Free-electron Laser Radiation Research Articles

Quantum description of free electron laser radiation and nonlinear amplitude equations

A relativistic quantum mechanical model to describe the quantum free electron laser dynamics has been developed. Neglecting the spin of electrons in the impacting beam, this model is based on the Klein–Gordon equation coupled to the Poisson equation for the space-charge potential and the wave equation for the transverse components of the radiation field. Furthermore, a system of coupled nonlinear envelope equations for the slowly varying amplitudes of the electron beam distribution and the radiation field has been derived. The fundamental system of basic equations has been cast into a suitable hydrodynamic formulation. In the framework of the hydrodynamic representation, a new dispersion relation has been derived and analyzed in both the quantum and the quasi-classical regimes, where the space-charge oscillations of the electron beam are taken into account.

Development of an APPLE III undulator prototype with three-dimensional force compensation for SHINE

The Shanghai high-repetition-rate XFEL and extreme light facility (SHINE) plans to install several elliptically polarizing undulators (EPUs) as afterburners behind the planar undulator section to obtain nearly saturated circularly polarized free-electron laser (FEL) radiation. Therefore, the SHINE R&D project needs to develop a 4-m-long EPU with an effective magnetic field of 1.5 T, a period of 68 mm, and a minimum gap of 3 mm. A magnetic force compensation EPU prototype has recently been built and tested at the Shanghai Synchrotron Radiation Facility (SSRF) based on the addition of permanent magnets. This prototype can realize magnetic compensation under various polarization modes in all working gaps, thereby reducing the deformation of the girders and maintaining sufficient rigidity of the mechanical structure. A girder deformation monitoring system was established to obtain real force compensation feedback. The final magnetic field measurement results meet the physical requirements, and the proposed scheme can be used as an alternative to the EPU with a high magnetic field and large magnetic force.

Ultrafast Roaming Mechanisms in Ethanol Probed by Intense Extreme Ultraviolet Free-Electron Laser Radiation: Electron Transfer versus Proton Transfer

Ultrafast H2+ and H3+ formation from ethanol is studied using pump-probe spectroscopy with an extreme ultraviolet (XUV) free-electron laser. The first pulse creates a dication, triggering H2 roaming that leads to H2+ and H3+ formation, which is disruptively probed by a second pulse. At photon energies of 28 and 32 eV, the ratio of H2+ to H3+ increases with time delay, while it is flat at a photon energy of 70 eV. The delay-dependent effect is ascribed to a competition between electron and proton transfer. High-level quantum chemistry calculations show a flat potential energy surface for H2 formation, indicating that the intermediate state may have a long lifetime. The ab initio molecular dynamics simulation confirms that, in addition to the direct emission, a small portion of H2 undergoes a roaming mechanism that leads to two competing pathways: electron transfer from H2 to C2H4O2+ and proton transfer from C2H4O2+ to H2.

Dependence of Pit Formation in Hard Tissue of Human Teeth on Free Electron Laser Pulse Structures

The purpose of this study was to compare and clarify the differences in pit formation between two modes of free-electron laser（FEL）irradiation with different beam cur rents and pulse structures and how the effects of these modes vary with the tissue structure of human teeth. FEL irradiation using the full-bunch mode（Fm）and burst mode（Bm）. The beam current was carried out at 200 mA for Fm and 2 A for Bm. The micro-pulse interval within the macro-pulse structure was 350 ps for Fm and 22.4 ns for Bm. The wavelength of the FEL was adjusted to 2.94 µm, and macro-pulse energy was adjusted to 6.0 mJ/macro-pulse by polarizing plate. The macro-pulse irradiation was set to 1 or 5 times for both modes. As the number of macro-pulse irradiations increased, the pits became deeper in Fm than in Bm, regardless of the site on the tooth. On the other hand, the bulge height was lower for Bm than for Fm, regardless of the site on the tooth or the number of macro-pulse irradiations. No scorch marks were observed in the pits for either mode. It was concluded that the differences in the pit including bulge formation were the macro-pulse structure between Fm and Bm. The intensity of the FEL irradiation caused plasma evaporation in the tooth hard tissue, and the tooth substance was ablated before thermal conduction occurred. The relation with the thermal relaxation time also confirmed that pits were formed without visible scorch marks.

Investigating ultrafast structural dynamics using high repetition rate x-ray FEL radiation at European XFEL

European XFEL is an international facility providing hard and soft x-ray free-electron laser radiation for user experiments with a wide range of scientific applications. Its superconducting linear accelerator enables high repetition rate experiments with a broad range of x-ray pulse delivery patterns. The combination of time-resolved experiments, providing access to the time-domain from sub-femtoseconds to milliseconds, with atomic resolution x-ray geometric and electronic structure determination methods is responsible for the bulk of scientific applications of European XFEL. In addition, the extreme x-ray intensities and coherence properties open new methods for studying matter out of equilibrium. After start of operation in 2017, the facility now harvests scientific applications with impact to the challenge areas climate and energy, health, environment and sustainability, and digitalization. Extensions of European XFEL aim to increase performance and capabilities for new scientific applications. An upgrade of the facility in the early 2030s will increase the applicability of European XFEL to solid materials and provide dedicated instruments for improved conditions in specific research fields.

IR/UV Double Resonance Study of the 2-Phenylallyl Radical and its Pyrolysis Products.

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.

The INFN-LNF present and future accelerator-based light facilities.

The INFN-Frascati National Laboratory (LNF) is nowadays running a 0.51 GeV electron-positron collider, DA NE, that also represents the synchrotron radiation source of the beamlines of the DA NE-Light facility. Not being DA NE dedicated to synchrotron radiations activities, the DA NE-Light facility can use it mainly in parasitic mode. Particle accelerators and high energy physics (HEP) have been and are the main core of the LNF research activity, but like other HEP international laboratories also LNF is now moving in the direction of developing a dedicated free electron laser (FEL) user facility, EuPRAXIA@SPARC_Lab, based on plasma acceleration. This new facility in the framework of the EuPRAXIA (European Plasma Research Accelerator with eXcellence in Applications) EU project should produce FEL radiation beams for a wide range of applications using a smaller accelerator compared to actual radio frequency-based accelerator sources dimensions.

Considered design and radiation calculation for a compact THz coherent radiation source

Since the light obtained from the Siam Photon Source (SPS) in Thailand is the high-intensity radiation in only the region of the infrared (IR) to high-energy X-rays, it has great attention in many areas of scientific research and industrial development. In order to increase in the number of users and applications, the terahertz (THz) region covering the frequency range from 0.1 to 10 THz needs to be fulfilled. Therefore, a compact accelerator based free-electron laser or coherent radiation is particularly interesting for providing an intense THz radiation. A 1.6-cell photocathode RF gun is used as an electron source to generate a high-brightness electron beam. The energy of electron beam can be raised up to meet the target of 30 MeV in a linear accelerator (Linac) before moving to a chicane bunch compressor. The THz radiation is produced when a short bunch electron beam propagates through an undulator. In order to achieve the intense narrow-band THz coherent radiation at tunable frequencies between 0.5 THz and 5.0 THz with MW peak power, a THz radiation source needs to be studied and developed. Design of high-power THz radiation source is described in this paper.

Features of the breakdown in heavy noble gases under the action of Novosibirsk free electron laser radiation

The results of experimental studies on the breakdown of noble gases (argon and krypton) by terahertz radiation from the Novosibirsk free electron laser (NovoFEL) are presented. For the first time, the breakdown thresholds of noble gases by terahertz radiation were measured in a wide pressure range (0.2-1.5 bar). Previous experiments to measure breakdown thresholds in the THz range in various gases were carried out for hundreds of GHz or at atmospheric pressure. Experimental breakdown thresholds are compared with calculated data using various simplified models. Keywords: THz radiation, gas breakdown, THz laser discharge.

Probing protein misfolding and dissociation with an infrared free-electron laser.

Misfolding is observed in the mutant proteins that are causative for neurodegenerative disorders such as polyglutamine diseases. These proteins are prone to aggregate in the cytoplasm and nucleus of cells. To reproduce cells with the aggregated proteins, gene expression system is usually applied, in which the expression construct having the mutated DNA sequence of the interest is transfected into cells. The transfected DNA is finally converted into the mutant protein, which is gradually aggregated in the cells. In addition, a simple method to prepare the cells having aggregates inside has been recently applied. Peptides were first aggregated by incubating them in water. The aggregates are spontaneously taken up by cells because aggregated proteins generally transfer between cells. Peptides with different degrees of aggregation can be made by changing the incubation times and temperatures, which enables to examine contribution of aggregation to the toxicity to the recipient cells. Moreover, such cells can be used for therapeutic researches of diseases in which aggregates are involved. In this chapter, we show methods to induce aggregation of peptides. The functional analyses of the cells with aggregates are also described. Then, experimental dissociation of the aggregates produced using this method by mid infrared free electron laser irradiation and its theoretical support by molecular dynamics simulation are introduced as the therapeutic research for neurodegenerative disorders.

Damage resistance of B4C reflective mirror irradiated by X-ray free-electron laser

In this paper, a simple theoretical model combining Monte Carlo simulation with the enthalpy method is provided to simulate the damage resistance of B4C/Si-sub mirror under X-ray free-electron laser irradiation. Two different damage mechanisms are found, dependent on the photon energy. The optimum B4C film thickness is determined by studying the dependence of the damage resistance on the film thickness. Based on the optimized film thickness, the damage thresholds are simulated at photon energy of 0.4–25 keV and a grazing incidence angle of 2 mrad. It is recommended that the energy range around the Si K-edge should be avoided for safety reasons.

An analytical study to determine the mechanical tolerances for the afterburner superconducting undulators at EuXFEL

At the European XFEL, a superconducting afterburner is planned for the SASE2 hard X-ray beamline. It will consist of 5 undulator modules plus a pre-series module called S-PRESSO. Within each module, two superconducting undulators (SCU) 2 m long are present. Such an afterburner will enable photon energies above 30 keV.The magnetic field of the SCU determines the quality of the electron beam trajectory and the free-electron laser (FEL) radiation. The mechanical accuracy of the SCU determines its magnetic field quality. In this contribution, we present the mechanical errors and an analytical study to determine the tolerances for our SCUs.

Stable Operation of a Free-Electron Laser Driven by a Plasma Accelerator.

The breakthrough provided by plasma-based accelerators enabled unprecedented accelerating fields by boosting electron beams to gigaelectronvolt energies within a few centimeters [1-4]. This, in turn, allows the realization of ultracompact light sources based on free-electron lasers (FELs) [5], as demonstrated by two pioneering experiments that reported the observation of self-amplified spontaneous emission (SASE) driven by plasma-accelerated beams [6,7]. However, the lack of stability and reproducibility due to the intrinsic nature of the SASE process (whose amplification starts from the shot noise of the electron beam) may hinder their effective implementation for user purposes. Here, we report a proof-of-principle experiment using plasma-accelerated beams to generate stable and reproducible FEL light seeded by an external laser. FEL radiation is emitted in the infrared range, showing the typical exponential growth of its energy over six consecutive undulators. Compared to SASE, the seeded FEL pulses have energies 2 orders of magnitude larger and stability that is 3 times higher.

Energy Transfer and Restructuring in Amorphous Solid Water upon Consecutive Irradiation.

Interstellar and cometary ices play an important role in the formation of planetary systems around young stars. Their main constituent is amorphous solid water (ASW). Although ASW is widely studied, vibrational energy dissipation and structural changes due to vibrational excitation are less well understood. The hydrogen-bonding network is likely a crucial component in this. Here, we present experimental results on hydrogen-bonding changes in ASW induced by the intense, nearly monochromatic mid-IR free-electron laser (FEL) radiation of the FELIX-2 beamline at the HFML-FELIX facility at the Radboud University in Nijmegen, The Netherlands. Structural changes in ASW are monitored by reflection-absorption infrared spectroscopy and depend on the irradiation history of the ice. The experiments show that FEL irradiation can induce changes in the local neighborhood of the excited molecules due to energy transfer. Molecular dynamics simulations confirm this picture: vibrationally excited molecules can reorient for a more optimal tetrahedral surrounding without breaking existing hydrogen bonds. The vibrational energy can transfer through the hydrogen-bonding network to water molecules that have the same vibrational frequency. We hence expect a reduced energy dissipation in amorphous material with respect to crystalline material due to the inhomogeneity in vibrational frequencies as well as the presence of specific hydrogen-bonding defect sites, which can also hamper the energy transfer.

Saturable Absorption of Free-Electron Laser Radiation by Graphite near the Carbon K-Edge.

The interaction of intense light with matter gives rise to competing nonlinear responses that can dynamically change material properties. Prominent examples are saturable absorption (SA) and two-photon absorption (TPA), which dynamically increase and decrease the transmission of a sample depending on pulse intensity, respectively. The availability of intense soft X-ray pulses from free-electron lasers (FELs) has led to observations of SA and TPA in separate experiments, leaving open questions about the possible interplay between and relative strength of the two phenomena. Here, we systematically study both phenomena in one experiment by exposing graphite films to soft X-ray FEL pulses of varying intensity. By applying real-time electronic structure calculations, we find that for lower intensities the nonlinear contribution to the absorption is dominated by SA attributed to ground-state depletion; our model suggests that TPA becomes more dominant for larger intensities (>1014 W/cm2). Our results demonstrate an approach of general utility for interpreting FEL spectroscopies.

Stacking Is Favored over Hydrogen Bonding in Azaphenanthrene Dimers.

N-Doped polycyclic aromatic hydrocarbons have recently emerged as potential organic electronic materials. The function of such materials is determined not only by the intrinsic electronic properties of individual molecules but also by their supramolecular interactions in the solid state. Therefore, a proper characterization of the interactions between the individual units is of interest to materials science since they ultimately govern properties such as excitons and charge transfer. Here, we report a joint experimental and computational study of two azaphenanthrene dimers to determine the structure and the nature of supramolecular interactions in the aggregates. IR/UV double-resonance experiments were carried out using far- and mid-infrared free-electron laser radiation. The experimental spectra are compared with quantum chemical calculations for the lowest-energy π-stacked and hydrogen-bonded structures. The data reveal a preference of the π-stacked structure for the benzo[f]quinoline and the phenanthridine dimer.

An application of a cavity-based beam arrival time measurement system: Beam energy measurement

A high-resolution cavity-based beam arrival/flight time measurement system has been established at SXFEL (Shanghai soft X-ray free-electron laser facility). During the development of the system, a correlation between the energy jitter and the beam flight time jitter was found which inspires and motivates us to further explore the possibility of using the beam arrival/flight time measurement system to measure the beam energy. On the one hand, beam energy is one of the most fundamental and critical parameters for accelerator facilities. Only precise handling of the beam energy enables precise control of the FEL (free-electron laser) radiation wavelength. On the other hand, there are only a few methods for beam energy measurement. The profile-based beam energy measurement method is commonly used but is interceptive. The BPM-based beam energy measurement scheme is non-interceptive but has a limited dynamic range and requires initial beam position calibration. This paper reports a promising application of a cavity-based high-resolution beam arrival/flight time measurement system: non-intercepting wide-range beam energy measurement. A brief introduction of the method principle and system implementation has been covered in this article. Moreover, a beam-based preliminary test has been performed and the test results show a linear correlation between the beam energy and the beam flight time under the current system configuration. The measured linear factor is 0.692 ps/MeV. In addition, the relative bunch energy measurement resolution is 0.0001.

Ionization phase retrieval by angular streaking from random shots of XUV radiation

We demonstrate an accurate phase retrieval of extreme ultraviolet (XUV) atomic ionization by streaking the photoelectron in a circularly polarized infrared (IR) laser field. Our demonstration is based on a numerical solution of the time-dependent Schr\"odinger equation. We test this technique using the hydrogen atom ionized by isolated attosecond XUV pulses across a wide range of the photon energies. Importantly, the proposed method works in a random shot mode when the time delay between the ionizing and streaking XUV and IR pulses may vary from shot to shot. This is a significant advantage over the existing interferometric techniques which require a systematic and controllable scan of the XUV and IR delay in one set of measurements. Such a scan may not be possible to take because of the arrival time jitter inherent to the stochastic nature of self-amplified free-electron laser radiation.

High field hybrid photoinjector electron source for advanced light source applications

The production of high spectral brilliance radiation from electron beam sources depends critically on the electron beam qualities. One must obtain very high electron beam brightness, implying simultaneous high peak current and low emittance. These attributes are enabled through the use of very high field acceleration in a radio-frequency (rf) photoinjector source. Despite the high fields currently utilized, there is a limit on the achievable peak current in high brightness operation, in the range of tens of Ampere. This limitation can be overcome by the use of a hybrid standing wave/traveling wave structure; the standing wave portion provides acceleration at a high field from the photocathode, while the traveling wave part yields strong velocity bunching. This approach is explored here in a C-band scenario, at field strengths ($>100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$) at the current state-of-the-art. It is found that one may arrive at an electron beam with many hundreds of Amperes with well-sub-micron normalized emittance. This extremely compact injector system also possesses attractive simplification of the rf distribution system by eliminating the need for an rf circulator. We explore the use of this device in a compact 400 MeV-class source, driving both inverse Compton scattering and free-electron laser radiation sources with unique, attractive properties.

Nonlinear terahertz transmission spectroscopy on Ga-doped germanium in high magnetic fields

The authors report the observation of cyclotron resonance transitions of holes in the magnetotransmission spectra of gallium-doped germanium at low temperatures, using intense, pulsed terahertz free-electron laser radiation with a photon energy lower than the ionization energy of the Ga dopants. The spectra are very sensitive to THz intensity and provide compelling results about ionization mechanisms and saturation of Landau level transitions. The spectroscopy technique introduced here offers an exciting perspective for researching nonlinear magneto-optical processes in semiconductors.