This contribution describes, on behalf of the HXRSS team, design, installation, commissioning and operation of the Hard X-Ray Self-Seeding (HXRSS) system at the SASE2 FEL line of the European XFEL. We have reached up to mJ-level self-seeded pulses at 9-10 keV and the tested operational range is 6-13 keV. The setup can work in burst mode, that is following the bunch pattern of the European XFEL. The peculiarities of the European XFEL, that are high-repetition rate and long, tuneable undulators will be discussed, together with the impact of two-chicanes simultaneous seeding on the crystal heat loading. A discussion on possible future developments, including the production of self-seeded radiation at a harmonic of the fundamental, will complement the description of the current performance of the system.

We report on the first observation of short, single-spike events generated at the SASE3 beamline of European XFEL via the “attosecond at harmonics method”. The approach was first proposed in [1]. We created bunching in the linear regime at around 0.5 keV and then, after bunching optimization by means of a magnetic chicane, we amplified the 4th harmonic bunching with a part of the undulator set to 2 keV. Due to the non-linear transformation of the bunching during the harmonic jump, radiation generated using this scheme occasionally exhibits single spike spectra (about a percent of the shots, which makes it attractive to use the method at high repetition-rate FELs). We expect those to correspond to single spikes in time-domain. We replicated the experiment numerically with the help of the GENESIS code.

We propose a computationally-efficient algorithm to calculate the field of partially coherent synchrotron radiation pulses from undulators. Wavefront propagation simulations play a pivotal role in designing beamline optics at new synchrotron radiation sources. However, they do not account for the stochastic behaviour of the initial radiation field, which is due to shot noise in the electron beam with finite transverse size and divergence. We present an algorithm that allows us to obtain and propagate radiation fields containing multiple transverse stochastic modes within undulator resonance. The proposed algorithm relies on a method for simulating Gaussian random fields. We initially generate the field as Gaussian white noise, and then we restrict its extent in the direct and in the reciprocal domains by using averaged radiation size and divergence. Strictly speaking, this procedure shapes the correct correlation function of the field only under the assumption of quasi-homogeneity. However, we show that the method can be applied with reasonable accuracy also outside of this assumption. We check consistency of the algorithm with the help of well-established approaches in simulating partially coherent undulator fields. Finally, the proposed method is well-suited for educational purposes.

More and more high-gain SASE FELs operate at high repetition rates, either in burst or in continuous wave mode of operation, offering an unprecedented number of electron bunches per second. External seeding techniques provide high quality FEL pulses of full coherence and shot-to-shot stability but cannot keep up with MHz repetition rates of such FELs due to their dependence on the seed laser repetition rate. One attractive solution to overcome this limitation is to employ an optical cavity to store radiation that acts as a seed for the electron bunches arriving at high repetition rates. Such a scheme not only allows seeded operation at multi-MHz repetition rates but also introduces the possibility to achieve seeded radiation at shorter wavelengths, overcoming the hurdle of insufficient power availability of seed laser systems in the vacuum ultraviolet (VUV) wavelength range. Here, we present different optical-cavity-based schemes and we give an overview of their unique capabilities together with simulation results.

Intense attosecond pulses generated by x-ray free-electron lasers (XFEL) are promising for attosecond science, for example, to study the quantum mechanical motion of electrons in molecules. This paper presents numerical simulations of the generation of attosecond soft and hard x-ray FEL pulses with the chirp-taper and Enhanced SASE schemes, based on the parameters of the European XFEL. To overcome the coherence time barrier, a modification of the chirp-taper scheme [1] is used in the case of soft x-rays. The results show that several hundred attosecond pulses can be obtained at photon energies of both 700 eV and 6 keV.

A major challenge in single-linac - multiple undulator setups like EuXFEL is the generation of individual shaped photon pulses, in particular, when working in a mode where a single pulse train, or cw stream, feeds all undulator lines. This work presents the experimental verification of a flexible delivery scheme producing photon pulses for each of the three undulator lines with their electron bunches individually shaped in charge, compression and optics on a single RF pulse burst.

European XFEL is a multi-beamline x-ray free-electron laser (FEL) user facility driven by a superconducting accelerator with a nominal photon energy range from 250 eV to 25 keV. An afterburner undulator based on superconducting undulator technology is currently being planned to enable extension of the photon energy range towards harder x-rays. This afterburner undulator would be installed downstream of the already operating SASE2 FEL beamline, emitting at the fundamental or at a harmonic of the upstream SASE2 undulator. In this contribution we present a first simulation study of the impact of undulator mechanical tolerances for operation of the afterburner undulator at the fundamental of SASE2.

The transverse coherence of the source is an important property for FEL experiments. Theory and simulations indicated different features for seeded and unseeded FELs but so far no direct comparison has been pursued experimentally on the same facility. At FERMI one has the unique possibility to test both configurations (SASE and seeding) within the same operating conditions. In this contribution we present the experimental results of the characterization of transverse coherence with special attention to the evolution of such property.

Attosecond pulse production is an important development focus for most major FEL facilities. Chirp/taper and eSASE schemes, both of which will shorten the pulses well below the femto-second level for both hard and soft x-rays, are proposed for implementation at EuXFEL. As a high repetition rate super conducting linac that feeds three 200m long undulator lines for parallel operation, EuXFEL presents distinct challenges but also unique opportunities for the proposed schemes.

Frequency mixing was studied experimentally at SASE3, the soft X-ray undulator of the European XFEL. Two frequencies were generated in the first part of the undulator in alternating K configuration. The mixing process occurred in the second part with detuned undulator segments used to generate R56. Finally, the difference frequency was radiated and amplified in a third part of the SASE3 undulator. Experiments were performed at several electron energies (11.5 GeV, 14 GeV, and 16.5 GeV) with frequency mixing generation at photon energies between 500 eV and 1.1 keV. Pulse energies were on the mJ level, depending on the length of the radiator part. A practical application of frequency mixing at European XFEL is a possible extension of the operating range of the SASE3 undulator towards lower photon energies, by using a relatively short afterburner with longer period.

The generation of x-ray pulses carrying orbital angular momentum from an x-ray free-electron laser (FEL) has attracted considerable attention due to the ability to directly change atomic states and develop new material characterization techniques. In this contribution, we report a new method for generating intense x-ray vortices. The method is based on the widely used self-amplified spontaneous emission scheme and does not require additional helical undulators or external laser systems. It can therefore in principle be employed by all existing XFEL facilities with limited hardware additions.

Phase-locked pulses are important for coherent control experiments. Here we present theoretical analyses and start-to-end simulation results for the generation of phase-locked pulses using the Hard X-ray Self-Seeding (HXRSS) system at the European XFEL. As proposed in Ref. [1], the method is based on a combination of self-seeding and fresh-slice lasing techniques. However, at variance with Ref. [1], here we exploit different transverse centroid offsets along the electron beam. In this way we may first utilize part of the electron beam to produce SASE radiation, to be filtered as seed and then generate HXRSS pulses from other parts of the beam applying appropriate transverse kicks. The final result consists in coherent radiation pulses with fixed phase difference and tunable time delay within the bunch length. This scheme should be useful for applications such as coherent x-ray pump-probe experiments.

Recently, Hard x-ray self-seeding (HXRSS) operations at the European X-ray free-electron laser (EuXFEL) opened a pathway towards the application of pulses with high spectral density (in terms of ph/eV per pulse) in the fields of applied physics, chemistry and biology, where the coherent radiation spectrum is essential. The spectrum of hard x-ray self seeding pulses is generally accompanied by a pedestal around the central seeded photon energy. The pedestal contains two separate components: normal self-amplified spontaneous (SASE) and sideband emissions that can be ascribed to long-wavelength modulations of the electron beam. The pedestal limits the spectral purity and can impact some user applications. In this report, we analyze the purity of HXRSS pulses in the presence of microbunching instability. We look at the spectral contents after and before saturation, and display the contribution of the pedestal in the HXRSS spectrum.