FEL pulses with an easily tunable duration are of great benefit to user experiments with high requirements on the temporal resolution. A transverse beam tilt is well suited to shorten the pulse duration in a controlled manner. We consider three methods of tilt generation: rf deflecting structures, lattice dispersion in combination with an energy chirp, and transverse wakefields from C-band accelerating cavities. We use monochromator scans in combination with an energy-chirped beam to diagnose the reduction in pulse duration.

X-ray absorption spectroscopy (XAS) with a SASE signal can be improved if the full XAS and reference spectrum are taken on a shot-to-shot basis, thus eliminating the impact of the intrinsic SASE fluctuations in the spectrum. This can be further improved if the FEL pulse has the frequency information encoded in its spatial position. The spatial encoding is achieved when a spatially tilted electron beam with a strong energy chirp is injected into a focusing-free undulator channel. We report on the demonstration of this mode at the hard X-ray beamline Aramis at SwissFEL. Possible applications and an outlook for further studies are discussed.

A novel and noninvasive method for two-color x-ray emission is demonstrated at SwissFEL. In the setup, a laser emittance spoiler pulse is overlapped with the primary photocathode laser to locally spoil the beam emittance and the FEL emission. This results, together with a chirped electron pulse, in X-ray emission at two colors. High-energy, high stability, independent control of the duration and of the intensity of the two colors is demonstrated. The laser emittance spoiler enables shot-to-shot selection between one and two-color FEL emission and further, it is compatible with high repetition-rate FELs, as it does not contribute to beam losses.

We report on the first demonstration of generating high-power and short FEL pulses using the fresh-slice multi-stage amplification scheme at Athos, the soft X-ray beamline of SwissFEL. We use a transversely tilted electron beam traveling through the unique Athos layout with magnetic chicanes between every two undulator modules. Our first results show the production of FEL radiation with pulse energies of few hundreds microjoules and pulse durations at the femtosecond level. This operation mode will allow us to advance the scientific opportunities of single-particle imaging experiments.

Laser pulses of sub-femtosecond duration can be used to track the motion of electrons in the inner shell, which is needed in a variety of advanced experiments. Although this has been accomplished in XUV and hard X-rays in a free-electron-laser facility, it remains a challenge in the soft X-ray region due to the relatively high photon energies and large slippage in the undulator. In this contribution, we present a method to achieve a pulse sequence of ∼ 120 attosecond each in average at 293.8 eV photon energy (4 nm wavelength), which covers the K-shell absorption of Carbon. The key is to create a linear undulator taper within each undulator module by rotating a transverse gradient undulator (TGU) at a small angle. The TGU technique is usually referred to minimise the energy spread effect in Laser-driven plasma accelerator, while in this paper we demonstrate that it can also be used to generate short pulses.

A tunable and multicolor light source with near Fourier-limited pulses, controlled delay, and fully coherent beam with precisely adjustable phase profiles enables state-of-the-art measurements and studies of femtosecond dynamic processes with high elemental sensitivity and contrast. The start-to-end simulations efforts aim to take advantage of the available global pool of software and past and present extensive efforts to provide realistic simulations, particularly for cases where precise and fine manipulation of the beam phase space is concerned. Since, for such cases, tracking of beams with billions of particles through magnetic structures and handover between multiple codes are required, extensive realistic studies for such cases are limited. Here we will describe a workflow that reduces the needed computational resources and share studies of the EEHG seed section for the FLASH2020+ [1] project.

In order to improve the brightness and coherence of the soft x-ray FEL line of SwissFEL (Athos), components for an Echo Enabled Harmonic Generation (EEHG) scheme are currently in preparation. The first components have been installed to allow first ESASE operation test in Spring 2022. This first stage consists in a 10 mJ class seed laser, a U200 modulator with individual control of each half period and a four electromagnets dipole chicane (R56 < 800 um). The large magnetic chicane and the second modulator are still in preparation for an installation by end 2022. This paper will give a technical description of the different systems as well as preliminary results of the commissioning with beam.

In the scope of the HERO ERC project, we are implementing a laser-based seeding scheme at the SwissFEL soft X-ray Athos beamline to generate fully coherent X-ray FEL pulses. With this perspective, we designed and built a new laser facility. It consists of a terawatt-class, femtosecond laser system based on Titanium Sapphire technology with wavelength tuning capability, an optical transfer line as well as a launching optical setup and diagnostics to spatially and temporally overlap the laser and the electron bunch inside the modulator, where the seeding process occurs. We present an overview of the facility with details of the laser performance as well as first commissioning results with the electron beam.

High-resolution measurements of the uncorrelated energy spread at SwissFEL indicate energy spread levels much larger than predicted by state-of-the-art particle tracking. This contribution presents measurements of the energy spread at the SwissFEL injector as a function of the electron bunch charge, the optics and the longitudinal dispersion of the lattice. The results indicate that both intra-beam scattering and micro-bunching, not covered in the conventional modeling of injectors, cause a blow-up of the energy spread. The work underlines the importance of considering the energy spread in the optimization and design of high-brightness electron beam sources and the need to develop new models to adequately understand and simulate the observed physics effects.

It is planned to extend SwissFEL by a third beamline, named Porthos, operating in the hard X-ray regime. Three bunches will be accelerated within one RF pulse and distributed into the different beamlines with resonant kickers operating at the bunch spacing of a few tens of nanoseconds. While the full extent of Porthos will not be realized before the end of this decade the extraction line from the main linac will also serve the P^3 experiment for the demonstration of a possible positron source for the FCC-ee project at CERN. We present the design of the switchyard, which will serve both purposes with only minimal changes.

With the beam synchronous readout of the beam position measurement at the hard X-ray FEL beamline Aramis at SwissFEL we analyze the intrinsic orbit jitter, using a classification algorithm and Principal Component Analysis (PCA). The method sorts the jitter in a set of eigenvectors and -values. With the magnitude of the eigenvalues the impact of the different jitter sources can be estimated. From the purely stochastic results we derive also a physical interpretation by matching the linear transport functions to the eigenvectors, reconstructing the orbit jitter in terms of the center of mass jitter of the electron bunch in the transverse positions, momenta, and the mean energy. Any deviation from the theoretical prediction indicates possible wrong set values of the transport magnets or errors in the BPM calibration (sign flip or faulty amplitude calibration). We present the results and give an outlook on extending the analysis to additional channels such as charge, compression and arrival time monitors as well as the FEL output signal.

Orbit response measurements in the soft X-ray beamline of Athos have shown coupling of the beam transport between the transverse planes, which is influenced by the on-axis field strength of the Apple-X undulator modules. A model reproduces this observation if a coupling term is included in the transport matrix of the undulator module. The presentation shows the estimate of the coupling strength as a function of beam energy, undulator field strength and orbit excitation.

In 2017, a collaboration between DESY, PSI and CERN was established with the aim of developing and building seven advanced X-Band Transverse Deflection Structure (TDS) with the new feature of variable polarization of the deflecting force. Seven deflectors were produced by PSI of which five were installed in three experiments at DESY, while the remaining two were installed in the ATHOS soft X-ray beamline in SwissFEL, with the goal to provide sub-fs resolution for soft X-ray pulse profiles. Early this year the X-band power source of the TDS for SwissFEL was completed and system commissioning started. This contribution summarizes the first deflection experiments performed.

X-ray absorption spectroscopy (XAS) enables the study of the electronic and geometric structural properties of matter. Such investigations can now be realized with femtosecond temporal resolution owing to the availability of X-ray free-electron lasers (XFELs) [1]. However, most XFELs currently utilize self-amplified spontaneous emission (SASE), which causes strong shot-to-shot fluctuations of their intensity and spectral distribution. Consequently, SASE fluctuations represent a challenge for the precise normalization of the measured absorption signal to the incident photon flux. Here, we have developed normalization schemes utilizing diffractive optics that overcome the SASE fluctuations. The diffractive optics are used to split the incoming XFEL SASE beam into two or three identical copies (±1 and 0th orders). By placing the (solid or liquid jet) sample in one of the diffracted beams, the absorption and reference signals are recorded simultaneously, thus enabling efficient data normalization on a shot-to-shot basis. In this contribution, we will present diffractive optics for two normalization schemes at SASE XFELs. First, a three beam geometry based on beam-splitting silicon off-axis zone plate [2] for soft XAS implemented at the Spectroscopy and Coherent Scattering beamline at the European XFEL to study L2,3 edges of transition metals will be presented. Secondly, a two-beam configuration for hard X-ray transient absorption spectroscopy of liquid jets (K-edge, an aqueous solution of [Fe(C2O4)3]3-) will be reported. Here, a beam-splitting transmission diamond grating for focused hard X-rays in combination with bent silicon <220> crystal was experimentally tested at the ALVRA station at the SwissFEL (ΔE/E ≈ 3% bandwidth). The results demonstrate high-quality K-edge transient XAS of [Fe(C2O4)3]3- solution without the need to scan the monochromator [3]. These normalization schemes pave the way for ultrafast L- and K-edge XAS measurements of transition metals at XFELs.

Temporal diagnostics of FEL pulses are generally of great benefit to FEL facilites, in particular to provide information to users and for the setup of special modes such as fresh-slice schemes. In this contribution we present FEL power profile measurements with femtosecond resolution at SwissFEL. The FEL temporal profiles are obtained from the longitudinal phase-space of the electrons after the undulator section. We use the transverse wakefields of a corrugated structure to horizontally streak the electron beam, and vertical dispersion to access the energy information. The advantages of this approach, in comparison to the standard streaking using transverse deflecting rf structures, are cost-effectiveness and stability against arrival time jitter.

Brilliant, ultrashort, and coherent X-ray FEL pulses allow investigations of dynamics at the inherent time and length scale of atoms. However, the user community still lacks access to phase-locked X-ray pulses, desirable for time domain correlation spectroscopies and coherent quantum control. Based on selective electron-bunch degradation in the accelerator, combined with two-stage, self-seeded photon emission, we propose an FEL mode generating subfemtosecond, phase-locked X-ray pulse pairs with up to 100 fs delay. Splitting the electron bunch in the accelerator, instead of photon pulses in the beamline, avoids relative phase jitter. This enables time-domain interferometry, such as the X-ray analog of the ubiquitous Fourier transform infrared spectrometer, and, more generally, all of nonlinear and quantum optics requiring coherent copies of beams.