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The 2nd stage of the FLASH2020+ project at DESY will be an upgrade of the FLASH1 beamline to enable HGHG and EEHG seeding with two modulator-chicane stages, and a radiator section with 11 Apple-III undulators to enable FEL radiation with controllable polarization. A key feature of FLASH, namely the capability of providing several thousand FEL pulses in the extreme UV and soft X-ray must not be compromised. Downstream of the radiator the beamline houses longitudinal diagnostics, a double bend (quasi-) achromat to separate the electrons from the photons and divert the electron beamline from the photon diagnostics, a post-compressor, a THz-Undulator (requires an electron beam that is compressed more strongly than for seeding), and finally the dumpline, capable of safely aborting up to 100 kW electron beam power. This article describes the conceptional and some technical details of the beamline with emphasis on the upstream part (modulators and radiator) designed for seeding.

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 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.

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.

The XUV and soft X-ray free-electron laser FLASH at DESY is capable of operating two undulator beamlines simultaneously with up to several thousand bunches per second. It is driven by a normal conducting RF photo-cathode gun and a superconducting L-band linac. FLASH is currently undergoing a substantial refurbishment and upgrade program (FLASH2020+). The first 9-months installation shutdown started in November 2021. Here we report on the operation in 2021 and present main upgrades during the ongoing shutdown., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand 13th Internation Particle Accelerator Conference, IPAC'22, Bangkok, Thailand, 12 Jun 2022 - 17 Jun 2022; [Geneva] : JACoW Publishing, Geneva, Switzerland 1010-1013 (2022). doi:10.18429/JACOW-IPAC2022-TUPOPT006, The 2nd stage of the FLASH2020+ project will be an upgrade of the FLASH1 beamline, downstream of the injector/linac section FLAH0 which is currently being upgraded. The currently existing beamline drives the original planar fixed gap SASE undulators from the TTF-2 setup, a THz undulator that uses the spent electron beam and deflects the e-beam into a dump beamline capable of safely dumping several thousand bunches per second. The updated beamline has been designed for EEHG seeding with 2 modulators, 3 chicanes, and a helical Apple-III undulator beamline as seeding radiator, followed by a transverse deflecting (S-band) structure for longitudinal diagnostics. The separation of the electron beam from the FEL beam will be moved upstream w.r.t. the old design to create more space for the photon diagnostics and will be achieved by a 5 deg double-bend-almost-achromat. To allow enable high power THz radiation output from a moderately compressed seeding beam, a post compressor will be installed. The capability of dumping the the long bunch trains safely may and will not be compromised by the design. This article describes the conceptional and some technical and details of the beamline., Published by JACoW Publishing, Geneva, Switzerland, [Geneva]

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]

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; JACoW Publishing, Geneva, Switzerland 4 pp. (2021). doi:10.18429/JACOW-IPAC2021-TUPAB102, The first stage of the FLASH2020+ project is an upgrade of the FLASH injector beamline. Within this framework, the 2nd bunch compression chicane (BCC) will be completely redesigned. The old S-chicane will be replaced with a new C-chicane which is 3.5m shorter thereby generating space a new section for re-matching the beam from the injector into the linac. The new BCC will be equipped with quad/skew-quad units in both legs of the chicane to compensate correlations of the transverse degrees of freedom with the longitudinal ones. Since quadrupoles tend to have a circular bore, the chicane is designed with movable round vacuum chambers and movable dipoles for maintaining full flexibility in choosing the compression parameters. This article describes the technical details and introduces a thin-lens model of BCCs which allows analytical estimates on the effects of powering the quad/skew-quad units on optics parameters as well as estimates on the required strengths of these magnets in order to remove correlations of the magnitudes typically observed at FLASH., 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 238-241 (2019). doi:10.18429/JACOW-FEL2019-TUP078, Seeded FELs enable the generation of fully coherent, transform-limited and high brightness FEL pulses, as the start-up process is driven by an external coherent light pulse. During the design process of such FELs, it is important to choose carefully the electron beam parameters to guarantee high performance. One of those parameters is the electron beam energy chirp. In this contribution, we show simulation results and we discuss how the electron beam energy chirp affects the final spectrum., 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

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

10th International Particle Accelerator Conference, IPAC'19, Melbourne, Australia, 19 May 2019 - 24 May 2019; JACoW Publishing, Geneva, Switzerland 3267-3270 (2019). doi:10.18429/JACOW-IPAC2019-WEPTS061, 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., Published by JACoW Publishing, Geneva, Switzerland

IPAC 2019, Melbourne, Australia, 19 May 2019 - 24 May 2019; 4 pp. (2019)., The proposed FLASH upgrade will rely on high quality electron beams provided to all undulator beam lines. Here we describe possible modifications to the FLASH lattice and the compression scheme that aim at improving the beam quality and the ability to control critical beam properties along the machine - simultaneously and independently for all beam lines.

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

This work presents an pump-probe setup for measuring the thermal evolution of diamond crystals at cryogenic temperatures under the heat load conditions of an X-ray free electron laser oscillator (XFELO). As the diamond Bragg reflectors of an XFELO are subjected to intense heat loads during operation, the correct understanding of the thermal evolution in diamond plays a major role in the correct modeling of an XFELO. Stoupin et al.* did a room temperature x-ray diffraction measurement on the nanosecond transient thermal response of diamond to an optical pulse. The measurements presented in this paper for the first time incorporate effects due to the very short penetration depth of only a few μm of an XFELO pulse in combination with the high mean free path in diamond at cryogenic temperatures. While at room temperature the heat equation based on Fourier's law accurately fits the measured results, this vastly changes due to the onset of ballistic processes at cryogenic temperatures. These changes, which are hard to predict theoretically, show the necessity of measurements of the thermal evolution in diamond with special regard to a correct mimicking of the heat load in an XFELO., 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

Free-electron laser(FEL) x-ray radiation can deliver pulses with a huge amount of energy in short time duration. X-ray optics like Bragg reflectors therefore must be chosen in a way that they can withstand radiation-material interaction without getting damaged so that they can maintain their technical functionality. Therefore thermal and mechanical reactions of Bragg reflectors to the radiation induced thermal strain and force (radiation pressure) have been considered in this study. The theory of thermoelasticity has been used to simulate the strain conditions at saturation of the amplifying process in an X-ray free-electron laser oscillator(XFELO). One aim of this study was to investigate, if the radiation pressure could be an effect that gives a considerable contribution to the strain propagation. The results of the simulations have shown that, if Bragg backscattering of the X-ray pulse by a diamond crystal with 99% reflectivity and 1% absorptivity is assumed, the value of the thermally induced strain is about two magnitudes higher than the radiation pressure induced strain., Proceedings of the 38th Int. Free Electron Laser Conf., FEL2017, Santa Fe, NM, USA

3rd European Advanced Accelerator Concepts Workshop, EAAC 2017, La Biodola, Isola d'Elba, Italy, 24 Sep 2017 - 30 Sep 2017, The beam-driven FLASHForward experiment 1 (X-1) aims at the generation of high-brightness electron bunches for photon science applications in several centimeters of plasma, with the plasma acting both as a cathode and accelerator.The 1 GeV electron-bunch with a peak current of 2.5 kA and a synchronized TW-laser system makes FLASHForward a unique facility[1] to study controlled electron-injection into plasma wakes.With density downramp injection, witness bunches of ~1 kA peak current at emittances well below 1 µm are achievable[2]. The sharp plasma density gradients are produced by means of controlled gas flow[3] or by localized laser ionization transverse to the electron-beam orbit[4]. Precise laser-to-electron-beam synchronization enables controlled injection as e.g. the Trojan Horse scheme[5], which is predicted to support sub-0.1-µm-emittance witness bunches.experimental installation status, planning, and prospects of the FLASHForward X-1 experiments are presented.

3rd European Advanced Accelerator Concepts Workshop, EAAC 2017, La Biodola, Isola d'Elba, Italy, 24 Sep 2017 - 30 Sep 2017, Staging of plasma-wakefield accelerators is essential to utilise them in particle physics or other applications requiring high energy beams.Quality preservation in external beam injection is one of the key missing milestones towards this goal.This and other topics related to the plasma booster will be studied at FLASHForward, a unique beam-driven plasma wakefield acceleration facility currently under construction at DESY (Hamburg, Germany),in the frame of the FLASHForward X-2 experiment.High-quality 1 GeV-class electron beams with $\mu m$-emittances from the free-electron laser FLASH will be utilised to generate driver-witness pairsby using a mask in a dispersive section.Alternatively, it is possible to generate two independent bunches directly in the photocathode electron-gun by using a double-pulse laser.In this contribution, the physics case and the current status of the FLASHForward X-2 experiment will be reviewed.The experimental installation will be described, with a focus on the electron beam line.Electron beam dynamics and Particle-in-Cell simulations will be presented

FLASH is the superconducting soft X-ray Free Electron Laser in Hamburg at DESY, Germany. A precise knowledge of the beam optics is a key aspect of the operation of a SASE FEL. A campaign of optics consolidation has started in 2013 when the second beam line FLASH2 was installed downstream of the FLASH linac. We give an update on progress of this effort and on recent results., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

38th International Free-Electron Laser Conference, FEL2017, Santa Fe, USA, 20 Aug 2017 - 25 Aug 2017; 4 pp. (2017)., FLASHForward is the Future-ORiented Wakefield Accelerator Research and Development project at the DESY free-electron laser (FEL) facility FLASH. It aims to produce high-quality, GeV-energy electron beams over a plasma cell of a few centimeters. The plasma is created by means of a 25 TW Ti:Sapphire laser system. The plasma wakefield will be driven by high-current-density electron beams extracted from the FLASH accelerator. The project focuses on the advancement of plasma-based particle acceleration technology through the exploration of both external and internal witness-beam injection schemes. Multiple conventional and cutting-edge diagnostic tools, suitable for diagnosis of short electron beams, are under development. The design of the post-plasma beamline sections will be finalized based on the result of these aforementioned diagnostics. In this paper, the status of the project, as well as the progress towards achieving its overarching goal of demonstrating FEL gain via plasma wakefield acceleration, is discussed.

8th International Particle Accelerator Conference, IPAC17, Copenhagen, Denmark, 15 May 2017 - 19 May 2017; Geneva : JACoW, 1692-1695 (2017). doi:10.18429/JACoW-IPAC2017-TUPIK006, FLASHForward is a beam-driven plasma wakefield acceleration facility, currently under construction at DESY (Hamburg, Germany), aiming at the stable generation of electron beams of several GeV with small energy spread and emittance. High-quality 1 GeV-class electron beams from the free-electron laser FLASH will act as the wake driver. The setup will allow studies of external injection as well as density-downramp injection. With a triangular-shaped driver beam electron energies of up to 5 GeV from a few centimeters of plasma can be anticipated. Particle-In-Cell simulations are used to assess the feasibility of each technique and to predict properties of the accelerated electron bunches. In this contribution the current status of FLASHForward, along with recent experimental developments and upcoming scientific plans, will be reviewed., Published by JACoW, Geneva

We present results on the commissioning of the two-module RadiaBeam / SLAC dechirper system at LCLS. This is the first installation and measurement of a corrugated dechirper at high energy (4.4 - 13.3 GeV), short pulses (< 200 fs) and while observing its effect on an operational FEL. Both the transverse and longitudinal wakefields allow more flexible electron beam tailoring. We verify that, for a single module at a given gap, the strength of the longitudinal wake on axis and the dipole near the axis agree well with the theoretical values. Using direct longitudinal phase space mapping and X-ray FEL spectrum measurements we demonstrate the energy chirp control capabilities., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

International Beam Instrumentation Conference, IBIC16, Barcelona, Spain, 11 Sep 2016 - 15 Sep 2016 ; (2016)., FLASHForward is a beam-driven plasma wakefield acceleration facility,currently under construction at DESY (Hamburg, Germany),aiming at the stable generation of electron beams of several \si{\GeV} with small energy spread and emittance.High-quality 1 GeV-class electron beams from the free-electron laser FLASH will act as the wake driver.The setup will allow studies on external injection as well as on various internal injection techniques, such as density-downramp or ionisation injection.With a triangular-shaped drive beam electron energies of up to 5 GeV from a few centimeters of plasma can be anticipated.Particle-In-Cell simulations are used to assess the feasibility of each technique and to predict properties of the accelerated electron bunches.In this contribution the physics case and the current status of FLASHForward will be reviewed.Concepts of the main components - the extraction beamline from the FLASH linac, the target area, the plasma cell and the post-plasma beam transport and diagnostics - will be described.

An X-ray free electron laser can be built in an oscillator (XFELO) configuration by employing an X-ray cavity with Bragg mirrors such as diamond*. An XFELO at the 5th harmonic frequency may be implemented at the LCLS II using its 4 GeV superconducting linac. The XFELO will provide stable, coherent, high-spectral-purity hard x-rays. In addition, portions of its output may be enhanced by the LCLS amplifier for stable pulses of ultrashort duration determined by the electron bunch length. Much progress has been made recently on the feasibility of an XFELO: Analytical and numerical methods have been developed to compute the performance of a harmonic XFELO. The energy spread requirement over a sufficient length of the bunch can be met by temporal shaping of the photo-cathode drive laser**. Experiments at the APS have shown that Be-compound refractive lenses are suitable for a low-loss focusing and that the synthetic diamond crystals can withstand the intense x-ray exposure, in accord with estimates based on molecular dynamics considerations***. A strain-free mounting of thin diamond crystal (< 100 microns) can be realized by shaping a thick diamond into a blind alley****., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

We discuss the possibility of using the RadiBeam/SLAC dechirper recently installed at LCLS for measuring the bunch length of very short bunches, less than 1 fs perhaps as short as 100 atto second. When a bunch travels close to one of the jaws the particles of the bunch get a transverse kick depends upon the position of a particle in a bunch. The tail particles get more kick. The transverse force also gets a nonlinear dependence on the transverse position. The stretched bunch can be measured at the YAG screen that is 100 m downstream the dechirper. The most important aspect of this measurement is that that no synchronization is needed. The Green's function for the transverse kick was evaluated based on the precise wake field calculations of the dechirper corrugated structure*. Using this function we can restore the longitudinal shape of the bunch. This may also help to see if a bunch has any micro-bunch structure., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

[Ebook] Proceedings of the 6th International Particle Accelerator Conference : (IPAC 2015) : Richmond, Virginia, USA : 03-08 May 2015 / Henderson, Stuart ; Ayers, E. ; Satogata, Todd ; Schaa, Volker R.W. , Geneva : JACoW, CERN, June 2015 6th International Particle Accelerator Conference, Richmond, Virginia, USA, 3 May 2015 - 8 May 2015 [Ebook] Proceedings of the 6th International Particle Accelerator Conference : (IPAC 2015) : Richmond, Virginia, USA : 03-08 May 2015 / Henderson, Stuart ; Ayers, E. ; Satogata, Todd ; Schaa, Volker R.W. , Geneva : JACoW, CERN, June 2015; Geneva : JACoW, 1488-1491(2015)., FLASH is the superconducting soft X-ray Free Electron Laser in Hamburg at DESY, Germany. Good control over the beam optics is a key aspect of the operation of a SASE FEL. In 2013 a second beam line, FLASH2, was assembled and the modifications necessary to feed the two beam lines were installed downstream of the FLASH linac. As reported before we started a campaign of optics consolidation. We give an update on the progress of this effort and on results., Published by JACoW, Geneva

The superconducting linac of the European XFEL Laboratory in Hamburg will produce electron bunch trains with a time structure that allow in principle the operation of an XFELO (X-ray FEL-Oscillator). The electron bunches of the European XFEL have an expected length between 2 and 180 fs (FWHM) with an expected arrival time jitter of about 30 fs (RMS). A jitter of the electron bunch arrival time leads to a detuning between the electron and photon pulse. Since an XFEL-Oscillator relies on a spatial overlap of electron and photon pulse, the influence of a lack of longitudinal overlap is studied. The simulations are performed for different bunch lengths and levels of arrival time jitter. The results of a simulation are presented where angular, transversal and arrival time jitter are taken into account simultaneously, assuming parameters expected for the European XFEL Linac., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea

FLASH2 is a newly build second beamline at FLASH, a soft X-ray FEL at DESY, Hamburg. Unlike the existing beamline FLASH1, it is equipped with variable gap undulators. This beamline is currently being commissioned. Both undulator beamlines of FLASH are driven by a common linear accelerator. Fast kickers and a septum are installed at the end of the linac to distribute the electron bunches of every train between FLASH1 and FLASH2. A specific beam optics in the extraction arc with horizontal beam waists in the bending magnets is mandatory in order to mitigate effects from coherent synchrotron radiation (CSR). We performed various beam optics measurements to ensure that the conditions for FEL operation at FLASH2 are fulfilled. Here we will show results of measurements., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

The LCLS-II with its high repetition rate and high quality beam will be capable of driving an X-ray free electron laser oscillator at higher harmonics in the hard X-ray regime (0.1 nm). The oscillator consists of a low loss X-ray crystal cavity using diamond Bragg crystals with meV bandwidth. The expected average spectral flux has been estimated to be at least two orders of magnitude greater than present synchrotron-based sources with highly stable, coherent pulses of duration 1 ps or less for applications in Mössbauer spectroscopy and inelastic x-ray scattering. A more detailed study of the start up of a fifth-harmonic X-ray FEL oscillator at LCLS-II will be presented with full, time-dependent simulations., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea

FLASH2, the second undulator beamline of the FLASH FEL user facility at DESY (Hamburg, Germany) is under commissioning. Its first lasing was achieved in August 2014. FLASH is the first soft X-ray FEL operating two undulator beamlines simultaneously. Both undulator beamlines are driven by a common linear superconducting accelerator with a beam energy of up to 1.25 GeV. Fast kickers and a septum are installed to distribute one part of the electron bunch train to FLASH1 and the other part to FLASH2 with full repetition rate. The commissioning of FLASH2 takes place primarily in parallel to FLASH1 user operation. Various beam optics measurements has been carried out in order to ensure the required electron beam quality for efficient SASE generation. This paper reports the status of the FLASH2 commissioning., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea