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Angle-resolved photoelectron spectrometers with microchannel plate detectors and fast digitizer electronics are versatile and powerful devices for providing non-invasive single-shot photon diagnostics at a MHz repetition rate X-ray free-electron lasers. In this contribution, we demonstrate and characterize the performance of our two operational photoelectron spectrometers for the application of hard X-rays and soft X-rays as well as new automation tools and online data analysis that enable continuous support for machine operators and instrument scientists. Customized software has been developed for the real-time monitoring of photon beam polarization and spectral distribution both in single-color and two-color operation. Hard X-ray operation imposes specific design challenges due to poor photoionization cross-sections and very high photoelectron velocities. Furthermore, recent advancements in machine learning enable resolution enhancement by training the photoelectron spectrometer together with an invasive high-resolution spectrometer, which generates a response function model. [ABSTRACT FROM AUTHOR]

Intense infrared (IR) rays can heat matters and evaporate waters thermally. One of the possible applications will be hair dryer, although the irradiation effects of IR rays on the hair have not been fully explored. In this study, we first examined the interaction of IR rays at various wavelengths from 3.0 µm (near IR) to 90 µm (far IR) with the surface structure of human hair by using IR free electron lasers (FELs). IR-FEL is an accelerator-based pico-second pulse laser, and the feature is the wavelength-tunability with the high-photon density. When one thread of hair was irradiated by the FEL of 6–7 mJ energy at 60 µm, the cleavage occurred, and the morphological destruction was observed on scanning-electron microscopy images after the irradiations at 70 µm and 6.1 µm (amide I). Synchrotron-radiation infrared microspectroscopy showed that those FEL irradiations decreased a shoulder band at 1710 cm−1 that corresponds to carboxyl group in melanin or fatty acids and increased absorption intensity at 500–600 cm−1. On the contrary, the FEL at 90 µm little changed either the surface morphology or the infrared absorption spectra. Interestingly, near-IR FELs at 3.0–3.5 µm induced bending of a hair, and 2D mapping of protein secondary conformations revealed that β-sheet was more increased than the other conformations on the surface of the bending area even at low pulse energy (1–3 mJ). As a result, the structural damage of the hair was least at 90 µm, which implies an ideal wavelength for drying hair mildly.Article highlights: A unique assessment of hair structure is demonstrated by using accelerator-based infrared laser system. A new far-infrared wavelength for drying hair softly is discovered by this research. Applicability of accelerator-based infrared laser for activities of human life is first shown by this research. [ABSTRACT FROM AUTHOR]

The capability to use extreme ultraviolet (EUV) light for generating transient gratings (TGs) has enabled the study of thermoelastic and magnetic dynamics at the nanoscale, in thin solid samples and surfaces, without the need of specially modifying them. However, the current mirror-based setup for generating EUV TG limits both its extension to the soft x-ray photon energy range and the attainment of few femtosecond time-resolution. Here we propose to overcome these limitations with an alternative experimental scheme based on diffractive optical elements that has become feasible with the current technology. In addition, some aspects of the discussed setup may facilitate the implementation of the EUV TG approach at table-top high-harmonic generation sources. [ABSTRACT FROM AUTHOR]

The maximum energy of photons from high-harmonic generation (HHG) increases with the wavelength of the driving laser. A free electron laser (FEL) is a continuously tunable light source in the mid-IR wavelength range and it is useful for investigating the extension of the accessible photon energy in HHG. Recently, the undulator magnets of the IR FEL at the Laboratory for Electron Beam Research and Application (LEBRA), Nihon University, have been replaced, and then the output power of the FEL has increased. Here, we evaluate the pulse duration and focal beam size of the FEL and show that the LEBRA FEL with 2 and 3 μ m laser wavelengths under a 44 MHz bunch repetition mode can drive HHG. [ABSTRACT FROM AUTHOR]

We investigate the effect of pre-bunched relativistic electron beams on the output power of a free electron laser by two electron beams in a theoretical study. In general, this highly coherent and stable pulse of laser is known as the best investigator of basic importance for present-day science. Free electron lasers are the best candidates for exploring nature at ultrasmall spatial and ultrafast temporal scales. In this paper, we have considered the distribution function of relativistic electron beams as a water-bag, and the configuration of the wiggler pump has assumed a helical magneto-static wiggler. By a set of nonlinear, 1D, coupled differential equations the wave-particle interaction problem has been analyzed. With the aid of MATLAB-based Runge-Kutta algorithm numerical solution has been obtained. Saturation profiles in different cases of pre-bunching phase region of beams have been plotted. Out-coming has shown that the saturation length decreases by about 7.2%. However, the amplitude of the output pulse increased by about 209.3% in comparison to the non-pre-bunched relativistic electron beams. [ABSTRACT FROM AUTHOR]

Irradiation of solid surfaces with high intensity, ultrashort laser pulses triggers a variety of secondary processes that can lead to the formation of transient and permanent structures over a large range of length scales from mm down to the nano‐range. One of the most prominent examples are LIPSS – Laser‐Induced Periodic Surface Structures. While LIPSS have been a scientific evergreen for of almost 60 years, experimental methods that combine ultrafast temporal with the required nm spatial resolution have become available only recently with the advent of short pulse, short wavelength free electron lasers. Here, the current status and future perspectives in this field are discussed by exploiting the unique possibilities of these 4th‐generation light sources to address by time‐domain experimental techniques the fundamental LIPSS‐question, namely why and how laser irradiation can initiate the transition of a "chaotic" (rough) surface from an aperiodic into a periodic structure. [ABSTRACT FROM AUTHOR]

The unique parameters of the Novosibirsk free electron laser (NovoFEL) allow unique experiments on the spectroscopy of molecules to be performed at the Shared Resource Center. The radiation parameters of terahertz NovoFEL are article described in detail. The molecular spectroscopy experiments performed on this laser are given with the corresponding references in which they are described. Possible but as yet unperformed experiments on terahertz NovoFEL in the field of ultra-high-resolution molecular spectroscopy are considered. [ABSTRACT FROM AUTHOR]

Beam stability and timing jitter in modern linear accelerators are becoming increasingly important. In particular, if a magnetic or radio-frequency (RF) compression regime is employed, the beam time of arrival jitter at the end of the linac can be strictly correlated with the phase noise of the accelerating fields of the RF structure working off-crest. For this reason, since 2008, an RF fast-feedback technique, which acts within each RF pulse, has been successfully employed at LNF-INFN (Laboratori Nazionali di Frascati dell'Istituto Nazionale di Fisica Nucleare) in the SPARC_LAB (Sources for Plasma Accelerators and Radiation Compton with Laser And Beam) facility on S-band (2856 MHz) klystrons powered by pulse-forming network (PFN) modulators, as reported in this paper. However, in order to meet the more stringent requirements of plasma wakefield acceleration schemes, some upgrades to this feedback system have been recently carried out. The first prototype has been experimentally tested on a C-band (5712 MHz) klystron, driven by a solid-state modulator, in order to investigate the possibility for additional improvement resulting from the inherently more stable power source. In this paper, the design, realization and the preliminary measurement results obtained at SPARC_LAB after such upgrades will be reviewed. [ABSTRACT FROM AUTHOR]

Angle-resolved photoelectron spectrometers with microchannel plate detectors and fast digitizer electronics are versatile and powerful devices for providing non-invasive single-shot photon diagnostics at a MHz repetition rate X-ray free-electron lasers. In this contribution, we demonstrate and characterize the performance of our two operational photoelectron spectrometers for the application of hard X-rays and soft X-rays as well as new automation tools and online data analysis that enable continuous support for machine operators and instrument scientists. Customized software has been developed for the real-time monitoring of photon beam polarization and spectral distribution both in single-color and two-color operation. Hard X-ray operation imposes specific design challenges due to poor photoionization cross-sections and very high photoelectron velocities. Furthermore, recent advancements in machine learning enable resolution enhancement by training the photoelectron spectrometer together with an invasive high-resolution spectrometer, which generates a response function model.

We study the Thomson scattering from highly oriented pyrolitic graphite excited by the extreme ultraviolet, coherent pulses of FERMI free electron laser (FEL). An apparent nonlinear behavior is observed and fully described in terms of the coherent nature of both exciting FEL beam and scattered radiation, producing an intensity-dependent enhancement of the Thomson scattering cross-section. The process resembles Dicke's superradiant phenomenon and is thus interpreted as the observation of superradiant Thomson scattering. The process also triggers the creation of coherent, low-q (< 0.3 Å-1), low energy phonons. The experimental data and analysis provide quantitative information on the sample characteristics, absorption, scattering factor, and coherent phonon energies and populations and open the route for the investigation of the deep nature of complex materials. [ABSTRACT FROM AUTHOR]

The European X-ray Free Electron Laser is a research facility located close to Hamburg, offering X-ray pulses with ultra-high brilliance and femtosecond duration at megahertz repetition rates. The detection systems necessary to unlock the full scientific potential made possible by this machine poses considerable challenges both in terms of data volume and rate, as well as the interpretation of their recorded signal. To provide optimal data quality, expert and detector-specific knowledge not easily accessible to external facility users is essential, and its implementation must cope with the generated volumes. We therefore aim to perform these preparatory processing steps and offer users a dataset suitable for further analysis as the primary data product. This work describes the machinery and workflows providing this data to users in an automatic, configurable and reproducible manner, both online during the experiment, and offline for scientific analysis afterward on the way to publication. [ABSTRACT FROM AUTHOR]

A study is made of the saturation mechanism of a single superradiant ‘spike’ of radiation in a Free Electron Laser. A one-dimensional (1D) computer model is developed using the Puffin, un-averaged FEL simulation code, which allows sub-radiation wavelength evolution of both the spike radiation field and the electron dynamics to be modelled until the highly non-linear saturation process of the spike is observed. Animations of the process from the start to the end of the interaction are available. The resultant saturated spike duration is at the sub-wavelength scale and has a broad spectrum. The electrons passing through the spike can both lose and gain energy many times greater than that of the ‘normal’ non-pulsed FEL interaction. A saturation mechanism is proposed and tested via a simple analysis of the 1D FEL equations. The scaling results of the analysis are seen to be in good agreement with the numerical results. A simple model of three dimensional diffraction effects of the radiation is applied to the results of the 1D simulations. This greatly reduces longer wavelengths of the power spectrum, which are seen to be emitted mainly after the electrons have propagated through the spike, and is seen to be in qualitative agreement with recent experimental results.

The computer simulation of a cavity-based (oscillator) Free-Electron Laser (FEL) requires the modelling of both the electron-light interaction in the FEL undulator and the radiation propagation within the optical cavity. The unaveraged 3D FEL simulation code Puffin has been coupled with the Optical Propagation Code (OPC) to allow a broadband, high temporal-resolution cavity FEL to be modelled for the first time. This requires the translation of the radiation field formats between the Puffin and OPC codes. This translation is described and the coupled codes are then used to model an example of a Regenerative Amplifier FEL operating in the VUV.

The European X-ray Free Electron Laser is a research facility located close to Hamburg, offering X-ray pulses with ultra-high brilliance and femtosecond duration at megahertz repetition rates. The detection systems necessary to unlock the full scientific potential made possible by this machine poses considerable challenges both in terms of data volume and rate, as well as the interpretation of their recorded signal. To provide optimal data quality, expert and detector-specific knowledge not easily accessible to external facility users is essential, and its implementation must cope with the generated volumes. We therefore aim to perform these preparatory processing steps and offer users a dataset suitable for further analysis as the primary data product. This work describes the machinery and workflows providing this data to users in an automatic, configurable and reproducible manner, both online during the experiment, and offline for scientific analysis afterward on the way to publication.

Free electron lasers (FELs) are becoming more and more popular as sources of radiation for research purposes in many fields of science. They are frequently employed in second harmonic generation (SHG) studies. SHG is an important effect of nonlinear responses of matter to irradiation. It should be separated from the second harmonic of the radiation source, FELs, for correct analysis of the response. Using an analytical model for harmonic powers evolution in a single-pass FEL, we demonstrate the dependence of FEL harmonics on the key parameters of beams and undulators and show that it is possible to reduce the second FEL harmonic content if the energy spread of an electron beam is increased. For LEUTL FEL radiation in the visible range, we analytically demonstrate a reduction in the second harmonic power by an order of magnitude if electron energy spread is increased twice, which still allows efficient bunching at the fundamental wavelength. The method is valid regardless of the photon energy and, therefore, the absorption edge of the target material, and as such, it is valid also in the X-ray band. [ABSTRACT FROM AUTHOR]

Using an analytical description of the evolution of harmonic power in free-electron laser (FEL) undulators, the influence of electron beam parameters on the fundamental characteristics of FEL radiation is investigated: gain length, spectrum, and harmonic power. The possibility of suppressing even harmonics in the FEL as a light source for material and surface studies using second harmonic generation (SHG) nonlinear response analysis is explored. The main factors affecting the emission of even harmonics in FELs are identified; the influence of the beam cross-section and the spread of electron energies on their radiation is analyzed. The example of the FEL LEUTL with visible light emission, which is used in the studies of optical properties of materials and the chemical state of molecules on surfaces and at interfaces, is considered. In this context, the possibility of suppressing the second harmonic in FELs is analyzed. [ABSTRACT FROM AUTHOR]

The development of efficient and reliable sensors operating at room temperature is essential to advance the application of terahertz (THz) science and technology. Pyroelectric THz detectors are among the best candidates, taking into account their variety, outstanding performance, ease of fabrication, and robustness. In this work, we compare the performance of six different detectors, based on either LaTiO3 crystal or different polymeric films, using monochromatic radiation of the Novosibirsk Free Electron Laser facility (NovoFEL) in the frequency range of 0.9–2.0 THz. The main characteristics, including noise equivalent power and frequency response, were determined for all of them. Possible reasons for the differences in the obtained characteristics are discussed on the basis of the main physicochemical characteristics and optical properties of the sensitive area. At least three detectors showed sufficient sensitivity to monitor the shape and duration of the THz macropulses utilizing only a small fraction of the THz radiation from the primary beam. This capability is crucial for accurate characterization of THz radiation during the main experiment at various specialized endstations at synchrotrons and free electron lasers. As an example of such characterization, the typical stability of the average NovoFEL radiation power at the beamline of the electron paramagnetic resonance endstation was investigated. [ABSTRACT FROM AUTHOR]

Currently, the terahertz-frequency range, which is on the border of the microwave and optical ranges, is being intensively utilized. One of the widely used materials in terahertz optics is indium antimonide (InSb), the plasma frequency of which depends on the degree of doping, temperature, and surface illumination. The possibility of generating surface plasmon polaritons, a type of surface electromagnetic waves, on the surface of an InSb sample using the attenuated-total-reflection method (ATR) (Otto scheme) is discussed. Using the scattering-matrix formalism, the conditions for the highest efficiency of the excitation of surface plasmon polaritons are established. If terahertz radiation with a frequency ω slightly less than ωp is used for this, the propagation length of such plasmons and the depth of their field penetration into the environment (air) are comparable to the radiation wavelength. It is possible to achieve surface plasmon resonance in the form of a sharp decrease in the intensity of monochromatic radiation reflected from the base of the ATR prism with a change in the angle of incidence and the size of the air gap. Test experiments were performed to observe the surface plasmon resonance on an InSb wafer using a high-resistance silicon prism and monochromatic radiation (λ = 141 μm) from the Novosibirsk free electron laser. The dependence of the resonant dip on the size of the air gap separating the prism from the sample surface is studied, and its optimal (in the case of resonance) value is established for semiconductors with a plasma frequency in the terahertz range. [ABSTRACT FROM AUTHOR]

Beam stability and timing jitter in modern linear accelerators are becoming increasingly important. In particular, if a magnetic or radio-frequency (RF) compression regime is employed, the beam time of arrival jitter at the end of the linac can be strictly correlated with the phase noise of the accelerating fields of the RF structure working off-crest. For this reason, since 2008, an RF fast-feedback technique, which acts within each RF pulse, has been successfully employed at LNF-INFN (Laboratori Nazionali di Frascati dell’Istituto Nazionale di Fisica Nucleare) in the SPARC_LAB (Sources for Plasma Accelerators and Radiation Compton with Laser And Beam) facility on S-band (2856 MHz) klystrons powered by pulse-forming network (PFN) modulators, as reported in this paper. However, in order to meet the more stringent requirements of plasma wakefield acceleration schemes, some upgrades to this feedback system have been recently carried out. The first prototype has been experimentally tested on a C-band (5712 MHz) klystron, driven by a solid-state modulator, in order to investigate the possibility for additional improvement resulting from the inherently more stable power source. In this paper, the design, realization and the preliminary measurement results obtained at SPARC_LAB after such upgrades will be reviewed.

The recently constructed vacuum ultraviolet (VUV) free electron laser (FEL) at the Dalian Coherent Light Source (DCLS) is yielding a wealth of new and exquisitely detailed information about the photofragmentation dynamics of many small gas-phase molecules. This Review focuses particular attention on five triatomic molecules—H2O, H2S, CO2, OCS and CS2. Each shows excitation wavelength-dependent dissociation dynamics, yielding photofragments that populate a range of electronic and (in the case of diatomic fragments) vibrational and rotational quantum states, which can be characterized by different translational spectroscopy methods. The photodissociation of an isolated molecule from a well-defined initial quantum state provides a lens through which one can investigate how and why chemical reactions occur, and provides numerous opportunities for fruitful, synergistic collaborations with high-level ab initio quantum chemists. The chosen molecules, their photofragments and the subsequent chemical reaction networks to which they can contribute are all crucial in planetary atmospheres and in interstellar and circumstellar environments. The aims of this Review are 3-fold: to highlight new photochemical insights enabled by the VUV-FEL at the DCLS, notably the recently recognized central atom elimination process that is shown to contribute in all of these triatomic molecules; to highlight some of the potential implications of this rich photochemistry to our understanding of interstellar chemistry and molecular evolution within the universe; and to highlight other and future research directions in areas related to chemical reaction dynamics and astrochemistry that will be enabled by increased access to VUV-FEL sources. [ABSTRACT FROM AUTHOR]

Ultrafast time-resolved extreme ultraviolet (EUV) reflectivity measurements of optically pumped amorphous carbon (a-C) have been performed with the FERMI free electron laser (FEL). This work extends the energy range used in previous reflectivity studies and adds polarization dependence. The EUV probe is known to be sensitive to lattice dynamics, since in this range the reflectivity is essentially unaffected by the photo-excited surface plasma. The exploitation of both s-and p-polarized EUV radiation permits variation of the penetration depth of the probe; a significant increase in the characteristic time is observed upon increasing the probing depth (1 vs. 5 ps) due to hydrodynamic expansion and consequent destruction of the excited region, implying that there is only a short window during which the probed region is in the isochoric regime. A weak wavelength dependence of the reflectivity is found, consistent with previous measurements and implying a maximum electronic temperature of 0.8 eV ± 0.4.

The European X-ray Free Electron Laser (EuXFEL) is a light source of the 4th generation which provides spatially coherent ultrashort X-ray pulses at high rate. The facility enabled unprecedented advancement in fundamental research, but also needed new detectors to fit the EuXFEL requirements, such as single photon resolution, large dynamic range, and high repetition rate: three 2D megapixel detectors have been developed to cope with the demanding environment. The DEPleted Field Effect Transistors (DEPFET) Sensor with Signal Compression (DSSC) detector is one of them. The parallel readout from each pixel is performed employing a dedicated Application Specific Integrated Circuit (ASIC). The generated data flow is read through a 2-stage Field Programmable Gate Array (FPGA)-based Data AcQuisition (DAQ) chain: the Input Output Board (IOB) controls the primary readout from 16 ASICs and serializes data on high-speed links that are sent to the Patch Panel Transceiver (PPT). It is also responsible for the clock distribution and timing control of the front-end and switched power channels. The PPT reorders and forwards the received data toward the EuXFEL back-end, and allows remote control over the whole DAQ. A first DSSC camera, based on miniaturized silicon drift detectors (mini-SDD) has been available for users’ experiments since 2019, and a second DEPFET-based camera is under construction. The IOB firmware has been thoroughly reviewed and modified to cope with the new DEPFET sensors’ physics and to improve the performance of both the mini-SDD and DEPFET versions. We present an overview of the DSSC, focusing on the DAQ, highlighting the main properties of the environment where the detector operates. We assess the firmware improvements introduced, with a particular focus on the IOB, and present the results obtained in comparison to the original firmware.

The pulse duration in short-pulse schemes for Self-Amplified Spontaneous Emission Free Electron Lasers (SASE FELs) is limited by the FEL coherence time. A recently proposed concept allows to overcome the coherence time barrier and to obtain much shorter pulses. When the lasing part of an electron bunch is much shorter than the coherence time, one can suppress the radiation in the long main undulator while preserving microbunching within that short lasing slice. Then, a short radiation pulse is produced in a relatively short radiator. A possible suppression method, an excessive reverse undulator taper, is discussed and illustrated numerically in this paper. We also performed the first experimental tests of this method at the soft X-ray FEL user facility FLASH. The measured pulse duration approaches 1 fs (FWHM) at the wavelength of 5 nm. [ABSTRACT FROM AUTHOR]

Precious metals, including Rh, are used practically in three-way catalytic converters of automobiles. An important role of Rh is the reduction of NOx to N2. Gas-phase clusters are used to study reactions relevant to heterogeneous catalysts because it is possible to define and detect the number of atoms included in the cluster and to obtain atomiclevel insights into the reactions. We have studied the adsorption forms of NO molecules on metal clusters by using infrared multiple photon dissociation (IRMPD) spectroscopy. We found that NO molecules adsorb on metal clusters molecularly or dissociatively depending on the element and the size of the clusters. For instance, NO adsorbs on Rhn+ clusters mainly in the molecular form (＞95％ on Rh6+), while more NO adsorbs dissociatively on Irn+ clusters (～50％ on Ir6+). In this review, the methodology and results of the identification of NO adsorption forms on Rh and related clusters are presented. [ABSTRACT FROM AUTHOR]

A project for a submillimeter free electron laser (FEL) based on a relativistic electron beam (REB) generated in a linear induction accelerator (LIA) is proposed for the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences, and the Institute of Applied Physics, Russian Academy of Sciences. A theoretical analysis shows that the electron beam generated in the LIA (energy MeV, current kA, normalized emittance ~ 1100 π mm mrad) is a suitable driver for generating subgigawatt pulses of coherent EM radiation in the submillimeter range of wavelengths (0.3–1 THz). The main proposals for developing a FEL based on the electron beam generated in a linear induction accelerator are presented, the main project tasks are outlined, and proposed ways of solving them are described. Results from electron-optical experiments on the formation of an electron beam intended for FEL applications are presented. [ABSTRACT FROM AUTHOR]

Free electron laser (FEL) is capable of producing ultra-short X-ray pulses. The estimation of X-ray pulse propagation is the key process of X-ray FEL beamline design. By using the Kostenbauder matrix approach, the evolution of an ultra-short pulse in a beamline system can be calculated. Therefore, it is of significant importance to investigate the Kostenbauder matrices of different kinds of X-ray optics. In this work, we derive a unified 6 × 6 optical matrix to describe various kinds of X-ray optical elements, including varied-line-spacing (VLS) toroidal grating, VLS spherical grating, VLS cylindrical grating, VLS plane grating, toroidal grating, spherical grating, cylindrical grating, plane grating, toroidal mirror, spherical mirror, cylindrical mirror, and plane mirror. These optics are usually adopted in soft X-ray regime. We apply this method to describe the transverse focusing, pulse front tilt, and pulse stretching after an X-ray pulse going through a VLS plane grating monochromator (VLS-PGM). We also use this approach to simulate a grating compressor which can be used to compress chirped soft X-ray pulse. This work is helpful in the design and optimization of X-ray beamline systems. [ABSTRACT FROM AUTHOR]

Chirped X-ray pulse compression is a promising approach for generating ultra-short X-ray free electron laser (XFEL) pulses. The design of X-ray pulse compressors requires the careful control of group delay dispersion (GDD), which plays a critical role in achieving optimal compression. However, the penetration dispersion of crystals and multilayers can induce an extra GDD, which may result in over-compression or under-compression. In this study, we investigate the penetration dispersion of crystals and multilayers theoretically and numerically. Our results indicate that the extra GDD induced by the penetration effect increases as the bandwidth of the rocking curve decreases. Moreover, the extra GDD is nonlinear and can be mitigated by optimizing the configuration of X-ray pulse compressors. This work provides insights into the dispersion compensation and configuration optimization of X-ray pulse compressors, which are essential for generating ultra-short XFEL pulses. [ABSTRACT FROM AUTHOR]

The amplification in a free-electron laser undulator of a short-wavelength subattosecond pulse formed in a laser plasma upon its interaction with a relativistic electron bunch produced by a petawatt laser is considered. The aim of this work is to find the parameters of the system at which it is possible to amplify ultrashort pulses with a duration of up to hundreds of zeptoseconds. [ABSTRACT FROM AUTHOR]

The High Duty Cycle (HDC) upgrade is a proposed improvement to the existing European X-ray Free Electron Laser (EuXFEL) to extend the pulsed RF duty factor from the actual value of around 1% to more than 5% up to Continuous Wave (CW). To implement this upgrade, the loaded quality factor (QL) of the superconducting cavities will increase by more than one order of magnitude. This will result in shrinking the cavity bandwidth to values as low as a few Hertz. Since the Lorentz Force Detuning (LFD) experienced during the accelerating field buildup is of hundreds of Hertz, the Low-Level RF (LLRF) system has to accurately track and control the cavity resonance frequency to obtain the desired accelerating gradient. Moreover, ponderomotive instabilities have to be suppressed to achieve stability during beam acceleration. Since LFD is a repetitive disturbance in cavity frequency, the correction to its effects can be implemented as a feedforward compensation on the piezoelectric tuners of the cavity. Initial results on the simulation of feedforward resonance control in the HDC regime are discussed in this proceeding.

Isolated 2‐phenylallyl radicals (2‐PA), generated by pyrolysis from a nitrite precursor, have been investigated by IR/UV ion dip spectroscopy using free electron laser radiation. 2‐PA is a resonance‐stabilized radical that is considered to be involved in the formation of polycyclic aromatic hydrocarbons (PAH) in combustion, but also in interstellar space. The radical is identified based on its gas‐phase IR spectrum. Furthermore, a number of bimolecular reaction products are identified, showing that the self‐reaction as well as reactions with unimolecular decomposition products of 2‐PA form several PAH efficiently. Possible mechanisms are discussed and the chemistry of 2‐PA is compared with the one of the related 2‐methylallyl and phenylpropargyl radicals. [ABSTRACT FROM AUTHOR]

Along with the development of FEL X-ray sources, the design and the construction of Compton Back-Scattering (CBS) devices for different applications are being pursued. These sources are designed to provide monochromatic, high peak brightness tunable light beams, with photon energies above 10 K e V . The source brightness is the most significant figure of merit, which specifies the suitability of X-ray beams for their application in a specific field of interest and also reflects the quality of the electron beam itself, which in turn depends on the charge, emittance, energy spread, etcetera. This article is intended as an introductory review to the field and we embed a few theoretical considerations and analytical formulae to develop simple tools for the design of CBS X-ray sources. The present attempt is analogous to previous efforts, put forward to model Free Electron Laser devices, using scaling relations accounting for the entanglement between the different physical quantities contributing to the output beam performances. We comment on the reliability of our analysis by a comparison with the wealth of results (either numerical and analytical) reported in the dedicated literature. [ABSTRACT FROM AUTHOR]

Coherent terahertz beams with radial polarization of the 1st, 2nd, and 3rd orders have been generated with the use of silicon subwavelength diffractive optical elements (DOEs). Silicon elements were fabricated by a technology similar to the technology used before for the fabrication of DOEs forming laser terahertz beams with pre-given mode content. The beam of the terahertz Novosibirsk Free Electron Laser was used as the illuminating beam. The experimental results are in good agreement with the results of the computer simulation. [ABSTRACT FROM AUTHOR]

One of the key problems of modern accelerator physics is an increase in the acceleration rate in linear electron accelerators. The physical limit of the strength of the accelerating field for ordinary and superconducting accelerating structures has practically been reached, therefore various new acceleration schemes are considered, primarily plasma acceleration and wake acceleration. It is proposed to consider the possibility of using a bunch generated in a laser-plasma channel for injection into a traditional structure based on microwave resonators. An electron source based on a cluster plasma can generate a short (from 0.1 to 1.0 ps) electron bunch with an energy of several hundred kiloelectronvolts, which makes it possible to consider such a source as an alternative to a photocathode. Next, the beam must be captured in the acceleration mode in a normally conducting section operating on a standing wave and accelerated to an energy of 50 MeV with the possibility of adjusting the energy. This article considers the features of such an accelerator, including the possible energy spectrum, the features of capturing an electron bunch with an extremely wide spectrum in the acceleration mode, as well as the electrodynamic characteristics of the accelerating structures. The simulation of the beam dynamics was carried out using the BEAMDULAC package developed at the Department of Electrophysical Installations of the National Research Nuclear University MEPhI. The main results of the development of electrodynamic models of the accelerating structures of this accelerator are also presented. [ABSTRACT FROM AUTHOR]

Currently, Russia is designing the SYLA (Synchrotron Laser) fourth-generation synchrotron radiation source, which will become one of the world's largest scientific centers. Its creation will make it possible to carry out experiments to study the structure of a wide range of objects in various disciplines at a qualitatively new level compared to the previous generation of sources. The general layout of the facility includes the main storage ring for an energy of 6 GeV and a linear electron accelerator (LEA) for full energy. It is proposed to use one LEA with two microwave guns. A microwave gun with a photocathode can be used to generate a beam for a free electron laser (FEL), a microwave gun with a thermionic cathode, for injection into a storage ring. Both injectors will work with the same regular part of the linear accelerator, consisting of 100–120 identical sections. The planned transverse emittance in the main storage ring will be 50–100 pm rad. The development of a general scheme of the LEA in order to minimize the spread of the beam energy and transverse emittance at the output, the optimization of the geometric and electrodynamic characteristics of the accelerating structures, and the analysis of the beam dynamics in this linear accelerator will be discussed in the article. The simulation of the beam dynamics was performed using the BEAMDULAC package developed at the Department of Electrophysical Installations of the National Research Nuclear University MEPhI. [ABSTRACT FROM AUTHOR]

The development of high-brightness X-ray free electron lasers (XFELs), such as hard X-ray self-seeding free electron lasers and XFEL oscillators (XFELOs), brings a severe challenge to the crystal monochromator due to a strong non-uniform thermal load. The distortion caused by spatial temperature gradients can severely affect the optical performance of crystals. Therefore, this paper presents a model to estimate the performance of non-uniform thermally distorted crystals. The model not only takes into account thermal strain, slope error and incident angle deviation, but also considers temperature-dependent factors such as the Debye--Waller factor and electric susceptibility. Our investigation indicates that the Debye--Waller factor reduces the height and bandwidth of rocking curves, and the impact of the electric susceptibility is tiny. The proposed model can describe the distortion of the reflectivity and transmissivity curves of non-uniform thermally loaded crystals and can be applied in the design of crystal monochromators, crystal splitters, crystal compressors and XFELOs. [ABSTRACT FROM AUTHOR]

Attosecond soft X-ray pulses are of great importance for the study of ultrafast electronic phenomena. In this paper, a feasible method is proposed to generate isolated fully coherent attosecond soft X-ray free electron laser via optical frequency beating. Two optical lasers with the opposite frequency chirps are used to induce a gradient frequency energy modulation, which helps to generate a gradually varied spacing electron pulse train. Subsequently, the undulator sections with electron beam delay lines are used to amplify the target ultra-short radiation. Numerical start-to-end simulations have been performed and the results demonstrate that an isolated soft X-ray pulse with the peak power of 330 GW and pulse duration of 620 as can be achieved by the proposed technique. [ABSTRACT FROM AUTHOR]

Comparative studies of characteristics of Bessel and "perfect" vortex beams with a topological charge 9, created using a binary silicon axicon and a "holographic" diamond axicon with continu-ous profile at a wavelength of 141 μm, are carried out. Beams with linear and radial polarization are investigated. An example of the use of a perfect radially polarized beam for the excitation of vortex plasmon-polaritons on a cylindrical conductor is given.

Abstract The X-ray Free Electron Laser of Pohang Accelerator Laboratory (PAL-XFEL) was opened for the user in 2017. PAL-XFEL was the third XFEL facility in the world and well known for the small timing jitter of FEL radiation. This success was possible because of the significant achievements in the accelerator technology in PAL-XFEL. They are not limited to a small part of the accelerator but cover from the injector for the electron beam generation to the undulator line for FEL radiation. In this review, we describe the details of newly developed devices that contributed to the successful construction of PAL-XFEL.

This paper presents the results of 15 years of studies in the field of terahertz holography at the Novosibirsk free electron laser. They cover two areas: research on obtaining holographic images in the terahertz range and the use of diffractive optical elements to form high-power terahertz radiation fields with specified characteristics (intensity, phase, and polarization), using well-studied and widely applied in the optical range methods of optical (analog), digital, and computer-generated holography. All experiments were performed with the application of high-power coherent monochromatic frequency-tunable radiation from the Novosibirsk free electron laser. The features of hologram registration in the terahertz range are described. Methods, technologies, and optical materials for terahertz holographic elements are discussed. A wide range of promising applications of high-power terahertz fields with a given spatial structure is considered. The results of the study of terahertz holograms recorded as digital holograms, as well as radiation-resistive optical elements realized as computer-synthesized holograms, are presented.

High-energy particles, including photons (x-ray, γ-ray, bremsstrahlung), electrons, and protons, possess the capability to penetrate materials and deposit energy within them. The degree of absorption depends on both the energy and type of particles, as well as the properties of the materials with which they interact. This energy deposition can manifest either at the material's surface or throughout its volume, potentially resulting in various failure modes. The primary aim of this paper is to establish a structured analysis methodology for evaluating the structural integrity of beam-intercepting devices when subjected to high-energy particles. The paper also reviews some of the underlying physics, pertinent to the scope of the thermomechanical analysis, potential failure modes, and introduces verification and validation methodologies. Engineers and researchers can utilize the guidelines presented in this paper to effectively plan the development of beam intercepting devices, thereby ensuring their reliability and performance in the presence of high-energy particle exposure. • The structural integrity of beam intercepting devices subjected to high-energy free electron lasers is investigated. • The underlying physics, and potential failure modes of beam intercepting devices are reviewed. • FEA verification and validation methodologies are presented. [ABSTRACT FROM AUTHOR]

Free electron lasers (FELs) are becoming more and more popular as sources of radiation for research purposes in many fields of science. They are frequently employed in second harmonic generation (SHG) studies. SHG is an important effect of nonlinear responses of matter to irradiation. It should be separated from the second harmonic of the radiation source, FELs, for correct analysis of the response. Using an analytical model for harmonic powers evolution in a single-pass FEL, we demonstrate the dependence of FEL harmonics on the key parameters of beams and undulators and show that it is possible to reduce the second FEL harmonic content if the energy spread of an electron beam is increased. For LEUTL FEL radiation in the visible range, we analytically demonstrate a reduction in the second harmonic power by an order of magnitude if electron energy spread is increased twice, which still allows efficient bunching at the fundamental wavelength. The method is valid regardless of the photon energy and, therefore, the absorption edge of the target material, and as such, it is valid also in the X-ray band.

Fluctuation X-ray scattering (FXS) is an emerging experimental technique in which X-ray solution scattering data are collected from particles in solution using ultrashort X-ray exposures generated by a free-electron laser (FEL). FXS experiments overcome the low data-to-parameter ratios associated with traditional solution scattering measurements by providing several orders of magnitude more information in the final processed data. Here we demonstrate the practical feasibility of FEL-based FXS on a biological multiple-particle system and describe data-processing techniques required to extract robust FXS data and significantly reduce the required number of snapshots needed by introducing an iterative noise-filtering technique. We showcase a successful ab initio electron density reconstruction from such an experiment, studying the Paramecium bursaria Chlorella virus (PBCV-1).