The invention of the laser transformed optics by providing intense, coherent light in the visible region, but extending this concept to X-rays has been hindered by a lack of suitable gain media and mirrors. Current hard X-ray free-electron laser (XFEL) facilities1-5 overcome this by amplifying shot noise from a high-peak-current electron bunch via self-amplified spontaneous emission6 in a single pass through long undulators, delivering very high brightness but with a noisy, multi-spiked temporal and spectral profile. Cavity-based XFELs (CBXFELs)7-9 were proposed to close this gap by recirculating spectrally filtered X-ray pulses in a Bragg-reflecting cavity synchronized to a high-repetition-rate electron beam. Here we show lasing with multi-pass gain at 6.952 keV in a 132.8-m round-trip diamond-based Bragg cavity10 at the European XFEL, matched to the 2.23-MHz bunch spacing of the superconducting accelerator5. Under stringent length and angular stability requirements, a ring-up in the cavity across successive bunches was observed, producing spectrally pure, microjoule-level pulses. This establishes the feasibility of CBXFELs in an accelerator environment and validates diamond Bragg optics for X-ray resonators. The demonstrated spectral purity opens a path to next-generation X-ray science, which demands highly coherent, stable sources.

We report on the operation of the first high peak and average power single-pass terahertz free-electron laser (THz FEL) based on a high-brightness photoinjector for developing a THz source for pump-probe experiments at advanced x-ray FELs (XFELs). Electron beams with 17 MeV/c momentum and charge up to 2.4 nC were used to generate narrow-band 3 THz radiation with pulse energies of more than 0.1 mJ. The novelty of the realized method is that the undulator radiation by electron beams is much longer than the radiation wavelength, with FEL starting from coherent beam contribution rather than shot noise. This proved to be not a simple scaling of the XFEL mechanism due to the significant role of the bunching factor in the considered case.

Development of an accelerator-based tunable THz source prototype for pump-probe experiments at the European XFEL is ongoing at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The proof-of-principle experiments on the THz SASE FEL are performed utilizing the LCLS-I undulator (on loan from SLAC) installed in the PITZ beamline. The first lasing at a center wavelength of 100 mu m was observed in the summer of 2022. The lasing of the narrowband THz source was achieved using an electron beam with an energy of similar to 17 MeV and a bunch charge up to several nC. Optimization of beam transport and matching resulted in the measurement of THz radiation with a pulse energy of tens of mu J, measured with pyroelectric detectors. The THz FEL gain curves were measured by means of specially designed short coils along the undulator. The results of the first characterization of the THz source at PITZ will be presented.

Bragg reflectors are essential for beam transport in X-ray free-electron laser (XFEL) facilities. On interaction with Bragg reflectors, a part of the pulse energy will be absorbed, causing the propagation of displacement waves due to rapid thermal expansion. It is suspected that these waves may cause stability problems for XFELs operating with megahertz repetition rates. Here, we experimentally investigate the displacement of a diamond Bragg reflector induced by an optical ultra-violet laser pulse, simulating XFEL pulses with mJ pulse energy, both at room temperature and cryogenic temperatures. Our experiment shows negligible damping of the displacement waves on µs timescales, which could cause disruption for subsequent XFEL pulses. We compare our measurements to a simulation framework based on the assumptions of local thermodynamic equilibrium and classical mechanics, observing reasonable agreement. Our results show that thermoelastic effects are critical for a reliable stability assessment of Bragg reflectors, but are often overlooked.

Abstract Advanced experiments using THz pump and X-ray probe pulses at modern free-electron lasers (FELs) like the European X-ray FEL require a frequency-tunable (from 0.1 THz to 30 THz), high-power (> 10 microjoule), narrow-band (~ 1–2%) THz source maintaining the repetition rate and pulse structure of the X-ray pulses. This paper reports the first results from a THz source, that is based on a self-amplified spontaneous emission (SASE) FEL operating with a central wavelength of 100 micrometers. The THz SASE FEL prototype is currently under development at the Photo Injector Test facility at DESY in Zeuthen (PITZ) and uses the same type of electron source as the European XFEL photo injector. The first proof-of-principle experiments were done at PITZ using an LCLS-I undulator to generate high-power, high-repetition-rate THz SASE FEL radiation. Electron bunches with a beam energy of ~ 17 MeV and a bunch charge of up to several nC are used to generate THz pulses with a pulse energy of several tens of microjoules. For example, for an electron beam with a charge of ~ 2.4 nC, 65 microjoules were measured at a central wavelength of 100 micrometers. These proof-of-principle experiments pave the way for a tunable, high-repetition-rate THz source providing pulses with energies in the millijoule range.

Modern X-ray free-electron laser (XFEL) sources can deliver photon pulses with millijoule pulse energies and megahertz repetition rate. As shown by the simulations in this work, for particular cases the dynamical heat load effects for Bragg reflectors could cause problems at these facilities. These problems would be underestimated if only quasi-static thermoelastic simulations are considered. Nevertheless, for the sake of simplicity the quasi-static approach is a common choice for estimating heat load effects. To emphasize the relevance of dynamical thermoelastic effects, the response to the partial absorption of an X-ray pulse, as provided by a saturated X-ray free-electron laser oscillator (XFELO) in a single crystal diamond with a thickness of 100 µm and lateral dimensions in the millimetre range, is discussed in this work. The outcome of the dynamic thermoelastic simulations indicates a clear dominance regarding the strain value reached, which is present for consecutive X-ray matter interactions with megahertz repetition rate.

In this article, the concept of a recently funded R&D project for the installation of a proof-of-concept cavity-based x-ray free electron laser (CBXFEL) demonstrator experiment at the European XFEL facility is presented, with the first results expected in 2024. It is composed of an x-ray cavity design in backscattering geometry with a 133 m round trip length using cryogenically cooled diamond crystals. It employs the concept of retroreflection to reduce the sensitivity to vibrations. The FEL radiation is produced in four undulator segments of 20 m total length. Simulations at 16 GeV beam energy and 250 pC bunch charge show that the expected x-ray pulses in saturation surpass state-of-the-art x-ray sources considering spectral flux and three-dimensional coherence. However, the stability of the proof of concept setup is severely challenged by the finite thermal transport in the diamond crystals. Therefore, suitable measures such as cooling the crystals to 70 K are explained in this paper and additional ones will have to be developed in the course of this project.

AbstractIt is well known from Maxwell theory that electromagnetic radiation is emitted whenever electric charges are accelerated in free space. This radiation assumes quite extraordinary properties whenever the charged particles move at ultrarelativistic speed: The radiation becomes very powerful and tightly collimated in space, and it may easily cover a rather wide spectrum ranging from the THz into the hard X-ray regime. When generation of such radiation is intended rather than being a side effect, the charged particles are normally electrons, thus kinetic energies are then typically in the multi-MeV range.

Free-electron lasers produce extremely brief, coherent, and bright laser-like photon pulses that allow to image matter at atomic resolution and at timescales faster than the characteristic atomic motions. In pulses of about 50 femtoseconds duration they provide as many photons as one gets in 1 s from modern storage ring synchrotron radiation facilities. FLASH, the Free-Electron Laser at DESY in Hamburg was the first FEL in the XUV/soft X-ray spectral range, started operation as a user facility in summer 2005, and was for almost 5 years the only short wavelength FEL facility worldwide. Hence, most of the technological developments as well as the scientific experiments performed by the user community were new and unique as outlined below. FLASH was driving FEL science and technology and paved the way for many new ideas. Because of using a linear accelerator in superconducting RF technology FLASH combines the extreme peak brightness characteristic for FELs with very high average brightness. It also was the prototype for the European XFEL located in the Hamburg metropolitan area, which started user operation in summer 2017.

Free-electron lasers (FELs) based on the self-amplified spontaneous emission (SASE) principle generate photon pulses with typically poor longitudinal coherence. FEL seeding techniques greatly improve longitudinal coherence by initiating FEL amplification in a controlled way using coherent light pulses. The sFLASH experiment installed at the FEL user facility FLASH at DESY in Hamburg is dedicated to the study of external seeding techniques. In this paper, the layout of the sFLASH seeding experiment is presented and an overview of recent developments is given.

Shaping the spectral phase of ultrafast pulses in the XUV region is technically challenging. One promising scheme is the manipulation of electron bunches of XUV Free-Electron Lasers (FEL) with conventional ultrafast UV lasers, imprinting some of the properties of the manipulating UV pulse to the XUV output pulse of the FEL. At the XUV FEL facility FLASH at DESY we use for our studies the “seeding” FEL beamline “sFLASH’ [3], where 267nm pulses of a Ti:sapphire technology ultrafast laser can be overlapped with about 700 MeV electron bunches in electro-magnetic undulator section (“modulator”). The resulting energy modulation in the 400 keV range can be converted to a current modulated electron beam. This beam is transported to a permanent magnet variable gap undulator section (“radiator”), producing “seeded” ultrafast XUV pulses of several uJ pulse energy and approximately 50fs pulse duration. This “high-gain harmonic generation” (HGHG) scheme is commonly used in FEL facilities to provided seeded XUV pulses. In this contribution we use THz radiation generated by the identical Ti:sapphire laser source to characterize the seeded XUV FEL pulses in time domain, pulse duration and temporal chirp. We show that chirped UV pulses produce chirped XUV radiation which is theoretically predicted and experimentally verified. [1,2]. In addition, it is presented that chirp manipulation of XUV pulses by tailored UV chirp pulses is feasible. The external manipulating laser with wavelength at 267 nm is generated via third harmonic generation (THG) process from NIR 800 nm pulses available from a commercial laser system (HIDRA, Coherent), which provides 50 fs pulses at 800 nm with 50 mJ energy/pulse. Part of those pulses are converted to UV and used for seeding the electron beam. Depending on the undulator gap settings, seeded XUV pulses with wavelengths at the 7th, 8th and 9th harmonic of the seed laser pulse are generated via HGHG operation regime [3]. Another part of laser pulses are used to generate THz field via optical rectification which is used to characterize the seeded FEL pulses. The generated THz field is used for direct temporal characterization of seeded FEL pulses via THz streaking technique [1]. There a single-shut time of flight (TOF) spectrometer is used to detect the variations of the electron kinetic energy of argon ions initiated by seeded FEL pulses using the THz streaking field. Using this technique, pulse duration and chirp of seeded FEL pulses could be measured via a non-invasive method. Figure 1 (a) and (b) shows the delay scan between seeded FEL and THz pulse, and the pulse duration measured by THz streaking technique and temporal deflection structure (TDS). The correlation between the group delay dispersion (GDD) of the seed laser pulses and the seeded XUV pulse duration is estimated using numerical simulation (Figire 1 (c)). Here, the variation of the XUV pulse duration in respect to the seed laser pulse chirp is shown. The simulation shows good agreement with the THz streaking measurements.

The FLASH free-electron laser (FEL) at DESY is currently operated in self-amplified spontaneous emission (SASE) mode in both beamlines FLASH1 and FLASH2. Seeding offers unique properties for the FEL pulse, such as full coherence, spectral and temporal stability. In this contribution, possible ways to carry the seeded FEL radiation to the user hall are presented with analytical considerations and simulations. For this, components of the sFLASH seeding experiment are used.

We report on a direct time-domain measurement of the temporal properties of a seeded free-electron laser pulse in the extreme ultraviolet spectral range. Utilizing the oscillating electromagnetic field of terahertz radiation, a single-shot THz streak-camera was applied for measuring the duration as well as spectral phase of the generated intense XUV pulses. The experiment was conducted at FLASH, the free electron laser user facility at DESY in Hamburg, Germany. In contrast to indirect methods, this approach directly resolves and visualizes the frequency chirp of a seeded free-electron laser (FEL) pulse. The reported diagnostic capability is a prerequisite to tailor amplitude, phase and frequency distributions of FEL beams on demand. In particular, it opens up a new window of opportunities for advanced coherent spectroscopic studies making use of the high degree of temporal coherence expected from a seeded FEL pulse.

Seeded free-electron lasers (FELs) produce intense, ultrashort and fully coherent X-ray pulses. These seeded FEL pulses depend on the initial seed properties. Therefore, controlling the seed laser allows tailoring the FEL radiation for phase-sensitive experiments. In this contribution, we present detailed simulation studies to characterize the FEL process and to predict the operation performance of seeded pulses. In addition, we show experimental data on the temporal characterization of the seeded FEL pulses performed at the sFLASH experiment in Hamburg.

The XUV free electron laser FLASH, has been recently operated in the high-gain harmonic generation (HGHG) mode. We characterized the laser-induced energy modulation, as well as the temporal profile of the seeded FEL pulses. FEL saturation was reached for the 7th harmonic of the 266 nm seed laser.

Free-electron lasers are unique sources of intense and ultra-short x-ray pulses that led to major scientific breakthroughs across disciplines from matter to materials and life sciences. The essential element of these devices are micrometer-sized electron bunches with high peak currents, low energy spread, and low emittance. Advanced FEL concepts such as seeded amplifiers rely on the capability of analyzing and controlling the electron beam properties with few-femtosecond time resolution. One major challenge is to extract tomographic slice parameters instead of projected electron beam properties. Here, we demonstrate that a radio-frequency deflector in combination with a dipole spectrometer not only allows for single-shot extraction of a seeded FEL pulse profile, but also provides information on the electron slice emittance and energy spread. The seeded FEL power profile can be directly related to the derived slice emittance as a function of intra-bunch coordinate with a resolution down to a few femtoseconds.