Free Electron Laser Applications Research Articles

Design and First Characterization of Active and Slim-Edge Planar Detectors for FEL Applications

This paper reports on the development of a dedicated technology for the fabrication of pixelated edgeless sensors to be used in X-ray imaging applications at free electron laser (FEL) facilities. The process was developed with the goal of producing planar sensors suitable for the tight FEL application requirements in terms of collection speed, spatial resolution, and radiation tolerance. At the same time, care has been taken to reduce the dead area at the borders of the sensors, thus minimizing the loss of information and distortion introduced when tiling several dies in a large area imager. Different active-edge and slim-edge terminations, designed with the aid of TCAD simulations, are discussed. Based on numerical simulations, a wafer layout was designed and devices with different configurations were fabricated. The experimental results from the electrical characterization of the produced p-on-n sensors and test structures are presented and discussed.

Normal planar undulators doubling as transverse gradient undulators

The transverse gradient undulator (TGU) has important application in the short-wavelength high-gain free electron lasers (FELs) driven by laser-plasma accelerators. However, the usual transversely tapered TGUs need special design and manufacture, and the transverse gradient cannot be tuned arbitrarily. In this paper we explore a new and simple method of using the natural transverse gradient of a normal planar undulator to compensate the beam energy spread effect. In this method, a vertical dispersion on the electron beam is introduced, then the dispersed beam passes through a normal undulator with a vertical off-axis orbit where the vertical field gradient is selected properly related to the dispersion strength and the beam energy spread. Theoretical analysis and numerical simulations for self-amplified spontaneous emission FELs based on laser plasma accelerators are presented, and indicate that this method can greatly reduce the effect of the beam energy spread, leading to a similar enhancement on FEL performance as the usual transversely tapered TGU, but with the advantages of economy, tunable transverse gradient and no demand of extra field for correcting the orbit deflection induced by the field gradient.Received 21 December 2016DOI:https://doi.org/10.1103/PhysRevAccelBeams.20.020707Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsSynchrotron radiation & free-electron lasersAccelerators & Beams

X-ray spectrometer based on a bent diamond crystal for high repetition rate free-electron laser applications.

A precise spectral characterization of every single pulse is required in many x-ray free-electron laser (XFEL) experiments due to the fluctuating spectral content of self-amplified spontaneous emission (SASE) beams. Bent single-crystal spectrometers can provide sufficient spectral resolution to resolve the SASE spikes while also covering the full SASE bandwidth. To better withstand the high heat load induced by the 4.5 MHz repetition rate of pulses at the forthcoming European XFEL facility, a spectrometer based on single-crystal diamond has been developed. We report a direct comparison of the diamond spectrometer with its Si counterpart in experiments performed at the Linac Coherent Light Source.

Feasibility study of a ``4H'' X-ray camera based on GaAs:Cr sensor

A multilayer stacked X-ray camera concept is described. This type of technology is called `4H' X-ray cameras, where 4H stands for high-Z (Z>30) sensor, high-resolution (less than 300 micron pixel pitch), high-speed (above 100 MHz), and high-energy (above 30 keV in photon energy). The components of the technology, similar to the popular two-dimensional (2D) hybrid pixelated array detectors, consists of GaAs:Cr sensors bonded to high-speed ASICs. 4H cameras based on GaAs also use integration mode of X-ray detection. The number of layers, on the order of ten, is smaller than an earlier configuration for single-photon-counting (SPC) mode of detection [1]. High-speed ASIC based on modification to the ePix family of ASIC is discussed. Applications in X-ray free electron lasers (XFELs), synchrotrons, medicine and non-destructive testing are possible.

X-ray diffraction gratings: Precise control of ultra-low blaze angle via anisotropic wet etching

Diffraction gratings are used from micron to nanometer wavelengths as dispersing elements in optical instruments. At shorter wavelengths, crystals can be used as diffracting elements, but due to the 3D nature of the interaction with light are wavelength selective rather than wavelength dispersing. There is an urgent need to extend grating technology into the x-ray domain of wavelengths from 1 to 0.1 nm, but this requires the use of gratings that have a faceted surface in which the facet angles are very small, typically less than 1°. Small facet angles are also required in the extreme ultra-violet and soft x-ray energy ranges in free electron laser applications, in order to reduce power density below a critical damage threshold. In this work, we demonstrate a technique based on anisotropic etching of silicon designed to produce very small angle facets with a high degree of perfection.

Widening the reach of structural biology.

An exciting future is outlined in which new approaches complementary to conventional X-ray crystallography will substantially widen the reach of structural biology. Primary among these are the recent advances in cryoelectron microscopy and the growing applications of free electron lasers.

Serial femtosecond crystallography opens new avenues for Structural Biology.

Free electron lasers (FELs) provide X-ray pulses in the femtosecond time domain with up to 10(12) higher photon flux than synchrotrons and open new avenues for the determination of difficult to crystallize proteins, like large complexes and human membrane proteins. While the X-ray pulses are so strong that they destroy any solid material, the crystals diffract before they are destroyed. The most successful application of FELs for biology has been the method of serial femtosecond crystallography (SFX) where nano or microcrystals are delivered to the FEL beam in a stream of their mother liquid at room temperature, which ensures the replenishment of the sample before the next X-ray pulse arrives. New injector technology allows also for the delivery of crystal in lipidic cubic phases or agarose, which reduces the sample amounts for an SFX data set by two orders of magnitude. Time-resolved SFX also allows for analysis of the dynamics of biomolecules, the proof of principle being recently shown for light-induced reactions in photosystem II and photoactive yellow protein. An SFX data sets consist of thousands of single crystal snapshots in random orientations, which can be analyzed now "on the fly" by data analysis programs specifically developed for SFX, but de-novo phasing is still a challenge, that might be overcome by two-color experiments or phasing by shape transforms.

Application of mid-infrared free-electron laser tuned to amide bands for dissociation of aggregate structure of protein.

A mid-infrared free-electron laser (FEL) is a linearly polarized, high-peak powered pulse laser with tunable wavelength within the mid-infrared absorption region. It was recently found that pathogenic amyloid fibrils could be partially dissociated to the monomer form by the irradiation of the FEL targeting the amide I band (C=O stretching vibration), amide II band (N-H bending vibration) and amide III band (C-N stretching vibration). In this study, the irradiation effect of the FEL on keratin aggregate was tested as another model to demonstrate an applicability of the FEL for dissociation of protein aggregates. Synchrotron radiation infrared microscopy analysis showed that the α-helix content in the aggregate structure decreased to almost the same level as that in the monomer state after FEL irradiation tuned to 6.06 µm (amide I band). Both irradiations at 6.51 µm (amide II band) and 8.06 µm (amide III band) also decreased the content of the aggregate but to a lesser extent than for the irradiation at the amide I band. On the contrary, the irradiation tuned to 5.6 µm (non-absorbance region) changed little the secondary structure of the aggregate. Scanning-electron microscopy observation at the submicrometer order showed that the angular solid of the aggregate was converted to non-ordered fragments by the irradiation at each amide band, while the aggregate was hardly deformed by the irradiation at 5.6 µm. These results demonstrate that the amide-specific irradiation by the FEL was effective for dissociation of the protein aggregate to the monomer form.

A mechanism of Cu work function reduction in CsBr/Cu photocathodes.

Thin films of CsBr deposited on Cu(100) have been proposed as next-generation photocathode materials for applications in particle accelerators and free-electron lasers. However, the mechanisms underlying an improved photocathode performance as well as their long-term stability remain poorly understood. We present Density Functional Theory (DFT) calculations of the work function reduction following the application of CsBr thin film coatings to Cu photocathodes. The effects of both flat and rough interface and van der Waals forces are examined. Calculations suggest that CsBr films can reduce the Cu(100) work function by about 1.5 eV, which would explain the observed increase in quantum efficiency (QE) of coated vs. uncoated photocathodes. A model explaining the experimentally observed laser activation of photocathodes is provided whereby the photo-induced creation of Br vacancies and Cs-Br di-vacancies and their subsequent diffusion to the Cu/CsBr interface lead to a further increase in QE after a period of laser irradiation.

Production of quasi ellipsoidal laser pulses for next generation high brightness photoinjectors

The use of high brightness electron beams in Free Electron Laser (FEL) applications is of increasing importance. One of the most promising methods to generate such beams is the usage of shaped photocathode laser pulses. It has already demonstrated that temporal and transverse flat-top laser pulses can produce very low emittance beams [1]. Nevertheless, based on beam simulations further improvements can be achieved using quasi-ellipsoidal laser pulses, e.g. 30% reduction in transverse projected emittance at 1nC bunch charge.In a collaboration between DESY, the Institute of Applied Physics of the Russian Academy of Science (IAP RAS) in Nizhny Novgorod and the Joint Institute of Nuclear Research (JINR) in Dubna such a laser system capable of producing trains of laser pulses with a quasi-ellipsoidal distribution, has been developed. The prototype of the system was installed at the Photo Injector Test facility at DESY in Zeuthen (PITZ) and is currently in the commissioning phase.In the following, the laser system will be introduced, the procedure of pulse shaping will be described and the last experimental results will be shown.

Effect of slope errors on the performance of mirrors for x-ray free electron laser applications.

In this work we point out that slope errors play only a minor role in the performance of a certain class of x-ray optics for X-ray Free Electron Laser (XFEL) applications. Using physical optics propagation simulations and the formalism of Church and Takacs [Opt. Eng. 34, 353 (1995)], we show that diffraction limited optics commonly found at XFEL facilities posses a critical spatial wavelength that makes them less sensitive to slope errors, and more sensitive to height error. Given the number of XFELs currently operating or under construction across the world, we hope that this simple observation will help to correctly define specifications for x-ray optics to be deployed at XFELs, possibly reducing the budget and the timeframe needed to complete the optical manufacturing and metrology.

Radiation hardness assessment of the charge-integrating hybrid pixel detector JUNGFRAU 1.0 for photon science.

JUNGFRAU (adJUstiNg Gain detector FoR the Aramis User station) is a two-dimensional hybrid pixel detector for photon science applications in free electron lasers, particularly SwissFEL, and synchrotron light sources. JUNGFRAU is an automatic gain switching, charge-integrating detector which covers a dynamic range of more than 10(4) photons of an energy of 12 keV with a good linearity, uniformity of response, and spatial resolving power. The JUNGFRAU 1.0 application-specific integrated circuit (ASIC) features a 256 × 256 pixel matrix of 75 × 75 μm(2) pixels and is bump-bonded to a 320 μm thick Si sensor. Modules of 2 × 4 chips cover an area of about 4 × 8 cm(2). Readout rates in excess of 2 kHz enable linear count rate capabilities of 20 MHz (at 12 keV) and 50 MHz (at 5 keV). The tolerance of JUNGFRAU to radiation is a key issue to guarantee several years of operation at free electron lasers and synchrotrons. The radiation hardness of JUNGFRAU 1.0 is tested with synchrotron radiation up to 10 MGy of delivered dose. The effect of radiation-induced changes on the noise, baseline, gain, and gain switching is evaluated post-irradiation for both the ASIC and the hybridized assembly. The bare JUNGFRAU 1.0 chip can withstand doses as high as 10 MGy with minor changes to its noise and a reduction in the preamplifier gain. The hybridized assembly, in particular the sensor, is affected by the photon irradiation which mainly shows as an increase in the leakage current. Self-healing of the system is investigated during a period of 11 weeks after the delivery of the radiation dose. Annealing radiation-induced changes by bake-out at 100 °C is investigated. It is concluded that the JUNGFRAU 1.0 pixel is sufficiently radiation-hard for its envisioned applications at SwissFEL and synchrotron beam lines.

Phase-only synthesis of ultrafast stretched square pulses.

We report the generation of square temporal-shape pulses with no loss of spectral bandwidth, using an analytic expression for the spectral phase modulation dependent only on the input spectrum and stretching factor. We demonstrate numerically and experimentally conversion of 40fs pulses into 150 times longer flat top pulses with sharp on and off fronts. Applications in pulse amplification and free electron lasers are considered.

Microfluidic sorting of protein nanocrystals by size for X-ray free-electron laser diffraction.

The advent and application of the X-ray free-electron laser (XFEL) has uncovered the structures of proteins that could not previously be solved using traditional crystallography. While this new technology is powerful, optimization of the process is still needed to improve data quality and analysis efficiency. One area is sample heterogeneity, where variations in crystal size (among other factors) lead to the requirement of large data sets (and thus 10–100 mg of protein) for determining accurate structure factors. To decrease sample dispersity, we developed a high-throughput microfluidic sorter operating on the principle of dielectrophoresis, whereby polydisperse particles can be transported into various fluid streams for size fractionation. Using this microsorter, we isolated several milliliters of photosystem I nanocrystal fractions ranging from 200 to 600 nm in size as characterized by dynamic light scattering, nanoparticle tracking, and electron microscopy. Sorted nanocrystals were delivered in a liquid jet via the gas dynamic virtual nozzle into the path of the XFEL at the Linac Coherent Light Source. We obtained diffraction to ∼4 Å resolution, indicating that the small crystals were not damaged by the sorting process. We also observed the shape transforms of photosystem I nanocrystals, demonstrating that our device can optimize data collection for the shape transform-based phasing method. Using simulations, we show that narrow crystal size distributions can significantly improve merged data quality in serial crystallography. From this proof-of-concept work, we expect that the automated size-sorting of protein crystals will become an important step for sample production by reducing the amount of protein needed for a high quality final structure and the development of novel phasing methods that exploit inter-Bragg reflection intensities or use variations in beam intensity for radiation damage-induced phasing. This method will also permit an analysis of the dependence of crystal quality on crystal size.

Numerical study of the laser-tip coupling in surface plasmon assisted stacked-double-gate field emitter arrays

Recently, sub-wavelength-pitch stacked double-gate metal nanotip arrays have been proposed to realize high current, high brightness electron bunches for ultrabright cathodes for x-ray free-electron laser applications. With the proposed device structure, ultrafast field emission of photoexcited electrons is efficiently driven by vertical incident near infrared laser pulses, via near field coupling of the surface plasmon polariton resonance of the gate electrodes with the nanotip apex. In this work, in order to gain insight in the underlying physical processes, the authors report detailed numerical studies of the proposed device. The results indicate the importance of the interaction of the double-layer surface plasmon polariton, the position of the nanotip, as well as the incident angle of the near infrared laser pulses.

Femtosecond all-optical synchronization of an X-ray free-electron laser.

Many advanced applications of X-ray free-electron lasers require pulse durations and time resolutions of only a few femtoseconds. To generate these pulses and to apply them in time-resolved experiments, synchronization techniques that can simultaneously lock all independent components, including all accelerator modules and all external optical lasers, to better than the delivered free-electron laser pulse duration, are needed. Here we achieve all-optical synchronization at the soft X-ray free-electron laser FLASH and demonstrate facility-wide timing to better than 30 fs r.m.s. for 90 fs X-ray photon pulses. Crucially, our analysis indicates that the performance of this optical synchronization is limited primarily by the free-electron laser pulse duration, and should naturally scale to the sub-10 femtosecond level with shorter X-ray pulses.

Two way street - complementary methods.

Two way street - complementary methods.

Design and fabrication of PPM and hybrid undulator for free electron laser applications

Two undulators for free electron laser applications are under fabrication at Insertion Device Development & Measurement Laboratory, DAVV, Indore. The first one is of PPM type with twenty periods, 50 mm each period. The second one is a hybrid undulator with twenty five periods, 20 mm each period. NdFeB magnets of rectangular cross section 12.5mm, 12.5mm & 6.25 mm, 6.25 mm will be used for the undulators respectively. Each magnet length is of 50 mm. Each magnet is scanned by a laser micrometer to ensure gap uniformity along the length of the undulator. In this paper we describe the technical design details of both the undulators.

Electron rearrangement dynamics in dissociating I(2)^(n+) molecules accessed by extreme ultraviolet pump-probe experiments.

The charge rearrangement in dissociating I_{2}^{n+} molecules is measured as a function of the internuclear distance R using extreme ultraviolet pulses delivered by the free-electron laser in Hamburg. Within an extreme ultraviolet pump-probe scheme, the first pulse initiates dissociation by multiply ionizing I_{2}, and the delayed probe pulse further ionizes one of the two fragments at a given time, thus triggering charge rearrangement at a well-defined R. The electron transfer between the fragments is monitored by analyzing the delay-dependent ion kinetic energies and charge states. The experimental results are in very good agreement with predictions of the classical over-the-barrier model demonstrating its validity in a thus far unexplored quasimolecular regime relevant for free-electron laser, plasma, and chemistry applications.

Research at the NSF STC for Biology with X-ray lasers

"The NSF BioXFEL Science and Technology Center (STC) is a new consortium of six research campuses devoted to the application of x-ray free-electron lasers (XFELs) to structural biology. Over the last four years a variety of approaches have been made to the observation of protein structure and dynamics for various classes of proteins. The Linac Coherent Light source at SLAC, the first hard-Xray EXFEL, provides intense coherent hard X-ray pulses at 120 Hz which vaporize protein when focussed to a sub-micron beam. Atomic-resolution Bragg diffraction patterns are nevertheless obtained using 50 fs pulses prior to the onset of significant damage, in this ""diffract-then-destroy"" mode, which outruns radiation damage. This use of short pulses instead of freezing samples to reduce radiation damage therefore opens the way to the study of protein dynamics at room temperature in a native environment. I'll review the work of several groups using a range of approaches to different types of sample, including the following: 1. Differences between the frozen sychrotron structure of GPCR proteins and the RT XFEL structure [1]. 2. Pump-probe dynamic structures in Photosynthesis [2]. 3. XFEL study of 2D protein crystals [3]. 4. Prospects for improved resolution in XFEL imaging from single particles such as viruses, where patterns can be obtained from a single virus. 5. New ideas - the Lipid Cubic Phase injector (which allows protein nanocrystals to be studied also at sychrotrons) [4], prospects for fast Laue diffraction using coherent attosecond X-ray lasers, ab-initio phasing [5], the use of angular correlation functions for analysis of fast solution scattering, and two-color opportunites for serial femtosecond crystallography (SFX). See [6] for a recent review of the field. 1. W.Liu et al Science 342, 1521 (2013) 2. A.Aquila et al Optics Express 20, 2706 (2012) 3. M.Frank et al IUCrJ (2014) In press. 4. U.Weierstall et al Nature Comms. (2014) In press. 5. J. Spence et al Optics Express 19, 2866 (2011). 6. J. Spence et al Rep. Prog. Phys. 75, 102601 (2012)."