Free-electron Laser Radiation Research Articles

The Radiation Beamline of Novosibirsk Free-Electron Laser Facility Operating in Terahertz, Far-Infrared, and Mid-Infrared Ranges

Unlike synchrotrons with multiple beamlines, free-electron lasers (FELs) are single-beam facilities, which nevertheless have a number of endstations. The latter requires development of complex radiation transport line, which should be efficient enough to avoid scaling down the FEL capabilities. Keeping the beam shape and radiation power level along the beamline is a challenge because the total length of the FEL radiation transport line can exceed a hundred meters. The FELs around the world differ from each other in both radiation parameters and endstations’ layout requiring individual design of their radiation transport lines. In this work, we describe the 120-m beamline for transporting the radiation of the Novosibirsk FEL facility, consisting of three FELs of the terahertz (THz), far-infrared, and mid-infrared ranges. The radiation of these three FELs is directed to the same beam transport channel, which is able to deliver the radiation to any of 14 endstations already commissioned. To compare the expected beam parameters with the actual ones, the radiation intensity distribution was measured in a number of places of the beamline that are THz radiation outputs for some endstations. Possible causes of the parameters’ mismatching observed for distant endstations are discussed. The problem of radiation absorption by water vapor is considered in detail.

Effects of the FEL Fluctuations on the 2s2p Li+ Auto-Ionization Lineshape

The photoionization of Lithium (Li+) via its doubly-excited state 2s2p 1P in intense free electron laser (FEL) radiation is studied. A recently developed perturbative statistical description of the atomic dynamics is used to calculate the ionization yield. It is observed that the FEL temporal fluctuations affect the lineshape significantly, strongly dependent on the product of the pulse’s coherence time with its intensity, ∼τcI0, which is a measure of the effect of the field in one correlation time. The weak-field long-pulse asymmetric resonant Fano-profile is broadened to resemble a Voight profile. As the intensity increases, the subsequent ionization of Li2+ takes over and causes further distortion of the lineshape for Li+.

Soft X-ray spectrometers based on aperiodic reflection gratings and their application

This paper is concerned with the history, properties, development, application, and prospects of soft X-ray (2 – 300 Å) VLS spectrometers, i.e., spectrometers with reflection diffraction gratings whose spacing varies monotonically across the aperture according to a prescribed law (so-called Varied Line-Space (VLS) gratings). An important feature of grazing-incidence VLS spectrometers is that the spectrum is formed on a nearly flat surface perpendicular (or slightly inclined) to the diffracted beams, making them perfectly compatible with modern CCD detectors. VLS spectrometers are employed for the spectroscopy of laboratory and astrophysical plasmas, including the diagnostics of relativistic laser-produced plasmas, for measuring the linewidth of an X-ray laser, for recording the high-order harmonics of laser radiation, and for recording the emission of fast electric discharges and other laboratory X-ray sources. Instruments with VLS gratings are employed to advantage in reflectometry/metrology, X-ray fluorescence analysis, and microscopy with the use of synchrotron, free-electron laser, and laser-produced plasma radiation, as well as in SXR emission spectroscopy, combined with an electron microscope (SXES). Recent years have seen the active development of VLS spectrometers dedicated to the investigation of the electronic structure of different materials and molecules by resonant inelastic X-ray scattering (RIXS) spectroscopy with synchrotron radiation. Among recent trends is the development of VLS gratings with a multilayer reflective coating and extension of the operating spectral range towards ‘tender’ X-rays (ℏω ∼ 1.5 – 6 keV), some projects aiming to achieve a resolving power λ/δ λ ∼ 105 in the region ℏω ∼ 1 keV.

Do Xylylenes Isomerize in Pyrolysis?

We report infrared spectra of xylylene isomers in the gas phase, using free electron laser (FEL) radiation. All xylylenes were generated by flash pyrolysis. The IR spectra were obtained by monitoring the ion dip signal, using a IR/UV double resonance scheme. A gas phase IR spectrum of para‐xylylene was recorded, whereas ortho‐ and meta‐xylylene were found to partially rearrange to benzocyclobutene and styrene. Computations of the UV oscillator strength for all molecules were carried out and provde an explanation for the observation of the isomerization products.

Design of an online spectrometer for the diagnosis of free-electron lasers.

A self-amplified spontaneous emission free-electron laser (FEL) is under construction at the Shanghai Soft X-ray Free-Electron Facility. Therefore, it is necessary to develop a suitable diagnostic tool capable of resolving the natural emission band of each FEL pulse. Thus, an online spectrometer with a plane mirror and plane variable-line-spacing grating at grazing incidence to monitor each single FEL pulse during the propagation of FEL radiation has been designed and is presented in this work. The method of ray tracing is used for monitoring incident radiation in order to understand spectral characteristics, and SHADOW, an X-ray optics simulation tool, and SRW, an X-ray optics wavefront tool, are applied to study the resolving power and focusing properties of the grating. The designed resolving power is ∼3 × 104 at 620 eV. Meanwhile, the effect of the actual slope error of mirrors on the ray-tracing results is also discussed. In order to provide further optimization for the choice of grating, a comparison of resolving powers between 2000 lines mm-1 and 3000 lines mm-1 gratings at different energies is analyzed in detail and radiation damage of mirrors as well as parameters such as the first-order diffraction angle β, the exit-arm length r2, and the tilt angle θ between the focal plane and the diffraction arm are studied and optimized. This work has provided comprehensive designing methods and detailed data for the design of diagnostic spectrometers in soft X-ray FELs and will be favorable to the design of other similar instruments.

Undulator and free-electron laser radiation with field harmonics and off-axis effects taken into account analytically

We present an analytic description of the generation of undulator radiation (UR) harmonics in a generalized elliptic undulator with field harmonics. The obtained analytic expressions for generalized Bessel and Airy functions describe the UR spectral lines and radiation intensity in the general and particular cases of two-frequency planar and helical undulators and also for other elliptic and planar undulators. We analytically describe the effect of finite electron beam size, beam emittance, off-axis deflection of electrons, electron energy spread, and beam-bending permanent magnetic components. The obtained analytic expressions distinguish the contributions made by each field component and by the beam and undulator parameters to the generation of radiation harmonics. Using an analytic model of single-pass free-electron lasers (FELs), we analyze the power evolution of harmonics in the LCLS, LEUTL, and SACLA FEL experiments. We discuss the effect that the beam and undulator parameters have on the generation of harmonics. It is shown that the strong second harmonics of radiation observed in experiments are caused by the detected beam deviation from the axis by 15μm over one gain length of 1.5 m in the X-ray LCLS FEL and by the large beam cross section of ∼ 200;μm in the LEUTL FEL. The results of modeling are fully consistent with experiments, which confirms the validity of the presented theoretical description of FEL power and radiation spectral density.

Излучение ондуляторов и лазеров на свободных электронах с аналитическим учётом гармоник поля и внеосевых эффектов

Излучение ондуляторов и лазеров на свободных электронах с аналитическим учётом гармоник поля и внеосевых эффектов

A MHz-repetition-rate hard X-ray free-electron laser driven by a superconducting linear accelerator

The European XFEL is a hard X-ray free-electron laser (FEL) based on a high-electron-energy superconducting linear accelerator. The superconducting technology allows for the acceleration of many electron bunches within one radio-frequency pulse of the accelerating voltage and, in turn, for the generation of a large number of hard X-ray pulses. We report on the performance of the European XFEL accelerator with up to 5,000 electron bunches per second and demonstrating a full energy of 17.5 GeV. Feedback mechanisms enable stabilization of the electron beam delivery at the FEL undulator in space and time. The measured FEL gain curve at 9.3 keV is in good agreement with predictions for saturated FEL radiation. Hard X-ray lasing was achieved between 7 keV and 14 keV with pulse energies of up to 2.0 mJ. Using the high repetition rate, an FEL beam with 6 W average power was created.

Thermal loading on self-seeding monochromators in x-ray free electron lasers

One approach to achieve longitudinally coherent hard X-ray free electron laser (XFEL) radiation is to introduce a single-crystal monochromator in the beam path to generate a co-propagating, time-synchronized self-seeding radiation pulse. For modern high-repetition rate XFELs, however, thermal loading on the crystal can degrade the quality of the seed pulse. To assess limitations on the maximum XFEL pulse repetition rate imposed by the seeding crystal, a comprehensive study of thermomechanical, and X-ray diffraction effects within the crystal is presented for two consecutive self-amplified spontaneous emission pulses incident on both reflective and transmissive crystal monochromators.

Soft X-ray diffraction patterns measured by a LiF detector with sub-micrometre resolution and an ultimate dynamic range.

The unique diagnostic possibilities of X-ray diffraction, small X-ray scattering and phase-contrast imaging techniques applied with high-intensity coherent X-ray synchrotron and X-ray free-electron laser radiation can only be fully realized if a sufficient dynamic range and/or spatial resolution of the detector is available. In this work, it is demonstrated that the use of lithium fluoride (LiF) as a photoluminescence (PL) imaging detector allows measuring of an X-ray diffraction image with a dynamic range of ∼107 within the sub-micrometre spatial resolution. At the PETRA III facility, the diffraction pattern created behind a circular aperture with a diameter of 5 µm irradiated by a beam with a photon energy of 500 eV was recorded on a LiF crystal. In the diffraction pattern, the accumulated dose was varied from 1.7 × 105 J cm-3 in the central maximum to 2 × 10-2 J cm-3 in the 16th maximum of diffraction fringes. The period of the last fringe was measured with 0.8 µm width. The PL response of the LiF crystal being used as a detector on the irradiation dose of 500 eV photons was evaluated. For the particular model of laser-scanning confocal microscope Carl Zeiss LSM700, used for the readout of the PL signal, the calibration dependencies on the intensity of photopumping (excitation) radiation (λ = 488 nm) and the gain have been obtained.

Demonstration of Large Bandwidth Hard X-Ray Free-Electron Laser Pulses at SwissFEL.

We have produced hard x-ray free-electron laser (FEL) radiation with unprecedented large bandwidth tunable up to 2%. The experiments have been carried out at SwissFEL, the x-ray FEL facility at the Paul Scherrer Institute in Switzerland. The bandwidth is enhanced by maximizing the energy chirp of the electron beam, which is accomplished by optimizing the compression setup. We demonstrate continuous tunability of the bandwidth with a simple method only requiring a quadrupole magnet. The generation of such broadband FEL pulses will improve the efficiency of many techniques such as x-ray crystallography and spectroscopy, opening the door to significant progress in photon science. It has already been demonstrated that the broadband pulses of SwissFEL are beneficial to enhance the performance of crystallography, and further SwissFEL users plan to exploit this large bandwidth radiation to improve the efficiency of their measurement techniques.

In cellulo crystallization of Trypanosoma brucei IMP dehydrogenase enables the identification of genuine co-factors

Sleeping sickness is a fatal disease caused by the protozoan parasite Trypanosoma brucei (Tb). Inosine-5’-monophosphate dehydrogenase (IMPDH) has been proposed as a potential drug target, since it maintains the balance between guanylate deoxynucleotide and ribonucleotide levels that is pivotal for the parasite. Here we report the structure of TbIMPDH at room temperature utilizing free-electron laser radiation on crystals grown in living insect cells. The 2.80 Å resolution structure reveals the presence of ATP and GMP at the canonical sites of the Bateman domains, the latter in a so far unknown coordination mode. Consistent with previously reported IMPDH complexes harboring guanosine nucleotides at the second canonical site, TbIMPDH forms a compact oligomer structure, supporting a nucleotide-controlled conformational switch that allosterically modulates the catalytic activity. The oligomeric TbIMPDH structure we present here reveals the potential of in cellulo crystallization to identify genuine allosteric co-factors from a natural reservoir of specific compounds.

Acousto-optic Modulation of Terahertz Radiation in Liquefied Sulfur Hexafluoride at Room Temperature

Acousto-optic modulation of terahertz free-electron laser radiation was investigated using diffraction on an acoustic wave in liquefied sulfur hexafluoride at room temperature. At the wavelength 130 μ m, the diffraction efficiency in Bragg regime of interaction 1% was obtained at 75 V driving electrical signal applied to the transducer terminals.

Advances in global optimization of high brightness beams

High brightness electron beams play an important role in accelerator-based applications such as driving X-ray free electron laser (FEL) radiation. In this paper, we report on advances in global beam dynamics optimization of an accelerator design using start-to-end simulations and a new parallel multi-objective differential evolution optimization method. The global optimization results in significant improvement of the final electron beam brightness.

Over-the-Horizon Terahertz Surface Plasmon Locator

The article demonstrates the possibility of creating an over-the-horizon locator with an information carrier of surface plasmon polaritons (SPPs), a kind of surface electromagnetic waves, of the terahertz (THz) spectral range. This possibility is based on the macroscopic propagation length (decimeters) of THz SPPs, as well as on their ability to propagate along convex surfaces of metals. Two schemes of location measurements on gold convex samples using THz SPPs were developed and tested for SPPs generated by free-electron laser radiation with a wavelength of 130 μm. It was found that the intensity of SPPs reflected by an object placed on a conducting surface about 4 cm beyond the horizon line depended on the height of the object above the surface, in accordance with the distribution of the SPP field in the air (a few millimeters or fractions of a millimeter). The distance, orientation, coordinates, and height of an object placed beyond the horizon line can be determined using the SPP locator, which can be applied for localization of hard-to-reach objects on a convex surface.

Generation and Characterization of Intense Ultralow-Emittance Electron Beams for Compact X-Ray Free-Electron Lasers.

The transverse emittance of the electron beam is a fundamental parameter in linac-based x-ray free-electron lasers (FELs). We present results of emittance measurements carried out at SwissFEL, a compact x-ray FEL facility at the Paul Scherrer Institute in Switzerland, including a description of the novel high-resolution measurement techniques and the optimization procedure. We obtained slice emittance values at the undulator entrance down to 200nm for an electron beam with a charge of 200pC and an rms duration of 30-40fs. Furthermore, we achieved slice emittances as low as 100nm for 10pC beams with few fs duration. These values set new standards for electron linear accelerators. The quality, verification, and control of our electron beams allowed us to generate high-power FEL radiation for a wavelength as short as 0.1nm using an electron beam with an energy of only 6GeV. The emittance values demonstrated at SwissFEL would allow producing hard x-ray FEL pulses with even lower-energy beams, thus paving the way for even more compact and cost-effective FEL facilities.

A guide to sample delivery systems for serial crystallography.

Crystallography has made a notable contribution to our knowledge of structural biology. For traditional crystallography experiments, the growth of crystals with large size and high quality is crucial, and it remains one of the bottlenecks. In recent years, the successful application of serial femtosecond crystallography (SFX) provides a new choice when only numerous microcrystals can be obtained. The intense pulsed radiation of X-ray free-electron lasers (XFELs) enables the data collection of small-sized crystals, making the size of crystals no longer a limiting factor. The ultrafast pulses of XFELs can achieve 'diffraction before destruction', which effectively avoids radiation damage and realizes diffraction near physiological temperatures. More recently, the SFX has been expanded to serial crystallography (SX) that can additionally employ synchrotron radiation as the light source. In addition to the traditional ones, these techniques provide complementary opportunities for structural determination. The development of SX experiments strongly relies on the advancement of hardware including the sample delivery system, the X-ray source, and the X-ray detector. Here, in this review, we categorize the existing sample delivery systems, summarize their progress, and propose their future prospectives.

A detailed investigation of single-photon laser enabled Auger decay in neon

Single-photon laser enabled Auger decay (spLEAD) is an electronic de-excitation process which was recently predicted and observed in Ne. We have investigated it using bichromatic phase-locked free electron laser radiation and extensive angle-resolved photoelectron measurements, supported by a detailed theoretical model. We first used separately the fundamental wavelength resonant with the Ne+ 2s–2p transition, 46.17 nm, and its second harmonic, 23.08 nm, then their phase-locked bichromatic combination. In the latter case the phase difference between the two wavelengths was scanned, and interference effects were observed, confirming that the spLEAD process was occurring. The detailed theoretical model we developed qualitatively predicts all observations: branching ratios between the final Auger states, their amplitudes of oscillation as a function of phase, the phase lag between the oscillations of different final states, and partial cancellation of the oscillations under certain conditions.

High power RF operations studies at TARLA facility

Turkish Accelerator and Radiation Laboratory (TARLA) is a facility capable of accelerating an electron beam up to 40 MeV. Two beamlines were proposed to generate free-electron laser radiation and bremsstrahlung. The accelerator employs two normal conducting cavities, so-called buncher cavities: subharmonic buncher (SHB) and fundamental buncher (FB), and two cryomodules that house two TESLA cavities each. SHB operates in 260 MHz and FB in 1.3 GHz, and is powered by 1.5 kW and 500 W radio frequency (RF) amplifiers, respectively. Each TESLA cavity is driven by 18 kW saturated high-power solid-state amplifiers (SSA). In addition, a L band pulse compressor system is designed and implemented at the facility to actively promote high-power RF research. Currently, setup of a resonant ring test bench is approved to test the RF components under high power RF conditions. This paper describes the TARLA high power RF, RF controller, and network structures. High power tests and measurements of the RF components of the TARLA beamline are given. Outcomes from the operation of the L band pulse compressor are explained, and the resonant ring test stand is stated as a summary.

Time-resolved studies of metalloproteins using X-ray free electron laser radiation at SACLA

BackgroundThe invention of the X-ray free-electron laser (XFEL) has provided unprecedented new opportunities for structural biology. The advantage of XFEL is an intense pulse of X-rays and a very short pulse duration (<10 fs) promising a damage-free and time-resolved crystallography approach. Scope of reviewRecent time-resolved crystallographic analyses in XFEL facility SACLA are reviewed. Specifically, metalloproteins involved in the essential reactions of bioenergy conversion including photosystem II, cytochrome c oxidase and nitric oxide reductase are described. Major conclusionsXFEL with pump-probe techniques successfully visualized the process of the reaction and the dynamics of a protein. Since the active center of metalloproteins is very sensitive to the X-ray radiation, damage-free structures obtained by XFEL are essential to draw mechanistic conclusions. Methods and tools for sample delivery and reaction initiation are key for successful measurement of the time-resolved data. General significanceXFEL is at the center of approaches to gain insight into complex mechanism of structural dynamics and the reactions catalyzed by biological macromolecules. Further development has been carried out to expand the application of time-resolved X-ray crystallography. This article is part of a Special Issue entitled Novel measurement techniques for visualizing ‘live’ protein molecules.