches, separated by 28 periods of the driving radiofrequency (at 1.3 GHz), timed for the temporal overlap of THz and X-ray pulses at the experimental station has been employed. In order to optimize conditions for a typical THz pump X-ray probe experiment, X-ray lasing of the first bunch to one-sixth of that of the second has been suppressed. Finally, synchronization of THz radiation pulses was measured to be ~20 fs (r.m.s.), and a solution for monitoring the arrival time for achieving higher temporal resolution ispresented.

[Ebook] FLS2018 : the 60th ICFA Advanced Beam Dynamics Workshop : March 5-9, 2018, Shanghai Institute of Applied Physics / Ho Chin, Yong , [Genf] : [CERN], June 2018, 60th ICFA Advanced Beam Dynamics Workshop on Future Light Sources, FLS2018, Shanghai, China, 5 Mar 2018 - 9 Mar 2018; Geneva, Switzerland : JACoW 68-73 (2018)., Harmonic lasing is a perspective mode of operation of X-ray FEL user facilities that allows to provide brilliant beamsof higher energy photons for user experiments. Another use-ful application of harmonic lasing is so called HarmonicLasing Self-Seeded Free Electron Laser (HLSS FEL) thatallows to improve spectral brightness of these facilities. Inthe past, harmonic lasing has been demonstrated in the FELoscillators in infrared and visible wavelength ranges, but notin high-gain FELs and not at short wavelengths. In this pa-per we report on the first evidence of the harmonic lasingand the first operation of the HLSS FEL at the soft X-rayFEL user facility FLASH in the wavelength range between4.5 nm and 15 nm., Published by JACoW, Geneva, Switzerland

The optimization of photoinjectors is crucial for the successful operation of linac-based free electron lasers, and beam dynamics simulations have shown that ellipsoidal photocathode laser pulses result in significantly lower electron beam emittance than that of conventional cylindrical pulses. Therefore, in collaboration with the Institute of Applied Physics (Nizhny Novgorod, Russia) and the Joint Institute of Nuclear Research (Dubna, Russia), a laser system capable of generating quasi-ellipsoidal laser pulses has been developed and installed at the Photo Injector Test facility at DESY, Zeuthen site (PITZ). The pulse shaping has been realized using the spatial light modulator technique, characterized by cross-correlation and spectrographic measurements, and is demonstrated with electron beam measurements. In this contribution the overall setup, operating principles, and results of first regular electron beam measurements will be presented together with corresponding beam dynamics simulations. Furthermore, the numerous improvements of the simplified re-design currently under construction shall be detailed., Proceedings of the 38th Int. Free Electron Laser Conf., FEL2017, Santa Fe, NM, USA

9th International Particle Accelerator Conference, IPAC'18, Vancouver, BC, Canada, 29 Apr 2018 - 4 May 2018; Geneva, Switzerland : JACoW 4 pp. (2018)., Recently FLASH (Free electron LASer in Hamburg) at DESY has been granted funding for a refurbishment project covering among others the replacement of two old SRF modules, an upgrade of the injector lasers and an upgrade of parts of the electron beam diagnostics.In addition we are proposing several possible upgrades and new features for the injector and the drive linac as well as in the undulator beamlines.Here we present options which are in our opinion technically feasible and at the same time operationally manageable., Published by JACoW, Geneva, Switzerland

Journal of synchrotron radiation 25(1), 39 - 43 (2018). doi:10.1107/S1600577517015442, FLASH, the X-ray free-electron laser in Hamburg, Germany, employs anarrowband high-field accelerator THz source for unique THz pump X-rayprobe experiments. However, the large difference in optical paths of the THzand X-ray beamlines prevents utilization of the machine’s full potential (e.g.extreme pulse energies in the soft X-ray range). To solve this issue, lasing ofdouble electron bunches, separated by 28 periods of the driving radiofrequency(at 1.3 GHz), timed for the temporal overlap of THz and X-ray pulses at theexperimental station has been employed. In order to optimize conditions for atypical THz pump X-ray probe experiment, X-ray lasing of the first bunch toone-sixth of that of the second has been suppressed. Finally, synchronization ofTHz radiation pulses was measured to be $\approx 20$ fs (r.m.s.), and a solution formonitoring the arrival time for achieving higher temporal resolution ispresented., Published by IUCr, Chester