Self-amplified Spontaneous Emission Free-electron Laser Research Articles

Differential Measurement of Electron Ejection after Two-Photon Two-Electron Excitation of Helium.

We report the measurement of the photoelectron angular distribution of two-photon single-ionization near the 2p^{2} ^{1}D^{e} double-excitation resonance in helium, benchmarking the fundamental nonlinear interaction of two photons with two correlated electrons. This observation is enabled by the unique combination of intense extreme ultraviolet pulses, delivered at the high-repetition-rate free-electron laser in Hamburg (FLASH), ionizing a jet of cryogenically cooled helium atoms in a reaction microscope. The spectral structure of the intense self-amplified spontaneous emission free-electron laser pulses has been resolved on a single-shot level to allow for post selection of pulses, leading to an enhanced spectral resolution, and introducing a new experimental method. The measured angular distribution is directly compared to state-of-the-art theory based on multichannel quantum defect theory and the streamlined R-matrix method. These results and experimental methodology open a promising route for exploring fundamental interactions of few photons with few electrons in general.

FEL Pulse Duration Evolution along Undulators at FLASH

Self-amplified spontaneous-emission (SASE) free-electron lasers (FELs) deliver ultrashort pulses with femtosecond durations. Due to the fluctuating nature of the radiation properties of SASE FELs, characterizing FEL pulses on a single-shot basis is necessary. Therefore, we use terahertz streaking to characterize the temporal properties of ultrashort extreme ultraviolet pulses from the free-electron laser in Hamburg (FLASH). In this study, pulse duration as well as pulse energy are measured in a wavelength range from 8 to 34 nm as functions of undulators contributing to the lasing process. The results are compared to one-dimensional and three-dimensional, time-dependent FEL simulations.

Data analysis procedures for time-resolved x-ray photoelectron spectroscopy at a SASE free-electron-laser

The random nature of self-amplified spontaneous emission (SASE) is a well-known challenge for x-ray core level spectroscopy at SASE free-electron lasers (FELs). Especially in time-resolved experiments that require a combination of good temporal and spectral resolution the jitter and drifts in the spectral characteristics, relative arrival time as well as power fluctuations can smear out spectral-temporal features. We present a combination of methods for the analysis of time-resolved photoelectron spectra based on power and time corrections as well as self-referencing of a strong photoelectron line. Based on sulfur 2p photoelectron spectra of 2-thiouracil taken at the SASE FEL FLASH2, we show that it is possible to correct for some of the photon energy drift and jitter even when reliable shot-to-shot photon energy data is not available. The quality of pump–probe difference spectra improves as random jumps in energy between delay points reduce significantly. The data analysis allows to identify coherent oscillations of 1 eV shift on the mean photoelectron line of 4 eV width with an error of less than 0.1 eV.

Construction and commissioning of mid-infrared self-amplified spontaneous emission free-electron laser at compact energy recovery linac.

The mid-infrared range is an important spectrum range where materials exhibit a characteristic response corresponding to their molecular structure. A free-electron laser (FEL) is a promising candidate for a high-power light source with wavelength tunability to investigate the nonlinear response of materials. Although the self-amplification spontaneous emission (SASE) scheme is not usually adopted in the mid-infrared wavelength range, it may have advantages such as layout simplicity, the possibility of producing a single pulse, and scalability to a short-wavelength facility. To demonstrate the operation of a mid-infrared SASE FEL system in an energy recovery linac (ERL) layout, we constructed an SASE FEL setup in cERL, a test facility of the superconducting linac with the ERL configuration. Despite the adverse circumstance of space charge effects due to the given boundary condition of the facility, we successfully established the beam condition at the undulators and observed FEL emission at a wavelength of 20 μm. The results show that the layout of cERL has the potential for serving as a mid-infrared light source.

Observation of harmonic lasing in the Angstrom regime at European X-ray Free Electron Laser

Harmonic lasing provides an opportunity to extend the photon energy range of existing and planned x-ray free electron laser (FEL) user facilities. Contrary to nonlinear harmonic generation, harmonic lasing can generate a much more intense, stable, and narrow-band FEL beam. Another interesting application is harmonic lasing self-seeding that allows to improve the longitudinal coherence and spectral power of a self-amplified spontaneous emission FEL. This concept was tested at the soft x-ray FEL user facility FLASH in the range of 4.5--15 nm and at Pohang accelerator laboratory X-ray Free Electron Laser (XFEL) at 1 nm. In this paper we present recent results from the European XFEL where we successfully demonstrated harmonic lasing at 5.9 Angstrom and 2.8 Angstrom. In the latter case we obtained both third and fifth harmonic lasing and, for the first time, operated a harmonic lasing cascade (fifth-third-first harmonics of the undulator). These results pave the way for reaching very high photon energies, up to 100 keV.

Study of temporal, spectral, arrival time and energy fluctuations of SASE FEL pulses.

Self-amplified spontaneous emission (SASE) pulses delivered by free electron lasers (FELs) are inherently fluctuating sources; each pulse varies in energy, duration, arrival time and spectral shape. Therefore, there is strong demand for a full characterization of the properties of SASE radiation, which will facilitate more precise interpretation of the experimental data taken at SASE FELs. In this paper, we present an investigation into the fluctuations of pulse duration, spectral distribution, arrival time and pulse energy of SASE XUV pulses at FLASH, both on a shot-to-shot basis and on average over many pulses. With the aid of simulations, we derived scaling laws for these parameters and disentangled the statistical SASE fluctuations from accelerator-based fluctuations and measurement uncertainties.

Об излучении гармоник в рентгеновских лазерах на свободных электронах с изменяемым параметром дипольности ондуляторов

X-ray free electron lasers (FELs) produce ultra short bursts of coherent radiation at the wavelengths ~1–100 . In the absence of ready seed in this range, the fundamental tone is usually radiated in self-amplified spontaneous emission (SASE) FELs. With the goal to achieve maximum performance in minimum size and cost of a FEL, we study the harmonic multiplication in resonating undulator cascades as viable alternative to the SASE fundamental radiation. The harmonic power evolution along FELs is analyzed for the SASE and the harmonic self-seed. The analytical results are compared with available data for soft X-rays FLASH2. The possibility of harmonic radiation is studied for FELs with variable deflection parameter k: FLASH2, SwissFEL and currently built LCLS-II. We demonstrate earlier growth of the harmonic power in a buncher as compared with the SASE regime at the same wavelength. The difference is explained theoretically by the harmonic bunching growth in the buncher, as compared with that in the SASE FEL at the same wavelength. Excellent characteristics of the SwissFEL allow amplification of its third harmonic. We propose the harmonic self-seed, which would allow 10 m shorter undulator length for saturation at the same power, and up to 30% shorter radiation wavelength with the existing electron beam. At the LCLS-II, good bunching at the third harmonic wavelength in a dedicated buncher allows early and strong third harmonic growth. However, further resonating amplifier requires small value of k in LCLS-II undulators, which makes the amplification slow. We propose using the full amplification of the third FEL harmonic in LCLS-II undulators with bunching disruption between them at the wavelength of the fundamental. We show that this scheme allows >1 GW FEL power at λ=0.25 nm wavelength achievable already at ~40 m of the undulators with the electron beam energy E=4 GeV.

Wigner distribution of self-amplified spontaneous emission free-electron laser pulses and extracting its autocorrelation.

The emerging concept of `beam by design' in free-electron laser (FEL) accelerator physics aims for accurate manipulation of the electron beam to tailor spectral and temporal properties of the radiation for specific experimental purposes, such as X-ray pump/X-ray probe and multiple wavelength experiments. `Beam by design' requires fast, efficient, and detailed feedback on the spectral and temporal properties of the generated X-ray radiation. Here a simple and cost-efficient method to extract information on the longitudinal Wigner distribution function of emitted FEL pulses is proposed. The method requires only an ensemble of measured FEL spectra and is rather robust with respect to accelerator fluctuations. The method is applied to both the simulated SASE spectra with known radiation properties as well as to the SASE spectra measured at the European XFEL revealing underlying non-linear chirp of the generated radiation. In the Appendices an intuitive understanding of time-frequency representations of chirped SASE radiation is provided.

Refractive Guide Switching a Regenerative Amplifier Free-Electron Laser for High Peak and Average Power Hard X Rays.

We present an x-ray regenerative amplifier free-electron laser design capable of producing fully coherent hard x-ray pulses across a broad tuning range at a high steady state repetition rate. The scheme leverages a strong undulator taper and an apertured diamond output-coupling cavity crystal to produce both high peak and average spectral brightness radiation that is 2 to 3 orders of magnitude greater than conventional single-pass self-amplified spontaneous emission free-electron laser amplifiers. Refractive guiding in the postsaturation regime is found to play a key role in passively controlling the stored cavity power. The scheme is explored both analytically and numerically in the context of the Linac Coherent Light Source II High Energy upgrade.

Analysis of ultra-short bunches in free-electron lasers

Free-electron lasers (FELs) operate at wavelengths from millimeter waves through hard x-rays. At x-ray wavelengths, FELs typically rely on self-amplified spontaneous emission (SASE). Typical SASE emission contains multiple temporal ‘spikes’ which limit the longitudinal coherence of the optical output; hence, alternate schemes that improve on the longitudinal coherence of the SASE emission are of interest. In this paper, we consider electron bunches that are shorter than the SASE spike separation. In such cases, the spontaneously generated radiation consists of a single optical pulse with better longitudinal coherence than is found in typical SASE FELs. To investigate this regime, we use two FEL simulation codes. One (MINERVA) uses the slowly-varying envelope approximation (SVEA) which breaks down for extremely short pulses. The second (PUFFIN) is a particle-in-cell simulation code that is considered to be a more complete model of the underlying physics and which is able to simulate very short pulses. We first anchor these codes by showing that there is substantial agreement between the codes in simulation of the SPARC SASE FEL experiment at ENEA Frascati. We then compare the two codes for simulations using electron bunch lengths that are shorter than the SASE slice separation. The comparisons between the two codes for short bunch simulations elucidate the limitations of the SVEA in this regime but indicate that the SVEA can treat short bunches that are comparable to the cooperation length.

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.

A hard X-ray self-amplified spontaneous emission free-electron laser optimizationusing evolutionary algorithms for dedicated user applications

Accelerator-based fourth-generation light sources are utilized in a wide range of interdisciplinary applications such as nanotechnology, materials science, biosciences, and medicine. A hard X-ray free-electron laser (FEL), as a state-of-the-art light source, was optimized using evolutionary algorithms for dedicated user applications such as X-ray Raman scattering (XRS), resonant inelastic X-ray scattering (RIXS), and X-ray emission spectroscopies (XES). Optimal parameter sets were obtained for an in-vacuum planar undulator driven by an 8 GeV electron beam. Performance parameters of self-amplified spontaneous emission (SASE) operation (i.e. optimized SASE performance parameters through evolutionary algorithms) were found to be consistent with operating X-ray FEL facilities around the world. It is shown that FEL characteristics for specific user experiments can be optimized by finding several evolutionary algorithm solutions within the range of 5 keV to 10 keV.

Hard X-ray self-seeding commissioning at PAL-XFEL.

A wake monochromator based on a large-area diamond single crystal for hard X-ray self-seeding has been successfully installed and commissioned in the hard X-ray free-electron laser (FEL) at the Pohang Accelerator Laboratory with international collaboration. For this commissioning, the self-seeding was demonstrated with a low bunch charge (40 pC) and the nominal bunch charge (180 pC) of self-amplified spontaneous emission (SASE) operation. The FEL pulse lengths were estimated as 7 fs and 29.5 fs, respectively. In both cases, the average spectral brightness increased by more than three times compared with the SASE mode. The self-seeding experiment was demonstrated for the first time using a crystal with a thickness of 30 µm, and a narrow bandwidth of 0.22 eV (full width at half-maximum) was obtained at 8.3 keV, which confirmed the functionality of a crystal with such a small thickness. In the nominal bunch-charge self-seeding experiment, the histogram of the intensity integrated over a 1 eV bandwidth showed a well defined Gaussian profile, which is evidence of the saturated FEL and a minimal electron-energy jitter (∼1.2 × 10-4) effect. The corresponding low photon-energy jitter (∼2.4 × 10-4) of the SASE FEL pulse, which is two times lower than the Pierce parameter, enabled the seeding power to be maximized by maintaining the spectral overlap between SASE FEL gain and the monochromator.

Generation of Coherent Radiation in the Near X-Ray Band by a Cascade FEL with a Two-Frequency Undulator

Generation of X-ray radiation in a cascade self-amplified spontaneous emission free-electron laser (SASE FEL) using the harmonics of a two-frequency undulator is studied. The advanced phenomenological model of a one-pass FEL that accounts for the main losses in real FELs is presented: the electron energy spread in the beam, the beam divergence, diffraction, and the fact that emission losses are greater at higher harmonics than in the main frequency range. The FEL mathematical model was performed using the Mathematica software and calibrated within the experiment carried out at the operating SPARC facility via complex three-dimensional numerical simulations. The phenomenological model is used to analyze FEL dynamics for generation of a high-energy X-ray emission at a relatively short length. It is proposed to use a two-frequency undulator for the initial electron grouping and subsequent frequency multiplication in a cascade FEL with higher harmonic amplification (HGHG). The advantages of the two-frequency undulator are presented for electron grouping at higher harmonics of the undulator radiation (UR). The operation of several types of FEL is simulated with amplification of the seed laser wave frequency in two and three cascades to generate the soft X-ray radiation. A seed laser with a wavelength of 11.43 nm corresponding to the peak reflectivity of mirror coatings with MoRu/Be is proposed for generating the intensive X-ray laser radiation with λ ~ 1.27–3.37 nm. Here, the intensive radiation power reaches 50 MW at a length of only 35 meters; the radiation shows good temporal coherence corresponding to the performance of a low-power seed laser with a lower frequency.

Dispersion-Based Fresh-Slice Scheme for Free-Electron Lasers.

The fresh-slice technique improved the performance of several self-amplified spontaneous emission free-electron laser schemes by granting selective control on the temporal lasing slice without spoiling the other electron bunch slices. So far, the implementation has required a special insertion device to create the beam yaw, called a dechirper. We demonstrate a novel scheme to enable fresh-slice operation based on electron energy chirp and orbit dispersion that can be implemented at any free-electron laser facility without additional hardware.

High-Power Femtosecond Soft X Rays from Fresh-Slice Multistage Free-Electron Lasers.

We demonstrate a novel multistage amplification scheme for self-amplified spontaneous-emission free electron lasers for the production of few femtosecond pulses with very high power in the soft x-ray regime. The scheme uses the fresh-slice technique to produce an x-ray pulse on the bunch tail, subsequently amplified in downstream undulator sections by fresh electrons. With three-stages amplification, x-ray pulses with an energy of hundreds of microjoules are produced in few femtoseconds. For single-spike spectra x-ray pulses the pulse power is increased more than an order of magnitude compared to other techniques in the same wavelength range.

High-harmonic x-ray undulator radiation for nanoscale-wavelength free-electron lasers

We explore the undulator radiation (UR) from relativistic electrons in two-frequency undulators intended for use in x-ray free-electron lasers (FELs). We use an exact analytical account for the major sources of spectral line broadening of the spontaneous UR to describe the UR spectrum, intensity and emission line shape. We also show possible partial compensation of the off-axis divergence by ad hoc imposed weak transverse constant magnetic components. The harmonic radiation of the two-frequency undulator is studied. High-harmonics amplification in the two-frequency undulator is underlined for possible use in x-ray FELs and the generation of powerful x-ray radiation is demonstrated. The evolution of high harmonic power is studied with the help of the developed phenomenological self-amplified spontaneous emission FEL model. The performance of a high-gain harmonic generation FEL with a two-frequency undulator–prebuncher is explored. Harmonic multiplication cascades are introduced in FELs to obtain nanometer x-ray FEL radiation in reduced size FELs at ~30 m with seed from commercially available excimer laser and ~0.5 GeV electrons.

Statistical properties of a free-electron laser revealed by Hanbury Brown–Twiss interferometry

We present a comprehensive experimental analysis of statistical properties of the self-amplified spontaneous emission (SASE) free-electron laser (FEL) FLASH at DESY in Hamburg by means of Hanbury Brown and Twiss (HBT) interferometry. The experiments were performed at the FEL wavelengths of 5.5 nm, 13.4 nm, and 20.8 nm. We determined the 2-nd order intensity correlation function for all wavelengths and different operation conditions of FLASH. In all experiments a high degree of spatial coherence (above 50%) was obtained. Our analysis performed in spatial and spectral domains provided us with the independent measurements of an average pulse duration of the FEL that were below 60 fs. To explain complicated behaviour of the 2-nd order intensity correlation function we developed advanced theoretical model that includes the presence of multiple beams and external positional jitter of the FEL pulses. By this analysis we determined that in most experiments several beams were present in radiating field and in one of the experiments external positional jitter was about 25% of the beam size. We envision that methods developed in our study will be used widely for analysis and diagnostics of the FEL radiation.

Normal planar undulators doubling as transverse gradient undulators

The transverse gradient undulator (TGU) has important application in the short-wavelength high-gain free electron lasers (FELs) driven by laser-plasma accelerators. However, the usual transversely tapered TGUs need special design and manufacture, and the transverse gradient cannot be tuned arbitrarily. In this paper we explore a new and simple method of using the natural transverse gradient of a normal planar undulator to compensate the beam energy spread effect. In this method, a vertical dispersion on the electron beam is introduced, then the dispersed beam passes through a normal undulator with a vertical off-axis orbit where the vertical field gradient is selected properly related to the dispersion strength and the beam energy spread. Theoretical analysis and numerical simulations for self-amplified spontaneous emission FELs based on laser plasma accelerators are presented, and indicate that this method can greatly reduce the effect of the beam energy spread, leading to a similar enhancement on FEL performance as the usual transversely tapered TGU, but with the advantages of economy, tunable transverse gradient and no demand of extra field for correcting the orbit deflection induced by the field gradient.Received 21 December 2016DOI:https://doi.org/10.1103/PhysRevAccelBeams.20.020707Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsSynchrotron radiation & free-electron lasersAccelerators & Beams

Single-atom response of helium atoms to pulses from an EUV free-electron laser: Implications for the subsequent development of superfluorescence

Using multilevel (161-level) numerical simulations beyond the rotating-wave approximation, we investigate the single-atom response of helium to intense pulses of radiation with properties characteristic of a self-amplified spontaneous emission (SASE) free-electron laser (FEL) source, and central wavelengths resonant with $1snp$ excitation ($n=3$,4). The results show a complete breakdown of the two-level approximation and suggest that, under typical experimental conditions, levels other than the resonant level are significantly populated. We discuss the implications for the subsequent development of superfluorescence (a collective effect) from the excited atomic medium and compare the results to recent experiments.