Free-electron Laser Beamlines Research Articles

Comparative study of the X-ray reflectivity and in-depth profile of a-C, B4C and Ni coatings at 0.1–2 keV

The use of soft X-rays near the carbon edge of absorption (270-300 eV) greatly enhances studies in various branches of science. However, the choice of reflecting coatings for mirrors operating in free-electron and X-ray free-electron laser (FEL and XFEL) beamlines in this spectral range is not so evident and experimental justifications of the mirror efficiency are rather limited. In the present paper it is demonstrated experimentally that the reflectivity of B4C- and Ni-coated grazing-incidence mirrors is high enough for their operation in FEL or XFEL beamlines near the carbon K-edge of absorption. The minimal reflectivity of both mirrors proves to exceed 80% near the carbon absorption edge at a grazing angle of 0.6°. An in-depth profile of the chemical elements composing the reflecting coatings is reconstructed based on analysis of a set of reflectivity curves measured versus the grazing angle at different photon energies in the soft X-ray spectral region. This allows us to predict correctly the mirror reflectivity at any X-ray energy and any grazing angle.

Exploring the conformational variability in the heme b propionic acid side chains through the effect of a biological probe: a study of the isolated ions.

The iron(III) protoporphyrin IX complex with imidazole, a biologically relevant ligand, occupying an axial position, has been studied by infrared multiple photon dissociation (IRMPD) spectroscopy. The complex has been delivered in gas-phase by electrospray ionization (ESI), mass selected in an ion trap, and assayed by IRMPD spectroscopy in two complementary frequency regions. The fingerprint range (900-1900 cm(-1)) has been scanned using the Orsay free-electron laser beamline (CLIO), while the X-H (X = C,N,O) stretching region (3000-3600 cm(-1)) has been inspected using a tabletop IR optical parametric oscillator/amplifier (OPO/OPA) laser source. DFT calculations have been performed to obtain a comprehensive pattern of the various potential conformers yielding optimized geometries, relative thermodynamic parameters, and respective IR spectra. The comparison between the IR spectra for representative conformers and the experimental IRMPD features suggests the coexistence of two families of conformers involving different degrees of folding and hydrogen bonding between the two propionic acid functionalities on the periphery of the protoporphyrin IX macrocycle in a ratio depending on environmental conditions such as ESI solvent and temperature. The observed conformational variability of the porphyrin substituents in the naked heme-imidazole complex is consistent with the fine-tuning of the reactivity properties of this important prosthetic group by the specific surroundings in the protein core.

Grid pulser for an electron gun with a thermionic cathode for the high-power operation of a terahertz free-electron laser

A grid pulser for a thermionic-cathode electron gun for an L-band electron linac with an RF frequency of 1.3GHz was developed in an effort to increase the power of a terahertz (THz) free-electron laser (FEL) based on the linac. The grid pulser can generate a train of electron pulses with a 5-ns duration at intervals of 36.9ns or at a repetition frequency of 27MHz, which is the 48th sub-harmonic of the RF frequency, and with a peak current of up to 2.4A or higher, which is four times higher than the current that can be obtained with the conventional grid pulser. In combination with the sub-harmonic buncher (SHB) system, whose fundamental frequency is 108MHz (the 12th sub-harmonic of the RF frequency), an electron beam that comprises electron bunches separated by 36.9-ns intervals for a period of 8μs can be successfully accelerated using the linac to an energy of 15MeV under the condition that the average beam current or, correspondingly, the beam loading in the acceleration tube of the linac is the same as that obtained using the conventional grid pulser. The time-resolved energy spectrum indicates that the energy is constant over the latter 6μs and that the energy spread is 1.2% (FWHM); this performance satisfies the requirements for the FEL. The bunch charge measured at the end of the FEL beamline is 4nC, which is four times higher than that obtained using the conventional grid pulser.

On the characterization of ultra-precise X-ray optical components: advances and challenges in ex situ metrology.

To fully exploit the ultimate source properties of the next-generation light sources, such as free-electron lasers (FELs) and diffraction-limited storage rings (DLSRs), the quality requirements for gratings and reflective synchrotron optics, especially mirrors, have significantly increased. These coherence-preserving optical components for high-brightness sources will feature nanoscopic shape accuracies over macroscopic length scales up to 1000 mm. To enable high efficiency in terms of photon flux, such optics will be coated with application-tailored single or multilayer coatings. Advanced thin-film fabrication of today enables the synthesis of layers on the sub-nanometre precision level over a deposition length of up to 1500 mm. Specifically dedicated metrology instrumentation of comparable accuracy has been developed to characterize such optical elements. Second-generation slope-measuring profilers like the nanometre optical component measuring machine (NOM) at the BESSY-II Optics laboratory allow the inspection of up to 1500 mm-long reflective optical components with an accuracy better than 50 nrad r.m.s. Besides measuring the shape on top of the coated mirror, it is of particular interest to characterize the internal material properties of the mirror coating, which is the domain of X-rays. Layer thickness, density and interface roughness of single and multilayer coatings are investigated by means of X-ray reflectometry. In this publication recent achievements in the field of slope measuring metrology are shown and the characterization of different types of mirror coating demonstrated. Furthermore, upcoming challenges to the inspection of ultra-precise optical components designed to be used in future FEL and DLSR beamlines are discussed.

Complex light source composed from subterahertz-wave coherent synchrotron radiation and an infrared free-electron laser at the Laboratory for Electron Beam Research and Application

We have successfully transported subterahertz-wave (sub-THz-wave) coherent synchrotron radiation (CSR) from an accelerator room to an experimental room by using an existing IR free-electron laser (FEL) beamline at the Laboratory for Electron Beam Research and Application at Nihon University. The power of the transported CSR was 50 nJ per macropulse, and it was available at frequencies of 0.1–0.3 THz. From two-dimensional imaging performed with the sub-THz-wave CSR, metallic structures concealed by plastic in a smart card were nondestructively detected at a spatial resolution of 1.4 mm. We also verified the existence of hydrated water in a crystal by using chemometric two-band spectroscopy with the sub-THz-wave CSR and IR FEL. Pioneering applications, such as more reliable material identification, can be anticipated from this complex light source composed from the sub-THz-wave CSR and IR FEL beams.

Fast optimization of a bimorph mirror using x-ray grating interferometry.

An x-ray grating interferometer was employed for in situ optimization of an x-ray bimorph mirror. Unlike many other at-wavelength techniques, only a single interferogram image, captured out of the focal plane, is required, enabling the optical surface to be quickly optimized. Moiré fringe analysis was used to calculate the wavefront slope error, which is proportional to the mirror's slope error. Using feedback from grating interferometry, the slope error of a bimorph mirror was reduced to <200 nrad (rms) in only two iterations. This technique has the potential to create photon beams with spatially homogeneous intensities for use in synchrotron and free electron laser beam lines.

Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser

We report on an experimental system designed to probe chemical reactions on solid surfaces on a sub-picosecond timescale using soft X-ray emission spectroscopy at the Linac Coherent Light Source (LCLS) free electron laser (FEL) at the SLAC National Accelerator Laboratory. We analyzed the O 1s X-ray emission spectra recorded from atomic oxygen adsorbed on a Ru(0001) surface at a synchrotron beamline (SSRL, BL13-2) and an FEL beamline (LCLS, SXR). We have demonstrated conditions that provide negligible amount of FEL induced damage of the sample. In addition we show that the setup is capable of tracking the temporal evolution of electronic structure during a surface reaction of submonolayer quantities of CO molecules desorbing from the surface.

Time-of-flight measurement of ionic species generated during ablation for optimization of focusing condition at free-electron laser beamline

Optimization of focusing conditions is important in free-electron laser applications. A time-of-flight mass analyzer has been designed and constructed for this purpose. The time-of-flight spectra of ionic species evolved from laser ablation of gold were measured. The yields of ionic species showed strong correlations with free-electron-laser intensity. This method conveniently allows for direct estimation of laser intensity on sample and determination of focusing position.

Experimental set-up and procedures for the investigation of XUV free electron laser interactions with solids

In this article, we describe the experimental station and procedures for investigating the interaction of short-wavelength free-electron lasers (FELs) pulses with solids. With the advent of these sources, a unique combination of radiation properties (including wavelength range from tens of nanometers down to sub-Angstroms, femtosecond pulse duration, and high pulse energy reaching milli-Joules level) creates new research possibilities for the systematic studies of radiation-induced structural changes in solids. However, the properties of the intense FEL radiation generate, apart from the new experimental opportunities, extreme demands on the experimental set-up (mostly in terms of radiation hardness of detectors and their saturation levels). Thus, radiation-induced phase transitions in solids, beyond the fundamental scientific interest, are of importance for the design of FEL beamlines and instruments which interact with the direct beam. In this report, we focus on the instrumentation and experimental techniques used in the recent studies performed at the FLASH facility in Hamburg.

X-ray beam-shaping via deformable mirrors: Analytical computation of the required mirror profile

X-ray mirrors with high focusing performances are in use in both mirror modules for X-ray telescopes and in synchrotron and FEL (Free Electron Laser) beamlines. A degradation of the focus sharpness arises in general from geometrical deformations and surface roughness, the former usually described by geometrical optics and the latter by physical optics. In general, technological developments are aimed at a very tight focusing, which requires the mirror profile to comply with the nominal shape as much as possible and to keep the roughness at a negligible level. However, a deliberate deformation of the mirror can be made to endow the focus with a desired size and distribution, via piezo actuators as done at the EIS-TIMEX beamline of FERMI@Elettra. The resulting profile can be characterized with a Long Trace Profilometer and correlated with the expected optical quality via a wavefront propagation code. However, if the roughness contribution can be neglected, the computation can be performed via a ray-tracing routine, and, under opportune assumptions, the focal spot profile (the Point Spread Function, PSF) can even be predicted analytically. The advantage of this approach is that the analytical relation can be reversed; i.e., from the desired PSF the required mirror profile can be computed easily, thereby avoiding the use of complex and time-consuming numerical codes. The method can also be suited in the case of spatially inhomogeneous beam intensities, as commonly experienced at synchrotrons and FELs. In this work we expose the analytical method and the application to the beam shaping problem.

Tyrosine nitration as evidenced by IRMPD spectroscopy

Protonated 3-nitrotyrosine ([nitroTyr + H] +) has been investigated by collision induced dissociation at variable energy and by IR multiple photon dissociation (IRMPD) spectroscopy in conjunction with quantum chemical calculations. Ultimately, this investigation is aimed at providing a diagnostic signature of protein tyrosine nitration, a post translational modification implied in pathological states. IRMPD spectroscopy of [nitroTyr + H] + ions has been examined in two different spectral regions, namely in the 1000–2000 cm −1 range, using the free electron laser beamline at the CLIO facility, and in the 3200–3700 cm −1 range, using a tabletop laser source. [nitroTyr + H] + ions have been assayed in parallel with intact [Tyr + H] + ions revealing characteristic bands at 1329 and 1540 cm −1. Mode assignments allowed by density functional theory calculations of the IR spectra of the most stable conformers indicate that these features are associated to vibrations of the nitro group and are endowed with enhanced activity. In the NH/OH stretching region, the remarkable difference between the IRMPD spectra is the strong absorption at 3641 cm −1 of the ring OH stretching of [Tyr + H] + which is absent in the nitrated ion due to a strong hydrogen bond engaging the ring OH and the ortho nitro group. These results suggest that IRMPD spectroscopy and tandem mass spectrometry may afford a selective method for the analysis and characterization of 3-nitrotyrosine, to be possibly extended to 3-nitrotyrosine-containing peptides.

Development of a multipurpose vacuum chamber for serial optical and diffraction experiments with free electron laser radiation

In this paper we present a development of a multipurpose vacuum chamber which primal function is to be used in pump/probe experiments with free electron laser (FEL) radiation. The chamber is constructed for serial diffraction and serial spectroscopy allowing a fast exchange of samples during the measurement process. For the fast exchange of samples, liquid jet systems are used. Both applications, utilizing soft x-ray FEL pulses as pump and optical laser pulses as probe and vice versa are documented. Experiments with solid samples as well as the liquid jet samples are presented. When working with liquid jets, a system of automatically refilled liquid traps for capturing liquids has been developed in order to ensure stable vacuum conditions. Differential pumping stages are placed in between the FEL beamline and the experimental chamber so that working pressure in the chamber can be up to four orders of magnitude higher than the pressure in the FEL beamline.

EUV multilayer coated mirrors for atto-physics, photolithography and space experiments: Software design procedure

Extreme ultraviolet multilayer coatings consist of periodic or aperiodic stack of two or more material layers in the nanometric scale. They are fundamental in a very wide range of applications: in atto-physics they allow manipulation of high order harmonics by compressing ultrafast pulses in the atto-second regime; in the free electron laser beamlines they can be used to select the proper harmonics while rejecting unwanted ones; in space experiments they are employed in astrophysical instrumentation dedicated to space weather studies for the observation of solar disk and coronal plasmas; in microelectronics industry they are used as coatings of mirrors and projection masks in extreme ultraviolet lithographic apparatus for integrated circuit production. The more and more sophisticated level of such experiments and apparatus required the realization of multilayers with very strict requirements; in particular high control of reflecting curve shape and phase, and stability with respect to deposition process can be achieved only by deposition of aperiodic structure. Optimization of such structures usually requires a quite complex mathematical approach; an innovative software tool to perform such designs is presented.

Sub-picosecond coherent VUV source on the Elettra storage ring

Taking advantage of the storage ring free electron laser beamline at Elettra, we have implemented an experimental setup for the generation of sub-picosecond (ps) coherent optical pulses in the VUV range. The setup is based on the frequency up-conversion of a high-power external signal (provided by a Ti:Sapphire laser) and makes use of a relativistic electron bunch as resonating medium. The produced VUV pulses have peak power in MW range, variable polarization, high shot to shot stability and control of the timing parameters at the ps level. In this paper, we present the first characterization of the temporal and spectral features of the emitted light. The radiation can be exploited for new experiments in the fields of dynamical phenomena, non-linear physics, magnetism and biology.

Wavefront tracking within the stationary phase approximation

A physical optics approach to wavefront tracking is presented based on the stationary phase approximation. It allows for modeling transmission of radiation through a system of optical elements including coherence properties with an acceptable numerical effort. The code PHASE incorporating these ideas assumes small angles as it is the case for most synchrotron radiation and the free electron laser beam lines.

Permanent magnet systems for free-electron lasers

We review uses of permanent magnets (PMs) in free-electron lasers (FELs). Recently, PMs have been considered to replace many of the electromagnet (EM) dipoles, quadrupoles, and sextupoles in FEL beamlines and linear collider quadrupoles. PM beamline optics offer several advantages over EMs (Proceedings of the Particle Acceleration Conference, 2001, p. 3218). They are more compact, need no power, and do not require cooling water. In addition, adjustable-strength quadrupoles may have a precisely tuneable magnetic centerline. High pole tip fields (1.5T in dipoles and 1.2T in quadrupoles) are easily achieved. PM technology opens up new bend design possibilities. We describe a new, high performance, all-PM bend, the Ballard bend, that is first-order isochronous and doubly achromatic. It is suitable for use in the exhaust beam leg of an energy recovery FEL. We have designed and built a compact, high field sector PM dipole. Measured field profiles agree to 10ppm of predictions. Compact PM quadrupoles were also designed and built. Measurements of field strength, axial profile, magnetic centerline tuning, and passive temperature compensation of strength and centerline shift agreed very well with predictions.