High Peak Brilliance Research Articles

Progress and Perspectives of Spectroscopic Studies on Carbon K-Edge Using Novel Soft X-ray Pulsed Sources

The development of novel coherent and brilliant sources, such as soft X-ray free electron laser (FEL) and high harmonic generation (HHG), enables new ultrafast analysis of the electronic and structural dynamics of a wide variety of materials. Soft X-ray FEL delivers high-brilliance beams with a short pulse duration, high spatial coherence and photon energy tunability. In comparison with FELs, HHG X-ray sources are characterized by a wide spectral bandwidth and few- to sub-femtosecond pulses. The approach will lead to the time-resolved reconstruction of molecular dynamics, shedding light on different photochemical pathways. The high peak brilliance of soft X-ray FELs facilitates investigations in a nonlinear regime, while the broader spectral bandwidth of the HHG sources may provide the simultaneous probing of multiple components. Significant technical breakthroughs in these novel sources are under way to improve brilliance, pulse duration, and to control spectral bandwidth, spot size, and energy resolution. Therefore, in the next few years, the new generation of soft X-ray sources combined with novel experimental techniques, new detectors, and computing capabilities will allow for the study of several extremely fast dynamics, such as vibronic dynamics. In the present review, we discuss recent developments in experiments, performed with soft X-ray FELs and HHG sources, operating near the carbon K-absorption edge, being a key atomic component in biosystems and soft materials. Different spectroscopy methods such as time-resolved pump-probe techniques, nonlinear spectroscopies and photoelectron spectroscopy studies have been addressed in an attempt to better understand fundamental physico-chemical processes.

PIONEER, a high-resolution single-crystal polarized neutron diffractometer.

PIONEER is a high Q-resolution, single-crystal, polarized neutron diffractometer at the Second Target Station (STS), Oak Ridge National Laboratory. It will provide the unprecedented capability of measuring tiny crystals (0.001 mm3, i.e., x-ray diffraction size), ultra-thin films (10nm thickness), and weak structural and magnetic transitions. PIONEER benefits from the increased peak brightness of STS cold-neutron sources and uses advanced Montel mirrors that are able to deliver a focused beam with a high brilliance transfer, a homogeneous profile, and a low background. Monte Carlo simulations suggest that the optimized instrument has a high theoretical peak brilliance of 2.9 × 1012 n cm-2 sr-1 Å-1 s-1 at 2.5 Å at the sample position, within a 5 × 5mm2 region and a ±0.3° divergence range. The moderator-to-sample distance is 60m, providing a nominal wavelength band of 4.3 Å with a wavelength resolution better than 0.2% in the wavelength range of 1.0-6.0 Å. PIONEER is capable of characterizing large-scale periodic structures up to 200 Å. With a sample-to-detector distance of 0.8m, PIONEER accommodates various sample environments, including low/high temperature, high pressure, and high magnetic/electric field. A large cylindrical detector array (4.0 sr) with a radial collimator is planned to suppress the background scattering from sample environments. Bottom detector banks provide an additional 0.4 sr coverage or can be removed if needed to accommodate special sample environments. We present virtual experimental results to demonstrate the scientific performance of PIONEER in measuring tiny samples.

Simulation Study of a Bright Attosecond γ-ray Source Generation by Irradiating an Intense Laser on a Cone Target

The interaction between an ultrastrong laser and a cone-like target is an efficient approach to generate high-power radiations such as attosecond pulses and terahertz waves. The objective is to study the γ-ray generation under this configuration with the help of 2D particle-in-cell simulations. It is deciphered that electrons experience three stages, including injection, acceleration and scattering, to emit high-energy photons via nonlinear Compton scattering (NCS). These spatial-separated attosecond γ-ray pulses own high peak brilliance (>1022 photons/(s·mm2·mrad2·0.1%BW)) and high energy (6 MeV) under the case of normalized laser intensity a0=30(I=2×1021 W/cm2). In addition, the cone target turns out to be an order of magnitude more efficient in energy transfer compared to a planar one.

Converting conventional electron accelerators to high peak brilliance Compton light sources

Several proposals have been put forward for converting electron accelerators to inverse Compton scattering (ICS) gamma sources. Typical approaches suggest combining near-IR solid-state lasers operating continuously at a multimegahertz repetition rate with e-beams when setting their interaction point inside a field-enhancement, Fabry-Perot optical cavity. We introduce here an alternative method of pairing particle accelerator beams with trains of long-wave-infrared, ${\ensuremath{\lambda}}_{L}\ensuremath{\approx}9--11\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ pulses from a picosecond ${\mathrm{CO}}_{2}$ laser of a novel architecture operating in a repetitive pulse-burst mode. Because of a considerable increase in the laser energy per pulse, combined with an order-of-magnitude higher number of laser photons per joule of laser energy, our approach allows us to increase the ICS peak flux and brilliance by 4 orders of magnitude compared to previous proposals while maintaining high (${10}^{11}--{10}^{12}\text{ }\text{ }\mathrm{ph}/\mathrm{s}$) average flux. This outcome is supported by the examples of the $\mathrm{DA}\mathrm{\ensuremath{\Phi}}\mathrm{NE}$ and CBETA accelerator facilities, where ${10}^{20}--{10}^{21}\text{ }\text{ }\mathrm{ph}/(\mathrm{s}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{mm}}^{2}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{mrad}}^{2}\ifmmode\cdot\else\textperiodcentered\fi{}0.1%BW)$ peak brilliances at 50--1000 keV photon energy range can be achieved and is comparable or exceeds the capabilities of contemporary synchrotron light sources at hard x rays. Such high-brightness ICS sources will find applications in pump-probe and other ultrafast studies that require building up meaningful datasets on a single x-ray pulse.

Generation of bright γ-ray/hard x-ray flash with intense femtosecond pulses and double-layer targets

We study a method to generate collimated γ-ray/hard X-ray flash with high energy conversion efficiency and high peak brilliance by shooting a double-layer target with a readily available ultra-intense laser pulse. The target is composed of a homogeneous tens-of-micrometer-thick slightly underdense plasma (SUP) slab in front of an ultrathin foil. For optimal targets, particle-in-cell numerical simulations reveal that nonlinear Thomson scattering from directly accelerated superponderomotive electrons leads to the generation of a γ-ray flash with a peak brilliance of 1.6×1022 photons/s/mm2/0.1%BW at the MeV energy level at a laser intensity of 8.5×1020W/cm2. The dependence of the emission on the target parameters is systematically studied. Compared with using slightly overdense plasma, the use of SUP results in the enhancement of the photon number and brightness of the γ-ray flash by a factor of 10 and 40, respectively. The underlying physics of the enhancement is discussed.

An inverse free electron laser acceleration-driven Compton scattering X-ray source

The generation of X-rays and γ-rays based on synchrotron radiation from free electrons, emitted in magnet arrays such as undulators, forms the basis of much of modern X-ray science. This approach has the drawback of requiring very high energy, up to the multi-GeV-scale, electron beams, to obtain the required photon energy. Due to the limit in accelerating gradients in conventional particle accelerators, reaching high energy typically demands use of instruments exceeding 100’s of meters in length. Compact, less costly, monochromatic X-ray sources based on very high field acceleration and very short period undulators, however, may enable diverse, paradigm-changing X-ray applications ranging from novel X-ray therapy techniques to active interrogation of sensitive materials, by making them accessible in energy reach, cost and size. Such compactness and enhanced energy reach may be obtained by an all-optical approach, which employs a laser-driven high gradient accelerator based on inverse free electron laser (IFEL), followed by a collision point for inverse Compton scattering (ICS), a scheme where a laser is used to provide undulator fields. We present an experimental proof-of-principle of this approach, where a TW-class CO2 laser pulse is split in two, with half used to accelerate a high quality electron beam up to 84 MeV through the IFEL interaction, and the other half acts as an electromagnetic undulator to generate up to 13 keV X-rays via ICS. These results demonstrate the feasibility of this scheme, which can be joined with other techniques such as laser recirculation to yield very compact photon sources, with both high peak and average brilliance, and with energies extending from the keV to MeV scale. Further, use of the IFEL acceleration with the ICS interaction produces a train of high intensity X-ray pulses, thus enabling a unique tool synchronized with a laser pulse for ultra-fast strobe, pump-probe experimental scenarios.

Sample delivery for serial crystallography at free-electron lasers and synchrotrons.

The high peak brilliance and femtosecond pulse duration of X-ray free-electron lasers (XFELs) provide new scientific opportunities for experiments in physics, chemistry and biology. In structural biology, one of the major applications is serial femtosecond crystallography. The intense XFEL pulse results in the destruction of any exposed microcrystal, making serial data collection mandatory. This requires a high-throughput serial approach to sample delivery. To this end, a number of such sample-delivery techniques have been developed, some of which have been ported to synchrotron sources, where they allow convenient low-dose data collection at room temperature. Here, the current sample-delivery techniques used at XFEL and synchrotron sources are reviewed, with an emphasis on liquid injection and high-viscosity extrusion, including their application for time-resolved experiments. The challenges associated with sample delivery at megahertz repetition-rate XFELs are also outlined.

Megahertz data collection from protein microcrystals at an X-ray free-electron laser

X-ray free-electron lasers (XFELs) enable novel experiments because of their high peak brilliance and femtosecond pulse duration. However, non-superconducting XFELs offer repetition rates of only 10–120 Hz, placing significant demands on beam time and sample consumption. We describe serial femtosecond crystallography experiments performed at the European XFEL, the first MHz repetition rate XFEL, delivering 1.128 MHz X-ray pulse trains at 10 Hz. Given the short spacing between pulses, damage caused by shock waves launched by one XFEL pulse on sample probed by subsequent pulses is a concern. To investigate this issue, we collected data from lysozyme microcrystals, exposed to a ~15 μm XFEL beam. Under these conditions, data quality is independent of whether the first or subsequent pulses of the train were used for data collection. We also analyzed a mixture of microcrystals of jack bean proteins, from which the structure of native, magnesium-containing concanavalin A was determined.

Pump laser-induced space-charge effects in HHG-driven time- and angle-resolved photoelectron spectroscopy

With the advent of ultrashort-pulsed extreme ultraviolet sources, such as free-electron lasers or high-harmonic-generation (HHG) sources, a new research field for photoelectron spectroscopy has opened up in terms of femtosecond time-resolved pump-probe experiments. The impact of the high peak brilliance of these novel sources on photoemission spectra, so-called vacuum space-charge effects caused by the Coulomb interaction among the photoemitted probe electrons, has been studied extensively. However, possible distortions of the energy and momentum distributions of the probe photoelectrons caused by the low photon energy pump pulse due to the nonlinear emission of electrons have not been studied in detail yet. Here, we systematically investigate these pump laser-induced space-charge effects in a HHG-based experiment for the test case of highly oriented pyrolytic graphite. Specifically, we determine how the key parameters of the pump pulse—the excitation density, wavelength, spot size, and emitted electron energy distribution—affect the measured time-dependent energy and momentum distributions of the probe photoelectrons. The results are well reproduced by a simple mean-field model, which could open a path for the correction of pump laser-induced space-charge effects and thus toward probing ultrafast electron dynamics in strongly excited materials.

New opportunities in structural biology with XFEL: from sources to structures

X-ray free electron lasers (XFEL) have shown the promise of providing new opportunities in structural biology research with their extremely high peak brilliance and short pulses. It is reaching the stage where biologically important questions can be tackled using XFEL based on the "diffract-before-destroy" concept. The first part of this presentation will focus on macromolecular crystallography using XFEL with results obtained at LCLS so far and future scope. R&D efforts being pursued at SLAC/LCLS include new beam modes, (two-color beam for de novo phasing, wider bandwidth for SAXS/WAXS and spectroscopy), beam multiplexing, a dedicated new station for in-air data collection, next generation detectors, data analysis incorporating pulse-by-pulse spectrometer measurements and post refinement. These projects are being pursued in collaboration with many groups locally and globally with a goal to provide integrated facilities for cutting edge structural biology research. For example, two-color self-seeded XFEL mode is being developed for simultaneous recording of diffraction data at two energies in order to optimize the dispersive difference between the two wavelengths for phasing. Another area of collaborative effort is a development of dedicated station for in-air data collection with a variety of sample delivery schemes. The second part will discuss a possible roadmap towards atomic resolution single particle imaging using XFEL. Here, key questions are ·Can XFEL single particle 3D structural analysis at atomic resolution be done? ·What is the pulse characteristics required? ·Can we overcome the radiation damage at soft X-ray regime? ·What is the highest resolution attainable in comparison with cryoEM? A workshop at LCLS is being organized to discuss these questions in 4 areas: radiation damage, image reconstruction algorithm, beam modes and instrumentation, sample delivery and heterogeneity. The outcome of the workshop and follow-up discussions will be presented.

Room Temperature Serial Crystallography at Synchrotrons

Serial Femtosecond Crystallography (SFX) is the most commonly used method for the emerging structure determination at X-ray free-electron lasers (FELs). The high peak brilliance of the FEL and the possibility of using femtosecond pulses afford use of nano-to-micron sized crystals in a diffraction-before-destruction approach for the acquisition of high-resolution undamaged diffraction data [1]. The crystals are obliterated upon exposure to an FEL X-ray pulse so only a single snapshot can be collected per crystal, necessitating a constant supply of fresh crystals. The crystals are therefore injected in a liquid microjet [2], [3]. We show that this serial method of data collection and the associated data analysis can be successfully adapted to serial crystallography (SX) measurements at synchrotrons, enabling room temperature studies using the unattenuated beam. Given the continuous supply of fresh crystals, the full tolerable dose can be used for each single crystal exposure, permitting analysis of small or weakly scattering crystals. FEL X-ray pulses are much shorter than the fraction of a second exposure time at a synchrotron, so SFX injection conditions are modified in SX such as to slow down the typically fast travelling crystals. By embedding the crystals in a viscous material the crystals remain in the beam long enough to yield measurable diffraction and smearing out of the diffraction peaks due to crystal tumbling is avoided. We demonstrate the successful application of room temperature SX at the Swiss Light Source at ambient pressure. Our experimental setup allows collection of both still and rotation data. Recent progress using model systems will be presented, establishing this high throughput, high dose rate approach as a new route to structure determination of macromolecules in their native environment and at room temperature.

Structural biology at the European X-ray free-electron laser facility.

The European X-ray free-electron laser (XFEL) facility, under construction in the Hamburg region, will provide high-peak brilliance (greater than 10(33) photons s(-1) mm(-2) mrad(-2) per 0.1% BW), ultrashort pulses (approx. 10 fs) of X-rays, with a high repetition rate (up to 27 000 pulses s(-1)) from 2016 onwards. The main features of this exceptional X-ray source, and the instrumentation developments necessary to exploit them fully, for application to a variety of scientific disciplines, are briefly summarized. In the case of structural biology, that has a central role in the scientific case of this new facility, the instruments and ancillary laboratories that are being planned and built within the baseline programme of the European XFEL and by consortia of users are also discussed. It is expected that the unique features of the source and the advanced features of the instrumentation will allow operation modes with more efficient use of sample materials, faster acquisition times, and conditions better approaching feasibility of single molecule imaging.

Compton scattering for spectroscopic detection of ultra-fast, high flux, broad energy range X-rays

Compton side-scattering has been used to simultaneously downshift the energy of keV to MeV energy range photons while attenuating their flux to enable single-shot, spectrally resolved, measurements of high flux X-ray sources to be undertaken. To demonstrate the technique a 1 mm thick pixelated cadmium telluride detector has been used to measure spectra of Compton side-scattered radiation from a Cobalt-60 laboratory source and a high flux, high peak brilliance X-ray source of betatron radiation from a laser-plasma wakefield accelerator.

The European X-ray free-electron laser facility in Hamburg

Activities on free-electron laser (FEL) X-ray sources, based on linear accelerators, to produce spatially coherent, ultra-short (∼100 fs) pulses with very high peak brilliance (10 28–10 32 photons/s/mm 2/mrad 2/0.1% BW) are summarized, with special emphasis on the European XFEL project in Hamburg. The scientific case includes time-resolved studies of dynamics on sub-ps scales, structural studies by imaging of non-periodic systems, and investigation of high energy-density phenomena such as non-linear X-ray optics and the production of warm dense matter. Examples are presented, with reference to the experience gained on the presently operational facilities, FLASH at DESY, Hamburg and LCLS in Stanford, California.

From 3rd- to 4th-generation light sources: Free-electron lasers in the X-ray range

Activities on free-electron laser (FEL) x-ray sources, based on linear accelerators, to produce spatially coherent, ultra-short (∼100 fs) pulses with very high peak brilliance (1028–1032 photons/s/mm2/mrad2/0.1% BW) are summarized. The scientific case includes time-resolved studies of dynamics on sub-ps scales, structural studies by imaging of non-periodic systems, and investigation of high energy-density phenomena such as non-linear x-ray optics and the production of warm dense matter. Examples are presented, with emphasis on the operational facilities, FLASH at DESY, Hamburg and LCLS in Stanford, California, and on the European XFEL project in Hamburg.

High quality electron beams from a laser wakefield accelerator

High quality electron beams have been produced in a laser-plasma accelerator driven by femtosecond laser pulses with a peak power of 26 TW. Electrons are produced with an energy up to 150 MeV from the 2 mm gas jet accelerator and the measured rms relative energy spread is less than 1%. Shot-to-shot stability in the central energy is 3%. Pepper-pot measurements have shown that the normalized transverse emittance is ∼1π mm mrad while the beam charge is in the range 2–10 pC. The generation of high quality electron beams is understood from simulations accounting for beam loading of the wakefield accelerating structure. Experiments and self-consistent simulations indicate that the beam peak current is several kiloamperes. Efficient transportation of the beam through an undulator is simulated and progress is being made towards the realization of a compact, high peak brilliance free-electron laser operating in the vacuum ultraviolet and soft x-ray wavelength ranges.

Characterization and applications of a tunable, laser-based, MeV-class Compton-scatteringγ-ray source

A high peak brilliance, laser-based Compton-scattering $\ensuremath{\gamma}$-ray source, capable of producing quasimonoenergetic photons with energies ranging from 0.1 to 0.9 MeV has been recently developed and used to perform nuclear resonance fluorescence (NRF) experiments. Techniques for characterization of $\ensuremath{\gamma}$-ray beam parameters are presented. The key source parameters are the size ($0.01\text{ }\text{ }{\mathrm{mm}}^{2}$), horizontal and vertical divergence ($6\ifmmode\times\else\texttimes\fi{}10\text{ }\text{ }{\mathrm{mrad}}^{2}$), duration (16 ps), and spectrum and intensity (${10}^{5}\text{ }\text{ }\mathrm{photons}/\mathrm{shot}$). These parameters are summarized by the peak brilliance, $1.5\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }\text{ }\mathrm{photons}/{\mathrm{mm}}^{2}/{\mathrm{mrad}}^{2}/\mathrm{s}/0.1%$ bandwidth, measured at 478 keV. Additional measurements of the flux as a function of the timing difference between the drive laser pulse and the relativistic photoelectron bunch, $\ensuremath{\gamma}$-ray beam profile, and background evaluations are presented. These results are systematically compared to theoretical models and computer simulations. NRF measurements performed on $^{7}\mathrm{Li}$ in LiH demonstrate the potential of Compton-scattering photon sources to accurately detect isotopes in situ.

The adaptive gain integrating pixel detector AGIPD a detector for the European XFEL

The European X-ray free electron laser is a new research facility currently under construction in Hamburg, Germany. Typical for XFEL machines is the high peak brilliance several orders of magnitudes above existing synchrotron facilities. With a pulse length below 100 fs and an extremely high luminosity of 30,000 flashes per second the European XFEL will have a worldwide unique time structure that enables researchers to record movies of ultrafast processes. This demands the development of new detectors tailored to the requirements imposed by the experiments while complying with the machine specific operation parameters. The adaptive gain integrating pixel detector (AGIPD) is one response to the need for large 2D detectors, able to cope with the 5 MHz repetition rate, as well as with the high dynamic range needed by XFEL experiments (from single photons to 10 4 12 keV photons per pixel per pulse). In addition, doses up to 1 GGy over three years are expected.

Performance simulation of a detector for 4th generation photon sources: The AGIPD

Future 4th generation photon sources, such as the European XFEL based in Hamburg, will deliver around 10 12 X-ray photons in less than 100 fs with full lateral coherence. These new sources will offer unprecedented possibilities in photon science. The high peak brilliance, combined with a 5 MHz repetition rate poses very high demands for the 2D detectors. In order to provide appropriate detectors during XFEL startup, three dedicated development projects have been initiated, one of them being the Adaptive Gain Integrating Pixel Detector (AGIPD) project which is a collaborative effort between DESY, PSI, University of Bonn, and University of Hamburg. An essential part of the AGIPD project is the development of a simulation tool for the complete detection system. The simulation tool as well as preliminary simulations of the detector characteristics is presented.

X radiation sources based on accelerators

Light sources based on accelerators aim at producing very high brilliance coherent radiation, tuneable from the infrared to X-ray range, with picosecond or femtosecond light pulses. The first synchrotron light sources were built around storage rings in which a large number of relativistic electrons produce “synchrotron radiation” when their trajectory is subjected to a magnetic field, either in bending magnets or in specific insertion devices (undulators), made of an alternating series of magnets, allowing the number of curvatures to be increased and the radiation to be reinforced. These “synchrotron radiation” storage rings are now used worldwide (there are more than thirty), and they simultaneously distribute their radiation to several tens of users around the storage ring. The most effective installations in term of brilliance are the so-called 3rd generation synchrotron radiation light sources. The radiation produced presents pulse durations of the order of a few tens of ps, at a high rate (of the order of MHz); it is tuneable over a large range, depending on the magnetic field and the electron beam energy and its polarisation is adjustable (in the VUV-soft-X range). Generally, a very precise spectral selection is made by the users with a monochromator. The single pass linear accelerators can produce very short electron bunches ( ∼ 100 femtosecond ). The beam of very high electronic density is sent into successive undulator modules, reinforcing the radiation's longitudinal coherence, produced according to a Free Electron Laser (FEL) scheme by the interaction between the electron bunch and a light wave. The very high peak brilliance justifies their designation as 4th generation sources. The number of users is smaller because an electron pulse produces a radiation burst towards only one beamline. Energy Recovery Linacs (ERL) let the beam pass several times in the accelerator structures either to recover the energy or to accelerate the electrons during several turns, and thus provide subpicosecond beams for a greater number of users. A state-of-the-art of X sources using conventional (and not laser plasma based) accelerators is given here, underlying the performance already reached or forecast and the essential challenges. To cite this article: M.-E. Couprie, J.-M. Filhol, C. R. Physique 9 (2008).