Free Electron Laser In Hamburg Research Articles

Comparison of wavefront sensing and ablation imprinting for FEL focus diagnostics at FLASH2

Extreme ultraviolet (EUV) photon beam characterization techniques, Hartmann wavefront sensing and single shot ablation imprinting, were compared along the caustic of a tightly focused free-electron laser (FEL) beam at beamline FL24 of FLASH2, the Free-electron LASer in Hamburg at DESY. The transverse coherence of the EUV FEL was determined by a Young’s double pinhole experiment and used in a back-propagation algorithm which includes partial coherence to calculate the beam intensity profiles along the caustic from the wavefront measurements. A very good agreement of the profile structure and size is observed for different wavelengths between the back-propagated profiles, an indirect technique, and ablation imprints. As a result, the Hartmann wavefront sensor including its software MrBeam is a very useful, single shot pulse resolved and fast tool for non-invasive determination of focal spot size and shape and also for beam profiles along the caustic.

Deep learning for laser beam imprinting.

Methods of ablation imprints in solid targets are widely used to characterize focused X-ray laser beams due to a remarkable dynamic range and resolving power. A detailed description of intense beam profiles is especially important in high-energy-density physics aiming at nonlinear phenomena. Complex interaction experiments require an enormous number of imprints to be created under all desired conditions making the analysis demanding and requiring a huge amount of human work. Here, for the first time, we present ablation imprinting methods assisted by deep learning approaches. Employing a multi-layer convolutional neural network (U-Net) trained on thousands of manually annotated ablation imprints in poly(methyl methacrylate), we characterize a focused beam of beamline FL24/FLASH2 at the Free-electron laser in Hamburg. The performance of the neural network is subject to a thorough benchmark test and comparison with experienced human analysts. Methods presented in this Paper pave the way towards a virtual analyst automatically processing experimental data from start to end.

Free-electron laser temporal diagnostic beamline FL21 at FLASH.

A beamline for temporal diagnostics of extreme ultraviolet (XUV) femtosecond pulses at the free-electron laser in Hamburg (FLASH) at DESY was designed, built and put into operation. The intense ultra-short XUV pulses of FLASH fluctuate from pulse to pulse due to the underlying FEL operating principle and demand single-shot diagnostics. To cope with this, the new beamline is equipped with a terahertz field-driven streaking setup that enables the determination of single pulse duration and arrival time. The parameters of the beamline and the diagnostic setup as well as some first experimental results will be presented. In addition, concepts for parasitic operation are investigated.

FLASH and the FLASH2020+ project—current status and upgrades for the free-electron laser in Hamburg at DESY

The free-electron laser in Hamburg, FLASH, is the first extreme ultra-violet and soft X-ray free-electron laser (FEL) user facility and has been continuously upgraded since its start in 2005. Further major works are currently underway within the FLASH2020+ project that pioneeringly implements full repetition rate external seeding at a superconducting accelerator facility. With fully tunable undulators providing variable polarization FEL pulses, we expect FLASH to turn into the ideal spectroscopy machine for ultrafast processes within the coming years.

Photon energy-resolved velocity map imaging from spectral domain ghost imaging

We present an approach that combines photon spectrum correlation analysis with the reconstruction of three-dimensional momentum distribution from velocity map images in an efficient, single-step procedure. We demonstrate its efficacy with the results from the photoionization of the 2 p-shell of argon using the Free-electron LASer in Hamburg free-electron laser (FEL). Distinct spectral features due to the spin-orbit splitting of Ar are resolved, despite the large average bandwidth of the ionizing pulses from the FEL. This demonstrates a clear advantage over the conventional analysis method, and it will be broadly beneficial for velocity map imaging experiments with FEL sources. The retrieved linewidth of the binding energy spectrum approaches the resolution limitation prescribed by the spectrometers used to collect the data. Our approach presents a path to extend spectral-domain ghost imaging to the case where the photoproduct observable is high-dimensional.

The soft X-ray and XUV split-and-delay unit at beamlines FL23/24 at FLASH2.

A split-and-delay unit for the extreme ultraviolet and soft X-ray spectral regions has been built which enables time-resolved experiments at beamlines FL23 and FL24 at the Free-electron LASer in Hamburg (FLASH). Geometric wavefront splitting at a sharp edge of a beam splitting mirror is applied to split the incoming soft X-ray pulse into two beams. Ni and Pt coatings at grazing incidence angles have been chosen in order to cover the whole spectral range of FLASH2 and beyond, up to hν = 1800 eV. In the variable beam path with a grazing incidence angle of ϑd = 1.8°, the total transmission (T) ranges are of the order of 0.48 < T < 0.84 for hν < 100 eV and T > 0.50 for 100 eV < hν < 650 eV with the Ni coating, and T > 0.06 for hν < 1800 eV for the Pt coating. For a fixed beam path with a grazing incidence angle of ϑf = 1.3°, a transmission of T > 0.61 with the Ni coating and T > 0.23 with a Pt coating is achieved. Soft X-ray pump/soft X-ray probe experiments are possible within a delay range of -5 ps < Δt < +18 ps with a nominal time resolution of tr = 66 as and a measured timing jitter of tj = 121 ± 2 as. First experiments with the split-and-delay unit determined the averaged coherence time of FLASH2 to be τc = 1.75 fs at λ = 8 nm, measured at a purposely reduced coherence of the free-electron laser.

Differential Measurement of Electron Ejection after Two-Photon Two-Electron Excitation of Helium.

We report the measurement of the photoelectron angular distribution of two-photon single-ionization near the 2p^{2} ^{1}D^{e} double-excitation resonance in helium, benchmarking the fundamental nonlinear interaction of two photons with two correlated electrons. This observation is enabled by the unique combination of intense extreme ultraviolet pulses, delivered at the high-repetition-rate free-electron laser in Hamburg (FLASH), ionizing a jet of cryogenically cooled helium atoms in a reaction microscope. The spectral structure of the intense self-amplified spontaneous emission free-electron laser pulses has been resolved on a single-shot level to allow for post selection of pulses, leading to an enhanced spectral resolution, and introducing a new experimental method. The measured angular distribution is directly compared to state-of-the-art theory based on multichannel quantum defect theory and the streamlined R-matrix method. These results and experimental methodology open a promising route for exploring fundamental interactions of few photons with few electrons in general.

FEL Pulse Duration Evolution along Undulators at FLASH

Self-amplified spontaneous-emission (SASE) free-electron lasers (FELs) deliver ultrashort pulses with femtosecond durations. Due to the fluctuating nature of the radiation properties of SASE FELs, characterizing FEL pulses on a single-shot basis is necessary. Therefore, we use terahertz streaking to characterize the temporal properties of ultrashort extreme ultraviolet pulses from the free-electron laser in Hamburg (FLASH). In this study, pulse duration as well as pulse energy are measured in a wavelength range from 8 to 34 nm as functions of undulators contributing to the lasing process. The results are compared to one-dimensional and three-dimensional, time-dependent FEL simulations.

Fragmentation Dynamics of Fluorene Explored Using Ultrafast XUV-Vis Pump-Probe Spectroscopy

We report on the use of extreme ultraviolet (XUV, 30.3 nm) radiation from the Free-electron LASer in Hamburg (FLASH) and visible (Vis, 405 nm) photons from an optical laser to investigate the relaxation and fragmentation dynamics of fluorene ions. The ultrashort laser pulses allow to resolve the molecular processes occurring on the femtosecond timescales. Fluorene is a prototypical small polycyclic aromatic hydrocarbon (PAH). Through their infrared emission signature, PAHs have been shown to be ubiquitous in the universe, and they are assumed to play an important role in the chemistry of the interstellar medium. Our experiments track the ionization and dissociative ionization products of fluorene through time-of-flight mass spectrometry and velocity-map imaging. Multiple processes involved in the formation of each of the fragment ions are disentangled through analysis of the ion images. The relaxation lifetimes of the excited fluorene monocation and dication obtained through the fragment formation channels are reported to be in the range of a few tens of femtoseconds to a few picoseconds.

A localized view on molecular dissociation via electron-ion partial covariance

Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental and theoretical results for the time-resolved electron spectra of the 4d3/2 and 4d5/2 atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site.

XUV-Initiated Dissociation Dynamics of Molecular Oxygen (O2)

We performed a time-resolved spectroscopy experiment on the dissociation of oxygen molecules after the interaction with intense extreme-ultraviolet (XUV) light from the free-electron laser in Hamburg at Deutsches Elektronen-Synchrotron. Using an XUV-pump/XUV-probe transient-absorption geometry with a split-and-delay unit, we observe the onset of electronic transitions in the O2+ cation near 50 eV photon energy, marking the end of the progression from a molecule to two isolated atoms. We observe two different time scales of 290 ± 53 and 180 ± 76 fs for the emergence of different ionic transitions, indicating different dissociation pathways taken by the departing oxygen atoms. With regard to the emerging opportunities of tuning the central frequencies of pump and probe pulses and of increasing the probe–pulse bandwidth, future pump–probe transient-absorption experiments are expected to provide a detailed view of the coupled nuclear and electronic dynamics during molecular dissociation.

Flexible ptychography platform to expand the potential of imaging at free electron lasers.

Ptychography, a scanning coherent diffraction imaging method, can produce a high-resolution reconstruction of a sample and, at the same time, of the illuminating beam. The emergence of vacuum ultraviolet and X-ray free electron lasers (FELs) has brought sources with unprecedented characteristics that enable X-ray ptychography with highly intense and ultra-fast short-wavelength pulses. However, the shot-to-shot pulse fluctuations typical for FEL pulses and particularly the partial spatial coherence of self-amplified spontaneous emission (SASE) FELs lead to numerical complexities in the ptychographic algorithms and ultimately restrict the application of ptychography at FELs. We present a general adaptive forward model for ptychography based on automatic differentiation, which is able to perform reconstructions even under these conditions. We applied this model to the first ptychography experiment at FLASH, the Free electron LASer in Hamburg, and obtained a high-resolution reconstruction of the sample as well as the complex wavefronts of individual FLASH pulses together with their coherence properties. This is not possible with more common ptychography algorithms.

Dark current studies of an L-band normal conducting RF gun

L-band normal conducting RF guns operating at long pulse mode and high gradient are installed at the European X-ray Free-Electron Laser (European XFEL) and the Free-electron LASer in Hamburg (FLASH) as high brightness electron sources. The dark current generated from the gun may cause severe issues, such as component activation, superconducting radio frequency (SRF) cavity quench and undulator degradation. The dark current is one of the parameters limiting the gun accelerating gradient and performance. In this paper, we report the dark current tracking simulations, including the detailed RF field distribution in the gap between cathode plug and copper hole in the backplane. The transmission ratios of the field emission from cathode regions were simulated for different copper hole geometries and different plug insertions. The dark current imaging measurements were conducted in an L-band normal conducting RF gun (gun 4.2) at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The dark current imaging comparison before and after a 180 degree cathode plug rotation shows the main dark current emissions come from the copper hole corner and Cs2Te film edge, which is consistent with the dark current tracking simulations.

Online Detuning Computation and Quench Detection for Superconducting Resonators

Superconducting cavities are responsible for beam acceleration in superconducting linear accelerators. Challenging cavity control specifications are necessary to reduce radio frequency (RF) costs and to maximize the availability of the accelerator. Cavity detuning and bandwidth are two critical parameters to monitor when operating particle accelerators. Cavity detuning is strongly related to the power required to generate the desired accelerating gradient. Cavity bandwidth is related to the cavity RF losses. A sudden increase in bandwidth can indicate the presence of a quench or multipacting event. Therefore, calculating these parameters in real time in the low-level RF (LLRF) system is highly desirable. A real-time estimation of the bandwidth allows for a faster response of the machine protection system in the case of quench events, whereas the estimation of cavity detuning can be used to drive piezoelectric tuner-based resonance control algorithms. In this article, a new field programmable gate array (FPGA)-based estimation component is presented. Such a component is designed to be used either in continuous wave (CW) or pulsed operation mode with loaded quality factors between $10^{6}$ and $10^{8}$ . Results of this component with free-electron LASer in Hamburg (FLASH), European X-ray free electron laser (EuXFEL), cryo module test bench (CMTB), and electron linac for beams with high brilliance and low emittance (ELBE) are presented.

Cavity tilt measurement using beam excited higher order mode signals at the Free Electron Laser FLASH

At the Free Electron Laser in Hamburg (FLASH), TESLA-type superconducting (SC) cavities are used for the acceleration of electron bunches. Higher Order Modes (HOMs) excited by electron bunches may cause emittance growth and decrease the FEL performance. The misalignment of the SC cavities housed inside cryo-modules is one of the main factors which enhance the coupling of HOMs to the bunches. We consider the cavity offset and tilt as the two most relevant misalignments. These can be measured by utilizing the beam excited HOM signals. The cavity offset has been measured before in several modules at FLASH, while the tilt has so far proved to be much more difficult to be measured. This is because the beam trajectory tilt excited modes are much weaker than the ones excited by offset. We used a method for measuring cavity tilt by carefully targeting the beam through the middle of a cavity. An efficient data processing routine based on signal fitting was applied. For the first time, the tilts of all cavities inside a cryo-module are measured. The ratio between the offset and angle dependence of the HOMs agrees well among the different cavities.

Pulse length effects on autoionizing states under the influence of intense SASE XUV fields

The Fano absorption line shape of an autoionizing state encodes information on its internal atomic structure and dynamics. When driven near-resonantly with intense extreme ultraviolet (XUV) electric fields, the absorption profile can be deliberately modified, including observable changes of both the line-shape asymmetry and strength of the resonance, revealing information on the underlying dynamics of the system in response to such external driving. We report on the influence of the XUV pulse parameters at high intensity that can be achieved with a free-electron laser (FEL) with statistically broadened spectra based on self-amplified spontaneous emission (SASE). More specifically, the impact of the FEL pulse duration is studied for the example of the doubly excited 2s2p resonance in helium, where line-shape modifications have been measured with XUV transient absorption spectroscopy in Fraunhofer-type transmission geometry. A computational few-level-model provides insight into the impact of different average pulse durations of the stochastic FEL pulses. These findings are supported by measurements performed at the Free-Electron Laser in Hamburg (FLASH) and provide further insight into XUV strong-coupling dynamics of resonant transitions driven by intense high-frequency FEL sources.

Parallel Broadband Femtosecond Reflection Spectroscopy at a Soft X-Ray Free-Electron Laser

X-ray absorption spectroscopy (XAS) and the directly linked X-ray reflectivity near absorption edges yield a wealth of specific information on the electronic structure around the resonantly addressed element. Observing the dynamic response of complex materials to optical excitations in pump–probe experiments requires high sensitivity to small changes in the spectra which in turn necessitates the brilliance of free electron laser (FEL) pulses. However, due to the fluctuating spectral content of pulses generated by self-amplified spontaneous emission (SASE), FEL experiments often struggle to reach the full sensitivity and time-resolution that FELs can in principle enable. Here, we implement a setup which solves two common challenges in this type of spectroscopy using FELs: First, we achieve a high spectral resolution by using a spectrometer downstream of the sample instead of a monochromator upstream of the sample. Thus, the full FEL bandwidth contributes to the measurement at the same time, and the FEL pulse duration is not elongated by a monochromator. Second, the FEL beam is divided into identical copies by a transmission grating beam splitter so that two spectra from separate spots on the sample (or from the sample and known reference) can be recorded in-parallel with the same spectrometer, enabling a spectrally resolved intensity normalization of pulse fluctuations in pump–probe scenarios. We analyze the capabilities of this setup around the oxygen K- and nickel L-edges recorded with third harmonic radiation of the free electron laser in Hamburg (FLASH), demonstrating the capability for pump–probe measurements with sensitivity to reflectivity changes on the per mill level.

Characterization of megahertz X-ray laser beams by multishot desorption imprints in PMMA.

Proper diagnostics of intense free-electron laser (FEL) X-ray pulses is indisputably important for experimental data analysis as well as for the protection of beamline optical elements. New challenges for beam diagnostic methods are introduced by modern FEL facilities capable of delivering powerful pulses at megahertz (MHz) repetition rates. In this paper, we report the first characterization of a defocused MHz 13.5-nm beam generated by the free-electron laser in Hamburg (FLASH) using the method of multi-pulse desorption imprints in poly(methyl methacrylate)(PMMA). The beam fluence profile is reconstructed in a novel and highly accurate way that takes into account the nonlinear response of material removal to total dose delivered by multiple pulses. The algorithm is applied to experimental data of single-shot ablation imprints and multi-shot desorption imprints at both low (10 Hz) and high (1 MHz) repetition rates. Reconstructed response functions show a great agreement with the theoretical desorption response function model.

Novel method for the generation of stable radiation from free-electron lasers at high repetition rates

For more than a decade free-electron lasers (FELs) have been in operation, providing scientists from many disciplines with the benefits of ultrashort, nearly transversely coherent radiation pulses with wavelengths down to the \AA{}ngstrom range. If no further techniques are applied, the FEL will only amplify radiation from the stochastic distributed electron density in the electron bunch. Contemporary developments aim at producing stable and single-mode radiation by preparing an electron bunch with favorable longitudinal electron density distributions using magnets and conventional laser pulses (seed), hence the name ``seeding.'' In recent years, short wavelength FELs at high electron beam energies and high repetition rates were proposed and built. At those repetition rates, an external seed with sufficient power to manipulate the electron beam cannot be provided by present state-of-the-art laser systems, thus no external seeding scheme could be applied yet. In this paper, we present ways to seed FELs to generate short wavelength radiation at high repetition rates, making use of tested electron beam manipulation schemes. For our parameter study, we used the parameters of FLASH, the free-electron laser in Hamburg. First simulations are presented, showing the feasibility of the method proposed.

THz streak camera performance for single-shot characterization of XUV pulses with complex temporal structures.

The THz-field-driven streak camera has proven to be a powerful diagnostic-technique that enables the shot-to-shot characterization of the duration and the arrival time jitter of free electron laser (FEL) pulses. Here we investigate the performance of three computational approaches capable to determine the duration of FEL pulses with complex temporal structures from single-shot measurements of up to three simultaneously recorded spectra. We use numerically simulated FEL pulses in order to validate the accuracy of the pulse length retrieval in average as well as in a single-shot mode. We discuss requirements for the THz field strength in order to achieve reliable results and compare our numerical study with the analysis of experimental data that were obtained at the FEL in Hamburg - FLASH.