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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

Structural dynamics 9(6), 064301 (2022). doi:10.1063/4.0000165, In the present contribution, we use x-rays to monitor charge-induced chemical dynamics in the photoionized amino acid glycine with femtosecond time resolution. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay. Temporal modulation of the Auger electron signal correlated with specific ions is observed, which is governed by the initial electronic coherence and subsequent vibronic coupling to nuclear degrees of freedom. In the time-resolved x-ray absorption measurement, we monitor the time-frequency spectra of the resulting many-body quantum wave packets for a period of 175 fs along different reaction coordinates. Our experiment proves that by measuring specific fragments associated with the glycine dication as a function of the pump-probe delay, one can selectively probe electronic coherences at early times associated with a few distinguishable components of the broad electronic wave packet created initially by the pump pulse in the cation. The corresponding coherent superpositions formed by subsets of electronic eigenstates and evolving along parallel dynamical pathways show different phases and time periods in the range of $(−0.3±0.1)𝜋≤𝜙≤(0.1±0.2)𝜋$ and $18.2 ^{+1.7}_{−1.4}≤𝑇≤23.9+1.2−1.1$ $fs.$fs. Furthermore, for long delays, the data allow us to pinpoint the driving vibrational modes of chemical dynamics mediating charge-induced bond cleavage along different reaction coordinates., Published by AIP Publishing LLC, Melville, NY

Science advances 8(22), eabn6848 (2022). doi:10.1126/sciadv.abn6848, Here, we use x-rays to create and probe quantum coherence in the photoionized amino acid glycine. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay and by photoelectron emission from sequential double photoionization. Sinusoidal temporal modulation of the detected signal at early times (0-25 fs) is observed in both measurements. Advanced ab initio many-electron simulations allow us to explain the first 25 fs of the detected coherent quantum evolution in terms of the electronic coherence. In the kinematically complete x-ray absorption measurement we monitor its dynamics for a period of 175 fs and observe an evolving modulation that may implicate the coupling of electronic to vibronic coherence at longer time scales. Our experiment provides a direct support for the existence of long-lived electronic coherence in photoionized biomolecules., Published by Assoc., Washington, DC [u.a.]

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

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

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

Transverse Deflecting RF-Structures (TDS) are successfully used for longitudinal diagnostic purposes at many Free-Electron Lasers (FEL) (LCLS, FLASH, EU-XFEL, FERMI). Moreover, by installing a TDS downstream of the FEL undulators and placing the measurement screen in a dispersive section, the temporal photon pulse structure can be estimated, as was demonstrated at LCLS and sFLASH. Here we describe the installation of a variable polarization X-band structure (PolariX TDS [1]) downstream of the FLASH2 undulators. The installation of such a TDS enables longitudinal phase space measurements and photon pulse reconstructions, as well as slice emittance measurements in both planes using the same cavity due to the unique variable polarization of the PolariX TDS., 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, 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

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

[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

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

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

High Gradient Workshop 2018, HG2018, Shanghai, SINAP, China, 4 Jun 2018 - 8 Jun 2018

8th International Particle Accelerator Conference, IPAC17, Copenhagen, Denmark, 15 May 2017 - 19 May 2017, The superconducting free-electron laser user facility FLASH at DESY in Hamburg, routinely produces several thousand photon pulses per second. The operational parameters cover a wavelength range from 90 nm down to 4 nm with pulse energies from several $\mu$J up to 1 mJ and with pulse durations of several hundred fs down to a few fs. The FLASH injector and linac drives two undulator beam lines (FLASH1, FLASH2) and therefore FLASH is capable of serving 2 independent experiments with photon pulse (sub-) trains of several 100 bunches at the full train repetition frequency of 10 Hz.We summarize here the highlights of the user operation at FLASH1/2 and the study program (machine development and FEL optimization) of the FLASH facility.

8th International Particle Accelerator Conference, IPAC17, Copenhagen, Denmark, 15 May 2017 - 19 May 2017 ; Geneva : JACoW, 2600-2603(2017). doi:10.18429/JACoW-IPAC2017-WEPAB017, The free-electron laser in Hamburg (FLASH) is a user facility, delivering soft X-ray radiation, consisting of two beam lines, FLASH1 and FLASH2. The injector and the main linac are shared between both beam lines. Starting in 2014, FLASH2 has been commissioned for user operation. Currently, there is no hardware installed for the direct measurement of the electron bunch length nor the photon pulse duration at FLASH2. Exact knowledge of the pulse duration is essential for time-resolved user experiments performed at FLASH. Therefore, we are designing a modified beam line, containing a new type of X-band deflecting cavity* and a dipole, downstream of the FLASH2 undulator, to map the longitudinal phase space onto a beam screen. Anticipating the feasibility of measuring the longitudinal phase space with high resolution, a study on optimizing the free-electron laser (FEL) performance for shortest bunches is ongoing., Published by JACoW, Geneva

8th International Particle Accelerator Conference, IPAC17, Copenhagen, Denmark, 15 May 2017 - 19 May 2017 ; Geneva : JACoW, 2621-2624(2017). doi:10.18429/JACoW-IPAC2017-WEPAB023, Harmonic lasing is a perspective mode of operation of X-ray FEL user facilities that allows to provide brilliant beams of higher energy photons for user experiments. Another useful application of harmonic lasing is so called Harmonic Lasing Self-Seeded Free Electron Laser (HLSS FEL) that allows to improve spectral brightness of these facilities. In the past, harmonic lasing has been demonstrated in the FEL oscillators in infrared and visible wavelength ranges, but not in high-gain FELs and not at short wavelengths. In this paper we report on the first evidence of the harmonic lasing and the first operation of the HLSS FEL at the soft X-ray FEL user facility FLASH in the wavelength range between 4.5 nm and 15 nm. Spectral brightness was improved in comparison with Self-Amplified Spontaneous emission (SASE) FEL by a factor of six in the exponential gain regime. A better performance of HLSS FEL with respect to SASE FEL in the post-saturation regime with a tapered undulator was observed as well. The first demonstration of harmonic lasing in a high-gain FEL and at a short wavelength paves the way for a variety of applications of this new operation mode in X-ray FELs., Published by JACoW, Geneva

The program of low charge mode of operation is under development at free electron laser FLASH aiming in single mode radiation pulses. A short pulse photoinjector laser has been installed at FLASH allowing production of ultrashort electron pluses with moderate compression factor of the beam formation system. Here we present pilot results of free electron laser FLASH operating at the wavelength of 13.1 nm and driven by 70 pC electron bunches. Relevant theoretical analysis has been performed showing good agreement with experimental results., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

9th International Particle Accelerator Conference, IPAC18, ; Journal of physics / Conference Series 1067(3), 032011 (2018). doi:10.1088/1742-6596/1067/3/032011, Transverse deflecting structures (TDS) are widely used in accelerator physics to measure the longitudinal density of particle bunches. When used in combination with a dispersive section, the whole longitudinal phase space density can be imaged. At the Linac Coherent Light Source (LCLS), the installation of such a device downstream of the undulators enables the reconstruction of the X-ray temporal intensity profile by comparing longitudinal phase space distributions with lasing on and lasing off. However, the resolution of this TDS is limited to around 1 fs rms (root mean square),and therefore, in most cases, it is not possible to resolve single self-amplified spontaneous emission (SASE) spikes within one photon pulse.By combining the intensity spectrum from a high resolution photon spectrometer and the temporal structure from the TDS, the overall resolution is enhanced, thus allowing the observation of temporal, single SASE spikes. The combined data from the spectrometer and the TDS is analyzed using an iterative algorithm to obtain the actual intensity profile.In this paper, we present the reconstruction algorithm as well as analyzed data obtained from simulations which shows the reliability of this method. Real data will be published at a later stage., Published by IOP Publ., Bristol

The free-electron laser facility FLASH at DESY (Hamburg, Germany) operates 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. The superconducting technology allows the acceleration of trains of several hundred microsecond spaced bunches with a repetition rate of 10 Hz. A fast kickers-septum system is installed to distribute one part of the electron bunch train to FLASH1 and the other part to FLASH2 keeping the full 10 Hz repetition rate for both beamlines. In order to deliver different beam properties to each beamline, the FLASH photoinjector uses two independent laser systems to generate different bunch pattern and bunch charges. One laser serves the FLASH1 beamline, the other the FLASH2 beamline. A third laser with adjus ö laser pulse duration is used to generate ultra-short bunches for single spike lasing., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea

Collective effects and instabilities due to longitudinal space charge and coherent synchrotron radiation can degrade the quality of the ultra-relativistic, high-brightness electron bunches driving free-electron lasers (FELs). In this contribution, we demonstrate suppression of FEL lasing induced by a laser-triggered microbunching instability at the free-electron laser FLASH. The interaction between the electron bunches and the 800-nm laser pulses takes place in an undulator upstream of the FEL undulators. A significant decrease of XUV photon pulse energies has been observed in coincidence with the laser-electron overlap in the modulator. We discuss the underlying mechanisms based on longitudinal space charge amplification (LSCA) [E.A. Schneidmiller and M.V. Yurkov, Phys. Rev. ST Accel. Beams 13, 110701 (2010)] and present measurements., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea

Journal of physics / Conference Series 874, 012023 (2017). doi:10.1088/1742-6596/874/1/012023, DESY’s free-electron laser FLASH provides soft X-ray pulses for scientific usersat wavelengths down to 4 nm simultaneously in two undulator beamlines. They are drivenby a common linear superconducting accelerator with a beam energy of up to 1.25 GeV. Thesuperconducting technology allows the acceleration of electron bunch trains of several hundredbunches with a spacing of 1 microsecond or more and a repetition rate of 10Hz. A fast kickerseptumsystem directs one part of the bunch train to FLASH1 and the other part to FLASH2keeping the full 10Hz repetition rate for both. The unique setup of FLASH allows independentFEL pulse parameters for both beamlines. In April 2016, simultaneous operation of FLASH1and FLASH2 for external users started. This paper reports on our operating experience withthis type of multi-beam, multi-beamline set-up., Published by IOP Publ., Bristol

At the Photo Injector Test facility at DESY, Zeuthen site (PITZ), high brightness electron sources for linac based Free Electron Lasers (FELs), like FLASH and the European XFEL are developed and characterized. The electrons are generated via the photoeffect at a cesium telluride (Cs₂Te) cathode and are accelerated by a 1.6-cell L-band RF-gun cavity with an accelerating gradient at the cathode of about 60 MV/m. The profile of the cathode laser pulse has been optimized yielding small emittances using laser pulse shaping methods. The transverse projected emittance is measured by a single slit scan technique. The measurement program in the last run period at PITZ concentrated on emittance measurements for the nominal 1 nC beam and emittance optimization for lower bunch charges. The recent results show that normalized projected emittances of about 1 mm-mrad for 1 nC charge and below 0.5 mm-mrad for 250 pC bunch charges can be realized at PITZ. The facility setup and measurement results including the uncertainty of the measured values will be reported and discussed in this contribution., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Recently, the free-electron laser in Hamburg (FLASH) at DESY has been upgraded considerably. Besides increasing the maximum energy to about 1.2 GeV and installation of a third harmonic rf cavity linearizing the longitudinal phase space distribution of the electron bunch, an FEL seeding experiment at wavelengths of about 35 nm has been installed. The goal is to establish direct FEL seeding employing coherent VUV pulses produced from a powerful drive laser by high-harmonic generation (HHG) in a gas cell. The project, called sFLASH, includes generation of the required HHG pulses, transporting it to the undulator entrance of a newly installed FEL-amplifier, controlling spatial, temporal and energy overlap with the electron bunches and setting up a pump-probe pilot experiment. Sophisticated diagnostics is installed to characterize both HHG and seeded FEL pulses, both in time and frequency domain. Compared to SASE-FEL pulses, almost perfect longitudinal coherence and improved synchronization possibilities for the user experiments are expected. In this paper the status of the experiment is presented., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

8th International Particle Accelerator Conference, IPAC17, Copenhagen, Denmark, 15 May 2017 - 19 May 2017 ; Geneva : JACoW, 184-187(2017). doi:10.18429/JACoW-IPAC2017-MOPAB044, Based on the success of the X-Band Transverse Deflecting Structure (TDS) diagnostic at LCLS*, a collaboration between DESY, PSI and CERN has formed with the aim of developing and building an advanced modular X-Band TDS system. The designed TDS has the new feature of providing variable polarization of the deflecting field**. The possibility of changing the orientation of the streaking field of the TDS to an arbitrary azimuthal angle allows for 3D characterization of the phase space using tomographic methods***. Moreover the complete 6D characterization of the beam phase space is possible by combining this technique with quadrupole scans and a dipole spectrometer. As this new cavity 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, the high precision tuning-free assembly procedures developed at PSI for the SwissFEL C-band accelerating structures will be used for the manufacturing****. The high-power rf system is based on the CERN-based X-band test stands. We summarize in this work the status of the projects and its main technical parameters., Published by JACoW, Geneva

sFLASH is a seeded FEL experiment at DESY, which uses a 38nm high harmonic gain (HHG)-based XUV-beam laser in tandem with FLASH electron bunches at the entrance of a 10m variable-gap undulator. The temporal overlap between the electron and HHG beams is critical to the seeding process. Use of a 3rd harmonic accelerating module provides a high current electron beam (at the kA level) with ~ 600fs FWHM bunch duration. The length of the HHG laser pulse will be ~30fs FWHM. The desired overlap is achieved in steps. First is the synchronization of the HHG drive laser (Ti: Sapphire, 800nm) and the incoherent spontaneous radiation from an upstream undulator. Next, the IFEL-modulated electron bunch will pass through a dispersive section, producing a density modulation in the beam. This in turn yields emission of coherent radiation from a downstream undulator or transition radiation screen when the longitudinal overlap of the two beams is achieved. The coherently enhanced light emitted will be then spectrally analyzed. The experimental layout, simulation results of generation and transport of both light pulses, and preliminary measurements are presented., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Seventh International Particle Accelerator Conference 2016, IPAC 2016, Busan, Republic of South Korea, 8 May 2016 - 13 May 2016 ; 729-731(2016)., FLASH, the superconducting free-electron laser at DESY delivers up to several thousand photon pulses per second with wavelengths ranging from 52 nm down to 4.2 nm and with pulse energies of up to 500 FLASH, the superconducting free-electron laser at DESY delivers up to several thousand photon pulses per second with wavelengths ranging from 52 nm down to 4.2 nm and with pulse energies of up to 500 $\mu$J to photon users at the FLASH1 beamline. In 2014 and 2015 a second beamline, FLASH2, has been commissioned in parallel to user operation at FLASH1. FLASH produces bunch trains of up to 800 bunches in 0.8 ms with a train repetition rate of 10 Hz. Each train can be split in sub-trains for FLASH1 and FLASH2, such that both beamlines receive bursts of bunches with full 10 Hz. Operational highlights are the SASE energy record of 600 $\mu$J at 15 nm in FLASH2, and the first simultaneous SASE lasing of three undulator systems: FLASH1 (13.7nm), sFLASH (38 nm), and FLASH2 (20 nm). sFLASH is the seeding experiment in the FLASH1 beamline. On April 8th 2016, FLASH has for the first time simultaneously served users at both beamlines, FLASH1 and FLASH2.$\mu$J to photon users at the FLASH1 beamline. In 2014 and 2015 a second beamline, FLASH2, has been commissioned in parallel to user operation at FLASH1. FLASH produces bunch trains of up to 800 bunches in 0.8 ms with a train repetition rate of 10 Hz. Each train can be split in sub-trains for FLASH1 and FLASH2, such that both beamlines receive bursts of bunches with full10 Hz. Operational highlights are the SASE energy record of 600 $\mu$J at 15 nm in FLASH2, and the first simultaneous SASE lasing of three undulator systems: FLASH1 (13.7nm), sFLASH (38 nm), and FLASH2 (20 nm). sFLASH is the seeding experiment in the FLASH1 beamline. On April 8th 2016, FLASH has for the first time simultaneously served users at both beamlines, FLASH1 and FLASH2.

The free-electron laser in Hamburg (FLASH) is a soft X-ray user facility, delivering radiation for photon science since 2005. Since 2014, a second beam line for user operation, FLASH2, has been in operation. The beam for FLASH2 is accelerated using the same accelerating modules as for the initial FLASH beam line (FLASH1), but then deflected downstream of this shared linac (linear accelerator). The electron bunches generated at a photocathode have an rms (root mean square) length of 6.5 ps or 4.5 ps when the standard injector lasers are used. These bunches are then compressed in two bunch compressors in the linac to about 30 fs to 200 fs. To shorten the bunch lengths even further, a short-pulse injector laser was installed at FLASH, which generates short, low charge electron bunches directly at the cathode. These short electron bunches with a length of about 1 ps can be further compressed in the linac to produce ultra-short FEL (Free-Electron Laser) pulses down to a few femtoseconds in the FLASH2 undulators. Measurements of such pulses as well as tracking simulations of the FLASH2 beam line are presented in this thesis. Both studies demonstrate the feasibility of ultra-short FEL pulses down to single-spike lasing at FLASH2. Up until now, no hardware to directly measure the electron bunch length has been installed in the FLASH2 beam line. As exact knowledge of the pulse duration is essential for time-resolved user experiments, the beam line downstream of the FLASH2 undulators has been redesigned for the installation of a variable polarization Transverse Deflecting Structure (TDS). In combination with a dipole magnet it is possible to map the longitudinal phase space density of the electron bunches onto a beam screen. Additionally, the photon pulse duration as well as the slice emittance in both transverse planes can be measured using such a TDS. This thesis presents the final layout of the beam line, the accelerator optics, and also simulations for the aforementioned measurements. However, the temporal resolution of an X-band TDS is limited to the order of 1 fs rms. In most cases, this is not sufficient to resolve single self-amplified spontaneous emission (SASE) spikes within one photon pulse for hard X-ray radiation. By adding the intensity spectrum from a high-resolution spectrometer to the temporal profile from the TDS the overall resolution can be enhanced. This is done by applying an iterative reconstruction algorithm that is presented in this thesis. Additionally, the algorithm is tested using simulation data of the Linac Coherent Light Source (LCLS) and applied to data taken at LCLS.

Free-electron lasers, like the free-electron laser in Hamburg (FLASH), provide brilliant photon pulses on a femto-second time scale. Generation of this pulses is done in a high-gain self-amplified spontaneous emission or seeded FEL process. This FEL process is driven by electron bunches with high peak current and high quality. The photon pulse length is determined by the electron bunch length inside the accelerator. A controlled and reproducible setup of accelerator parameter for certain photon pulse settings requires longitudinal electron bunch diagnostics which is able to measure electron bunch duration on the femto-second time scale. Currently, two diagnostic techniques, one in time domain and the other in frequency domain, can be used routinely for this task. The time domain diagnostics utilizes a transverse deflecting cavity and the other spectral resolved intensity measurements.Longitudinal form factor is the Fourier transform of the longitudinal electron bunch density distribution. The form factor modulus is accessible from spectral resolved intensity measurements, while the phase information is lost. This loss of phase information prevents a unique determination of the longitudinal electron bunch density by application of the inverse Fourier transform to the longitudinal form factor. Instead, phase reconstruction techniques have to be used in order to obtain the longitudinal electron bunch density distribution.A new time profile reconstruction algorithm is presented in this thesis. This algorithm is a further development of the Gerchberg-Saxton iterative reconstruction algorithm. The new algorithm is based on statistical and clustering analysis of a results pool, which is generated using multiple independent iterative reconstruction algorithm starts.Behaviour of the new time profile reconstruction algorithm is tested on model profiles and on measurements at FLASH. Further, a comparison with another time profile reconstruction technique based on the Kramers-Kronig relation is done.Since reconstruction of time domain profiles from spectral resolved intensity measurements suffer from ambiguities due to loss of phase information, a comparison of longitudinal diagnostics utilizing spectral resolved intensity measurements of coherent transition radiation with complementary time domain diagnostics technique has been performed at FLASH.

Dissertation, Universität Hamburg, 2017; 144 pp. (2018). = Dissertation, Universität Hamburg, 2017, High-gain Free-Electron Lasers (FEL) constitute a powerful source of intense, short radiation pulses in the XUV and X-ray range that are used in a wide variety of research fields. They typically work in the Self-Amplified Spontaneous Emission (SASE) Mode. As a consequence of the statistical nature of this process, the radiation pulses exhibit a low degree of longitudinal coherence. There are basically two approaches for improving the longitudinal coherence. First, the so-called seeding of the FEL process, where the FEL radiation properties are controlled by a coherent seed-pulse coupled into the beam line. Second, the generation of very short FEL pulses that contain only a single longitudinal mode of the radiation. This thesis deals with the latter method and with the attempt to establish it at the Free-Electron Laser in Hamburg (FLASH), in order to offer photon pulses to the users that transversely as well as longitudinally exhibit a high degree of coherence.For this purpose, analytical and numerical studies were performed to determine the required parameters of the accelerator (essentially bunch length, a modification of the photo-injector laser). These parameters have been successfully realized at FLASH and first measurements of the generated FEL pulses have been conducted. It could be verified that the photon pulses indeed contain little more than a single coherent mode in average.