Self-amplified Spontaneous Emission Mode Research Articles

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.

Bright Ångstrom and picometer free electron laser based on the Large Hadron electron Collider energy recovery linac

The Large Hadron electron Collider (LHeC) is a proposed future particle-physics project colliding 60 GeV electrons from a six-pass recirculating energy-recovery Linac (ERL) with 7 TeV protons stored in the LHC. The ERL technology allows for much higher beam current and, therefore, higher luminosity than a traditional Linac. The high-current, high-energy electron beam can also be used to drive a free electron laser (FEL). In this study, we investigate the performance of an LHeC-based FEL, operated in the self-amplified spontaneous emission mode using electron beams after one or two turns, with beam energies of, e.g., 10, 20, 30 and 40 GeV, and aim at producing X-ray pulses at wavelengths ranging from 8~\AA\ to 0.5~\AA . In addition, we explore a possible path to use the 40 GeV electron beam for generating photon pulses at much lower wavelengths, down to a few picometre. We demonstrate that such ERL-based high-energy FEL would have the potential to provide orders of magnitude higher average brilliance at \AA\ wavelengths than any other FEL either existing or proposed. It might also allow a pioneering step into the picometre wavelength regime.

High-brightness self-seeded X-ray free-electron laser covering the 3.5 keV to 14.6 keV range

A self-seeded X-ray free-electron laser (XFEL) is a promising approach to realize bright, fully coherent free-electron laser (FEL) sources in the hard X-ray domain that have been a long-standing issue with longitudinal coherence remaining challenging. At the Pohang Accelerator Laboratory XFEL, we have demonstrated a hard X-ray self-seeded XFEL with a peak brightness of 3.2 × 1035 photons s–1 mm–2 mrad–2 0.1% bandwidth (BW)–1 at 9.7 keV. The bandwidth (0.19 eV) is about 1/70 times as wide (close to the Fourier transform limit) and the peak spectral brightness is 40 times higher than in self-amplified spontaneous emission (SASE), with substantial improvements in the stability of self-seeding and noticeably suppressed pedestal effects. We could reach an excellent self-seeding performance at a photon energy of 3.5 keV (lowest) and 14.6 keV (highest) with the same stability as the 9.7 keV self-seeding. The bandwidth of the 14.6 keV seeded FEL was 0.32 eV, and the peak brightness was 1.3 × 1035 photons s–1 mm–2 mrad–2 0.1%BW–1. We show that the use of seeded FEL pulses with higher reproducibility and a cleaner spectrum results in serial femtosecond crystallography data of superior quality compared with data collected using SASE mode.

Delay-detune scan for longitudinal electron beam characterization at free-electron lasers

We present first experimental results of a novel method to study the energy chirp of the electron beam lasing window at a free-electron laser operating in self-amplified spontaneous emission mode. The method requires a single magnetic chicane splitting the FEL undulator into two parts of several gain lengths each. Undulator segments should have variable gaps. By scanning both the delay in the chicane and detuning in a part of the undulators, one can retrieve the linear component of both the radiation frequency chirp and the electron energy chirp. In addition, such scan improves the accuracy of the previously proposed autocorrelation-based pulse duration measurement technique. The proposed method targets facilities that lack direct diagnostics of the electron beam longitudinal phase space distribution. It also can be extended to diagnose transverse tilts of the beam.

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.

Compact undulator line for a high-brilliance soft-X-ray free-electron laser at MAX IV.

The optimal parameter space for an X-ray free-electron laser (FEL) in the self-amplified spontaneous emission (SASE) operation mode is examined. This study focuses on FEL operation with a shorter undulator period and higher undulator strength made available through recent developments in in-vacuum, cryogenic and superconducting undulators. Progress on short-period undulator technologies is surveyed and FEL output characteristics versus undulator parameters are computed. The study is performed on a case of the planned soft-X-ray FEL at the MAX IV Laboratory in Sweden. An extension of the SASE mode into the harmonic lasing self-seeded mode is also analysed.

一种新型keV波段超高分辨光栅光谱仪设计

This paper presents a novel grating spectrometer design that combines a convexly cylindrical mirror with a concavely cylindrical varied line spacing grating to achieve an extreme resolving power of 33000-70000 in the photon energy range of 0.6-1.5 keV for a Gaussian light source with effective meridional size of 30 μm (root mean square) at an upstream initial object distance of 10 m. This type of spectrometer can be used to diagnose the radiation spectrum of a soft X-ray free electron laser and properly measure the spectral distribution and the detailed structure of the self-amplified spontaneous emission mode due to its extremely high resolving power. The overall optical aberrations of the system are well analyzed and compensated, providing an excellent flat field at the detector domain throughout the entire spectral range. A machine learning support vector machine algorithm is introduced into explore the optimal system to deliver the desired resolving performance. In addition, to enhance the spectral intensity and detection efficiency simultaneously, a sagittal confinement focusing mirror is introduced into the optical path of the spectrometer to reduce the astigmatism in spectral imaging. © 2019, Chinese Lasers Press. All right reserved.

On spectral and temporal coherence of x-ray free-electron laser beams.

A model for the coherence properties of free-electron lasers (FELs) in time and frequency domains is introduced within the framework of classical second-order coherence theory of nonstationary light. An iterative phase-retrieval algorithm is applied to construct an ensemble of field realizations in both domains, based on single-pulse spectra measured at the Linac Coherent Light Source (LCLS) in self-amplified spontaneous emission mode. Such an ensemble describes the specific FEL pulse train in a statistically averaged sense. Two-time and two-frequency correlation functions are constructed, demonstrating that the hard X-ray free-electron laser at LCLS in this case behaves as a quasistationary source with low spectral and temporal coherence. We also show that the Gaussian Schell model provides a good description of this FEL.

Effects of self-seeding and crystal post-selection on the quality of Monte Carlo-integrated SFX data.

Serial femtosecond crystallography (SFX) is an emerging method for data collection at free-electron lasers (FELs) in which single diffraction snapshots are taken from a large number of crystals. The partial intensities collected in this way are then combined in a scheme called Monte Carlo integration, which provides the full diffraction intensities. However, apart from having to perform this merging, the Monte Carlo integration must also average out all variations in crystal quality, crystal size, X-ray beam properties and other factors, necessitating data collection from thousands of crystals. Because the pulses provided by FELs running in the typical self-amplified spontaneous emission (SASE) mode of operation have very irregular, spiky spectra that vary strongly from pulse to pulse, it has been suggested that this is an important source of variation contributing to inaccuracies in the intensities, and that, by using monochromatic pulses produced through a process called self-seeding, fewer images might be needed for Monte Carlo integration to converge, resulting in more accurate data. This paper reports the results of two experiments performed at the Linac Coherent Light Source in which data collected in both SASE and self-seeded mode were compared. Importantly, no improvement attributable to the use of self-seeding was detected. In addition, other possible sources of variation that affect SFX data quality were investigated, such as crystal-to-crystal variations reflected in the unit-cell parameters; however, these factors were found to have no influence on data quality either. Possibly, there is another source of variation as yet undetected that affects SFX data quality much more than any of the factors investigated here.

Experimental demonstration of enhanced self-amplified spontaneous emission by an optical klystron.

We report the first experimental evidence of enhancement of self-amplified spontaneous emission, due to the use of an optical klystron. In this free-electron laser scheme, a relativistic electron beam passes through two undulators, separated by a dispersive section. The latter converts the electron-beam energy modulation produced in the first undulator in density modulation, thus enhancing the free-electron laser gain. The experiment has been carried out at the FERMI facility in Trieste. Powerful radiation has been produced in the extreme ultraviolet range, with an intensity a few orders of magnitude larger than in pure self-amplified spontaneous emission mode. Data have been benchmarked with an existing theoretical model.

A high-harmonic generation source for seeding a free-electron laser at 38 nm

Direct seeding with a high-harmonic generation (HHG) source can improve the spectral, temporal, and coherence properties of a free-electron laser (FEL) and shall reduce intensity and arrival-time fluctuations. In the seeding experiment sFLASH at the extreme ultraviolet FEL in Hamburg FLASH, which operates in the self-amplified spontaneous emission mode (SASE), the 21st harmonic of an 800 nm laser is refocused into a dedicated seeding undulator. For seeding, the external light field has to overcome the noise level of SASE; therefore, an efficient coupling between seed pulse and electron bunch is mandatory. Thus, an HHG beam with a proper divergence, width, beam quality, Rayleigh length, pointing stability, single-shot pulse energy, and stability in the 21st harmonic is needed. Here, we present the setup of the HHG source that seeds sFLASH at 38.1 nm, the optimization procedures, and the necessary diagnostics.

Self-Amplified Spontaneous Emission Free-Electron Laser with an Energy-Chirped Electron Beam and Undulator Tapering

We report the first experimental implementation of a method based on simultaneous use of an energy chirp in the electron beam and a tapered undulator, for the generation of ultrashort pulses in a self-amplified spontaneous emission mode free-electron laser (SASE FEL). The experiment, performed at the SPARC FEL test facility, demonstrates the possibility of compensating the nominally detrimental effect of the chirp by a proper taper of the undulator gaps. An increase of more than 1 order of magnitude in the pulse energy is observed in comparison to the untapered case, accompanied by FEL spectra where the typical SASE spiking is suppressed.

Partial-coherence method to model experimental free-electron laser pulse statistics

A general numerical approach is described that allows obtaining model sets of temporal pulse shapes of free-electron lasers (FELs) operating in the self-amplified spontaneous emission mode. Based on a random partial-coherence approach, sets of pulse shapes can be calculated that satisfy statistical criteria of FEL light predicted by established FEL theory. Importantly, the numerically retrieved sets of pulses reproduce the experimentally accessible FEL light characteristics as measured at the Free-electron LASer at Hamburg (FLASH), such as the average spectrum, single-shot spectral shape, and pulse duration. The high-precision agreement with the experimental average spectral shape, without further knowledge of FEL machine parameters, makes this approach a convenient tool for the analysis and theoretical modeling of nonlinear optical or pump-probe experiments with FEL light.

Coherence properties of the European XFEL

The European x-ray free-electron laser (XFEL) provides x-ray self-amplified spontaneous emission (SASE) FEL radiation in the wavelength range from 0.1 to 3 nm using three undulator systems. The SASE mode of operation at the European XFEL defines specific behavior of longitudinal and transverse coherence properties. In this paper, we describe the evolution of the temporal and transverse correlation functions along the undulator length, and we extract the corresponding evolution of coherence time and degree of transverse coherence as typical figures of merit. Generation of coherent radiation inside the FEL undulators is followed by beam transport to the experiments. During transport, the total number of coherent modes is preserved, but the wavefront can be disturbed, and we analyze the conditions under which this occurs. It is emphasized that the development of experimental observables for the degree of coherence and wavefront properties will be important for experiments using coherent x-ray radiation.

Laser assisted electron beam conditioning for free electron lasers

A new method for electron beam conditioning in free electron lasers is proposed. It uses the electron beam interaction with a laser light in two wiggler magnets separated by a strong-focusing channel. The effect of the conditioning is illustrated by the example of a hypothetical single-pass, high-gain free electron laser operating in the self-amplified spontaneous emission mode with the x-ray emission at ${\ensuremath{\lambda}}_{x}=1.5\text{ }\AA{}$. The proposed conditioner is relatively compact and can be used as a practical add-on device to short x-ray wavelength free electron lasers.

Demonstration of gain saturation and controlled variation of pulse length at the TESLA test facility FEL

We report on experimental evidence that the free-electron laser at the TESLA Test Facility has reached the maximum power gain of 107 while operating in the Self-Amplified Spontaneous Emission mode. Following the request of the first scientific users, the TTF FEL has been tuned in the wavelength range between 80 and 120nm (vacuum ultraviolet, VUV), and saturation has been achieved in this entire wavelength range. At saturation the FEL emits short pulses with GW peak power and a high degree of transverse coherence. Also, the paper reports on the demonstration of controlled pulse length variation between 30 and 100fs and on detailed intensity fluctuation studies in this operation mode.

Study of the statistical properties of the radiation from a VUV SASE FEL operating in the femtosecond regime

The Free-Electron Laser (FEL) at the TESLA Test Facility at DESY operates in the self-amplified spontaneous emission mode and generates sub-100-fs radiation pulses in the vacuum ultraviolet spectral region. During operation in the saturation regime, radiation pulses with GW peak power are produced. The statistical properties of the FEL radiation have been studied for different amplification regimes as well as behind a narrow-band monochromator and found to be in good agreement with the results of numerical simulations. Information about the spectral and temporal structure of the FEL radiation has been deduced from the statistical properties. The pulse duration of the FEL radiation can be varied by tailoring the electron bunch that drives the FEL.

Self-Amplified Spontaneous Emission and Bunching at 3 GHz in a Microwave Free-Electron Laser

The CEA/CESTA free-electron laser amplifier has been operated in the self-amplified spontaneous emission mode. This waveguide laser has two resonant frequencies for given beam energy and wiggler field, at approximately 3 and 35 GHz, respectively, for our operating conditions. Highly reproducible microwave power levels of 40 MW and pronounced electron bunching are seen at the lower frequency. Streak camera photographs of the time dependence of the bunching mechanism at the lower frequency are presented. The bunching is accompanied by a significant loss of electrons from the beam.

Saturnus: the UCLA high-gain infrared FEL project

We present the status of Saturnus: an infrared FEL operating in the 10 μm wavelength region, driven by a compact 20 MeV linac with a photoinjector, under construction at UCLA. The 1.5 cm period, 0.5 T peak-field undulator is being built at the Kurchatov IAE. The FEL is being designed to operate primarily in the self-amplified spontaneous emission mode. We plan to study the startup from noise, optical guiding, saturation, sidebands and superradiance, with emphasis on the effects important for future short-wavelength operation of FELs. The photoinjector follows closely the Brookhaven design. Electrons are injected into an accelerating section based on the plane-wave transformer design developed by Swenson at SAIC. Simulation of the linac and FEL show a gain length of 10 cm, and saturation power of 50 MW.