Self-amplified Spontaneous Emission Research Articles

Hard X-ray operation of X-ray gas monitors at theEuropean XFEL.

X-ray gas monitors (XGMs) are operated at the European XFEL for non-invasive single-shot pulse energy measurements and average beam-position monitoring. The underlying measurement principle is the photo-ionization of rare gas atoms at low gas pressures and the detection of the photo-ions and photo-electrons created. These are essential for tuning and sustaining self-amplified spontaneous emission (SASE) operation, machine radiation safety, and sorting single-shot experimental data according to pulse energy. In this paper, the first results from XGM operation at photon energies up to 30 keV are presented, which are far beyond the original specification of this device. Here, the Huge Aperture MultiPlier (HAMP) is used for single-shot pulse energy measurements since the standard X-ray gas monitor detectors (XGMDs) do notprovide a sufficient signal-to-noise ratio, even at the highest operating gas pressures. A single-shot correlation coefficient of 0.98 is measured between consecutive XGMs operated with HAMP, which is as good as measuring with the standard XGMD detectors. An intra-train non-linearity of the HAMP signal is discovered, and operation parameters to mitigate this effect are studied. The upper repetition rate limit of HAMP operation at 2.25 MHz is also determined. Finally, the possibilities and limits for future XGM operation at photon energies up to 50 keV are discussed.

Efficient prediction of attosecond two-colour pulses from an X-ray free-electron laser with machine learning

X-ray free-electron lasers are sources of coherent, high-intensity X-rays with numerous applications in ultra-fast measurements and dynamic structural imaging. Due to the stochastic nature of the self-amplified spontaneous emission process and the difficulty in controlling injection of electrons, output pulses exhibit significant noise and limited temporal coherence. Standard measurement techniques used for characterizing two-coloured X-ray pulses are challenging, as they are either invasive or diagnostically expensive. In this work, we employ machine learning methods such as neural networks and decision trees to predict the central photon energies of pairs of attosecond fundamental and second harmonic pulses using parameters that are easily recorded at the high-repetition rate of a single shot. Using real experimental data, we apply a detailed feature analysis on the input parameters while optimizing the training time of the machine learning methods. Our predictive models are able to make predictions of central photon energy for one of the pulses without measuring the other pulse, thereby leveraging the use of the spectrometer without having to extend its detection window. We anticipate applications in X-ray spectroscopy using XFELs, such as in time-resolved X-ray absorption and photoemission spectroscopy, where improved measurement of input spectra will lead to better experimental outcomes.

Three-dimensional, time-dependent simulation of tapered EUV FELs with phase shifters

In this article, we have developed and evaluated a method for obtaining extreme ultraviolet (EUV) radiation by amplifying the third harmonic of the free-electron laser (FEL) radiation using phase shifters and tapering. Simulations of self-amplified spontaneous emission (SASE) FELs and seeding schemes are presented. We have demonstrated that the combination of phase shifts and tapering results in a more efficient technique that reduces electron beam energy by half. The stability of the scheme in relation to phase shift errors has been analyzed, and the implementation of the method has been discussed. The proposed FEL design enables a decrease in facility energy and a reduction in the cost of both new and existing projects in the EUV band.

Transient X-Ray Absorption Near Edge Structure Spectroscopy Using Broadband Free-Electron Laser Pulses.

A new method for time-resolved X-ray absorption near edge structure (XANES) spectroscopy that enables faster data acquisition and requires smaller sample quantities for high-quality data, thus allowing the analysis of more samples in a shorter time is introduced. The method uses large bandwidth free electron laser pulses to measure laser-excited XANES spectra in transmission mode. A beam-splitting grating configuration allows simultaneous measurements of the spectra of the incoming X-ray Free Electron Laser (XFEL) pulses and transmission XANES, which is crucial for compensating the pulse-dependent intensity and spectrum fluctuations due to the self-amplified spontaneous emission operation. The implementation of this new methodology is applied on a liquid solution of ammonium iron(III) oxalate jet and is compared to previous results, showing great improvements in the speed of acquisition and spectral resolution, and the ability to measure a large 2-D spectral-time map quickly.

Photon Diagnostics for the High-Gain THz FEL at PITZ

Research and development of an accelerator-based THz source prototype for pump-probe experiments at the European XFEL are ongoing at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). Proof-of-principle experiments have been performed to generate a high-gain THz Free-electron Laser (FEL) based on the Self-Amplified Spontaneous Emission scheme. The FEL radiation pulses with a central wavelength of about 100 μm (3 THz) and single pulse energy of several tens μm can be generated. In this paper, we present and discuss the photon diagnostic setup for the THz FEL together with examples of diagnostic results, including pulse energy and an FEL gain curve. The upgraded photon diagnostic setup, capable of measuring pulse energy, transverse distribution, and spectral distribution, is expected to be operational in the spring of 2023.

Sustainable early-stage lasing in a low-emittance electron storage ring

In this Letter, we report on the concept and analysis of a low-emittance electron storage ring, in which the electron beams undergo an early-stage self-amplified spontaneous emission lasing process on a turn-by-turn basis. The lasing process for each pass through a long undulator in the ring is terminated when the radiated power is still negligible compared to the total synchrotron loss of each circulation, and the electron beams can be maintained in an equilibrium state that supports sustainable lasing. A self-consistent model is derived for evaluation of the properties of the electron beams, and a design with numerical modeling is presented that demonstrates the feasibility of generating short wavelength radiation at the kW power level.

Prediction on X-ray output of free electron laser based on artificial neural networks

Knowledge of x-ray free electron lasers’ (XFELs) pulse characteristics delivered to a sample is crucial for ensuring high-quality x-rays for scientific experiments. XFELs’ self-amplified spontaneous emission process causes spatial and spectral variations in x-ray pulses entering a sample, which leads to measurement uncertainties for experiments relying on multiple XFEL pulses. Accurate in-situ measurements of x-ray wavefront and energy spectrum incident upon a sample poses challenges. Here we address this by developing a virtual diagnostics framework using an artificial neural network (ANN) to predict x-ray photon beam properties from electron beam properties. We recorded XFEL electron parameters while adjusting the accelerator’s configurations and measured the resulting x-ray wavefront and energy spectrum shot-to-shot. Training the ANN with this data enables effective prediction of single-shot or average x-ray beam output based on XFEL undulator and electron parameters. This demonstrates the potential of utilizing ANNs for virtual diagnostics linking XFEL electron and photon beam properties.

Investigation of the stray radiation origin and composition at the European XFEL undulators with gafchromic films measurements and Geant4 simulations

At EuXFEL, a stray field of ionizing radiation is present near undulator magnets which eventually can lead to their demagnetization and decreased efficiency of the Self-Amplified Spontaneous Emission (SASE) process. In this work we investigate both the origin and composition of the radiation field at the undulator magnets with gafchromic film measurements and Geant4 Monte Carlo simulations. Both measurements and simulations suggest that the radiation field in the upstream undulator cells comes from high-energy electrons striking the beam pipe. In addition, horizontal dose distributions at the entrance of the undulator segments depend on where the electrons hit the beam pipe. Geant4 simulations show that the radiation fluence near magnets is dominated by photons, while charged secondary electrons and positrons contribute almost entirely to the ionizing dose absorbed by magnets.

Toward a fully coherent tender and hard X-ray free-electron laser via cascaded EEHG in fourth-generation synchrotron light sources.

Free-electron-laser-based beamlines utilize fully coherent laser pulses with extremely narrow bandwidth allowing direct use of X-rays without monochromators. This could be very beneficial for all users of current and future fourth-generation diffraction-limited synchrotron light sources (DL-SLSs) who need narrowband full-coherence high-brightness X-ray pulses. Based on our previous finding, i.e. that separating the two stages of echo-enabled harmonic generation (EEHG) with a few extra bending-magnet sections provides an effective way to increase the momentum compaction of chicane 1, one can simultaneously achieve adequate prebunching at extremely high harmonics as well as keep the energy modulation to the ideal minimum. This could open the door for cascaded EEHG, toward fully coherent tender and hard X-ray wavelengths. Built on our compact design of a twin-pulse seeding electron beam with an adjustable delay and timing jitter at the level of a few femtoseconds, a cascaded EEHG can be implemented, which includes two EEHG beamlines, where the radiation pulse generated by the first beamline with harmonic h1 could be used as the input seed laser pulse to the second beamline with harmonic h2. Hence, the second radiator could potentially reach very high harmonics [h = h1(20)h2(25-100)] from 500 to 2000, corresponding to tender and hard X-ray wavelengths. It is demonstrated that the cascaded EEHG scheme is compatible with almost any current or planned fourth-generation DL-SLS, with significant benefits for space-limited storage rings in particular. The main advantage is that this scheme requires almost no change of the storage-ring lattice and is fully compatible with other beamlines. Current proposals for rings with much longer straight sections would add self-amplified spontaneous emission as another viable option for storage-ring-based free-electron lasers.

Generation of Ultrashort Pulses in XUV and X-ray FELs via an Excessive Reverse Undulator Taper

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.

Status and future of the soft X-ray free-electron laser beamline at the SHINE

The Shanghai High repetition rate XFEL and Extreme light facility (SHINE) is under construction and aims at generating X-rays between 0.4 and 25 keV with three FEL beamlines at repetition rates of up to 1 MHz. The soft X-ray FEL beamline, FEL-II, will be ready for commissioning in 2025. It is designed to cover the photon energy from 0.4 to 3 keV, in which the baselines of the FEL operation modes are self-amplified spontaneous emission (SASE), self-seeding, echo-enabled harmonic generation (EEHG), and polarization control. Therefore, a high repetition-rate external seed laser, large period length modulator, soft X-ray monochromator, planar undulator, and elliptically polarized undulator have been adopted in the FEL-II beamline. Several potentials such as an ultra-short pulse mode and a multi-color mode are also foreseeable without significant equipment changes in the follow-up operation. A dual-period undulator design is suggested for the echo-enabled harmonic generation (EEHG) commissioning, and it has great potential to break through the unreachable energy of the fully coherent X-ray in the future. The FEL-II beamline will deliver SASE radiation and fully coherent radiation in all the wavelengths of interest.

High-spectral-resolution absorption measurements with free-electron lasers using ghost spectroscopy

We demonstrate a simple and robust high-resolution ghost spectroscopy approach for x-ray and extreme ultraviolet absorption spectroscopy at free-electron laser sources. Our approach requires an on-line spectrometer before the sample and a downstream bucket detector. We use this method to measure the absorption spectrum of silicon, silicon carbide and silicon nitride membranes in the vicinity of the silicon L2,3-edge. We show that ghost spectroscopy allows the high-resolution reconstruction of the sample spectral response using a coarse energy scan with self-amplified spontaneous emission radiation. For the conditions of our experiment the energy resolution of the ghost-spectroscopy reconstruction is higher than the energy resolution reached by scanning the energy range by narrow spectral bandwidth radiation produced by the seeded free-electron laser. When we set the photon energy resolution of the ghost spectroscopy to be equal to the resolution of the measurement with the seeded radiation, the measurement time with the ghost spectroscopy method is shorter than scanning the photon energy with seeded radiation. The exact conditions for which ghost spectroscopy can provide higher resolution at shorter times than measurement with narrow band scans depend on the details of the measurements and on the properties of the samples and should be addressed in future studies.

X-ray Free Electron Laser Accelerator Lattice Design Using Laser-Assisted Bunch Compression

We report the start-to-end modeling of our accelerator lattice design employing a laser-assisted bunch compression (LABC) scheme in an X-ray free electron laser (XFEL), using the proposed Matter-Radiation Interactions in Extremes (MaRIE) XFEL parameters. The accelerator lattice utilized a two-stage bunch compression scheme, with the first bunch compressor performing a conventional bulk compression enhancing the beam current from 20 A to 500 A, at 750 MeV. The second bunch compression was achieved by modulating the beam immediately downstream of the first bunch compressor by a laser with 1-μm wavelength in a laser modulator, accelerating the beam to the final energy of 12 GeV, and compressing the individual 1-μm periods of the modulated beam into a sequence of microbunches with 3-kA current spikes by the second bunch compressor. The LABC architecture presented had been developed based on the scheme of enhanced self-amplified spontaneous emission (ESASE), but operated in a disparate regime of parameters. Enabled by the novel technology of the cryogenic normal conducting radiofrequency photoinjector, we investigated an electron beam with ultra-low emittance at the starting point of the lattice design. Our work aimed at mitigating the well-known beam instabilities to preserve the beam emittance and suppress the energy spread growth.

The MING proposal at SHINE: megahertz cavity enhanced X-ray generation

The cavity-based X-ray free-electron laser (XFEL) has promise in producing fully coherent pulses with a bandwidth of a few meV and very stable intensity, whereas the currently existing self-amplified spontaneous emission (SASE) XFEL is capable of generating ultra-short pulses with chaotic spectra. In general, a cavity-based XFEL can provide a spectral brightness three orders of magnitude higher than that of the SASE mode, thereby opening a new door for cutting-edge scientific research. With the development of superconducting MHz repetition-rate XFEL facilities such as FLASH, European-XFEL, LCLS-II, and SHINE, practical cavity-based XFEL operations are becoming increasingly achievable. In this study, megahertz cavity enhanced X-ray generation (MING) is proposed based on China’s first hard XFEL facility - SHINE, which we refer to as MING@SHINE.

Analytical study of higher harmonic bunching and matrix formalism in linear high-gain free-electron laser model

Short-wavelength single-pass free electron lasers (FEL) are capable of generating transverse-coherent and high-power radiations. Compared with the self-amplified spontaneous emission (SASE) FEL scheme, seeded FEL schemes can, in principle, improve the longitudinal coherence of radiation pulses. In recent years, the research community has pursued the realization of high-repetition-rate (MHz) and high-average-power (kW) seeded FELs. To alleviate the requirements of external seeding lasers, many novel seeding schemes have been proposed, which are currently limited by the state-of-the-art laser systems. An analytical study of a recently proposed seeding scheme based on direct-amplification-enabled harmonic generation (DEHG) is presented herein. In contrast to the complete numerical simulations and optimization design, this study starts from one-dimensional FEL equations of motion and derives an analytical formula for the harmonic bunching factor at the chicane exit after the electron beam traverses the modulator undulator. Additionally, to obtain a quick estimate of downstream high-gain FEL performance, we constructed a linear high-gain FEL transport matrix, including the three-dimensional effects incorporated by the Ming-Xie formula. As an application of the analytical formulas, we discuss three different cases for designing and optimizing the seeding scheme. We expect this analytical approach to shed light on seeding designs that aim to produce the desired high-brightness mirobunched electron beam.

Seeded free-electron laser driven by a compact laser plasma accelerator

Free-electron lasers generate high-brilliance coherent radiation at wavelengths spanning from the infrared to the X-ray domains. The recent development of short-wavelength seeded free-electron lasers now allows for unprecedented levels of control on longitudinal coherence, opening new scientific avenues such as ultra-fast dynamics on complex systems and X-ray nonlinear optics. Although those devices rely on state-of-the-art large-scale accelerators, advancements on laser-plasma accelerators, which harness gigavolt-per-centimetre accelerating fields, showcase a promising technology as compact drivers for free-electron lasers. Using such footprint-reduced accelerators, exponential amplification of a shot-noise type of radiation in a self-amplified spontaneous emission configuration was recently achieved. However, employing this compact approach for the delivery of temporally coherent pulses in a controlled manner has remained a major challenge. Here we present the experimental demonstration of a laser-plasma accelerator-driven free-electron laser in a seeded configuration, where control over the radiation wavelength is accomplished. Furthermore, the appearance of interference fringes, resulting from the interaction between the phase-locked emitted radiation and the seed, confirms longitudinal coherence. Building on our scientific achievements, we anticipate a navigable pathway to extreme-ultraviolet wavelengths, paving the way towards smaller-scale free-electron lasers, unique tools for a multitude of applications in industry, laboratories and universities.

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.

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.

Development of photo-diodes for Pohang-Accelerator-Laboratory X-ray free-electron laser

The Pohang-Accelerator-Laboratory X-ray free-electron laser (PAL-XFEL) provides intense ultra-short X-ray pulses via a self-amplified spontaneous emission (SASE) process. Since fluctuations are induced by the intrinsic characteristics of the SASE process, we designed and fabricated photo-diodes (PDs) to be used in quadrant beam position and pop-in monitoring systems for measuring intensity and position, and beam size, respectively. Six 500μm-thick high-resistivity silicon wafers were used for different fabrication processes to vary the thicknesses of bias electrode and anti-reflective coating layers optimized for the quantum efficiency (QE) at typical wavelengths of 266 and 400 nm used for pump–probe experiments at the PAL-XFEL. In this paper, we present the electrical characteristics, QE, signal-to-noise ratio, energy resolution of the fabricated PDs, and compared them with those of commercial PD.

Statistical analysis of hard X-ray radiation at the PAL-XFEL facility performed by Hanbury Brown and Twiss interferometry

A Hanbury Brown and Twiss interferometry experiment based on second-order correlations was performed at the PAL-XFEL facility. The statistical properties of the X-ray radiation were studied within this experiment. Measurements were performed at the NCI beamline at 10 keV photon energy under various operation conditions: self-amplified spontaneous emission (SASE), SASE with a monochromator, and self-seeding regimes at 120 pC, 180 pC and 200 pC electron bunch charge. Statistical analysis showed short average pulse duration from 6 fs to 9 fs depending on the operational conditions. A high spatial degree of coherence of about 70-80% was determined in the spatial domain for the SASE beams with the monochromator and self-seeding regime of operation. The obtained values describe the statistical properties of the beams generated at the PAL-XFEL facility.