Undulator beamline optimization with integrated chicanes for X-ray free-electron-laser facilities

Demonstration of full polarization control of soft X-ray pulses with Apple X undulators at SwissFEL using recoil ion momentum spectroscopy

The ability to freely control the polarization of X-rays enables measurement techniques relying on circular or linear dichroism, which have become indispensable tools for characterizing the properties of chiral molecules or magnetic structures. Therefore, the demand for polarization control in X-ray free-electron lasers is increasing to enable polarization-sensitive dynamical studies on ultrafast time scales. The soft X-ray branch Athos of SwissFEL was designed with the aim of providing freely adjustable and arbitrary polarization by building its undulator solely from modules of the novel Apple X type. In this paper, the magnetic model of the linear inclined and circular Apple X polarization schemes are studied. The polarization is characterized by measuring the angular electron emission distributions of helium for various polarizations using cold target recoil ion momentum spectroscopy. The generation of fully linear polarized light of arbitrary angle, as well as elliptical polarizations of varying degree, are demonstrated.

Enhanced X-ray free-electron laser performance with optical klystron and helical undulators

This article presents a demonstration of the improved performance of an X-ray free-electron laser (FEL) using the optical klystron mechanism and helical undulator configuration, in comparison with the common planar undulator configuration without optical klystron. The demonstration was carried out at Athos, the soft X-ray beamline of SwissFEL. Athos has variable-polarization undulators, and small magnetic chicanes placed between every two undulators to fully exploit the optical klystron. It was found that, for wavelengths of 1.24 nm and 3.10 nm, the required length to achieve FEL saturation is reduced by about 35% when using both the optical klystron and helical undulators, with each effect accounting for about half of the improvement. Moreover, it is shown that a helical undulator configuration provides a 20% to 50% higher pulse energy than planar undulators. This work represents an important step towards more compact and high-power FELs, rendering this key technology more efficient, affordable and accessible to the scientific community.

An X-ray free-electron laser with a highly configurable undulator and integrated chicanes for tailored pulse properties

X-ray free-electron lasers (FELs) are state-of-the-art scientific tools capable to study matter on the scale of atomic processes. Since the initial operation of X-ray FELs more than a decade ago, several facilities with upgraded performance have been put in operation. Here we present the first lasing results of Athos, the soft X-ray FEL beamline of SwissFEL at the Paul Scherrer Institute in Switzerland. Athos features an undulator layout based on short APPLE-X modules providing full polarisation control, interleaved with small magnetic chicanes. This versatile configuration allows for many operational modes, giving control over many FEL properties. We show, for example, a 35% reduction of the required undulator length to achieve FEL saturation with respect to standard undulator configurations. We also demonstrate the generation of more powerful pulses than the ones obtained in typical undulators. Athos represents a fundamental step forward in the design of FEL facilities, creating opportunities in FEL-based sciences.

Versatile modulators for laser-based FEL seeding at SwissFEL

The Paul Scherrer Institute is implementing laser-based seeding in the soft X-ray beamline (Athos) of its free-electron laser, SwissFEL, to enhance the temporal and spectral properties of the delivered photon pulses. This technique requires, among other components, two identical modulators for coupling the electron beam with an external laser with a wavelength range between 260 and 1600 nm. The design, magnetic measurements results, alignment, operation and also details of the novel and exotic magnetic configuration of the prototype are described.

The Swiss Light Source and SwissFEL at the Paul Scherrer Institute

At the Paul Scherrer Institute, two electron accelerator-based photon sources are in operation, namely a synchrotron source, the swiss light source (SLS), and an X-ray free-electron laser, SwissFEL. SLS has been operational since 2001 and SwissFEL since 2017. In this time, unique and world-leading scientific programs and methods have developed from the SLS and the SwissFEL in fields as diverse as macromolecular biology, chemical and physical sciences, imaging, and the electronic structure and behaviour of novel and complex materials. To continue the success, a major upgrade of SLS, the SLS2.0 project, is ongoing and at SwissFEL further endstations are under construction.

Laser-Driven Modulation of Electron Beams in a Dielectric Micro-Structure for X-Ray Free-Electron Lasers

We describe an application of laser-driven modulation in a dielectric micro-structure for the electron beam in a free-electron laser (FEL). The energy modulation is transferred into longitudinal bunching via compression in a magnetic chicane before entering the undulator section of the FEL. The bunched electron beam comprises a series of enhanced current spikes separated by the wavelength of the modulating laser. For beam parameters of SwissFEL at a total bunch charge of 30 pC, the individual spikes are expected to be as short as 140 as (FWHM) with peak currents exceeding 4 kA. The proposed modulation scheme requires the electron beam to be focused into the micrometer scale aperture of the dielectric structure, which imposes strict emittance and charge limitations, but, due to the small interaction region, the scheme is expected to require ten times less laser power as compared to laser modulation in a wiggler magnet, which is the conventional approach to create a pulse train in FELs.

The SwissFEL soft X-ray free-electron laser beamline: Athos

The SwissFEL soft X-ray free-electron laser (FEL) beamline Athos will be ready for user operation in 2021. Its design includes a novel layout of alternating magnetic chicanes and short undulator segments. Together with the APPLE X architecture of undulators, the Athos branch can be operated in different modes producing FEL beams with unique characteristics ranging from attosecond pulse length to high-power modes. Further space has been reserved for upgrades including modulators and an external seeding laser for better timing control. All of these schemes rely on state-of-the-art technologies described in this overview. The optical transport line distributing the FEL beam to the experimental stations was designed with the whole range of beam parameters in mind. Currently two experimental stations, one for condensed matter and quantum materials research and a second one for atomic, molecular and optical physics, chemical sciences and ultrafast single-particle imaging, are being laid out such that they can profit from the unique soft X-ray pulses produced in the Athos branch in an optimal way.

A simple and compact scheme to enhance the brightness of self-amplified spontaneous emission free-electron-lasers

A simple and compact scheme that enhances the brightness of self-amplified spontaneous-emission (SASE) free-electron lasers is presented. The method combines the high-brightness SASE scheme and the optical klystron concept to increase the temporal coherence of the produced radiation and to reduce the required length of the undulator beamline at the same time. The scheme is very simple and only requires compact chicanes between the modules of the undulator beamline. Simulations show that, in comparison with SASE, the brightness can be improved by up to a factor of ten and the required length to achieve saturation can be reduced by 20% or more.

Compact coherence enhancement by subharmonic self-seeding in X-ray free-electron laser facilities

X-ray free-electron lasers (FELs) are cutting-edge scientific tools able to generate transversely coherent radiation with very high power and ultra-short pulse durations. The self-seeding mechanism has been proven to increase the longitudinal coherence of the FEL radiation but its efficiency could be significantly improved, especially for soft X-rays. This paper proposes the enhancement of the performance of self-seeding by combining it with the harmonic generation mechanism. In particular, by starting the process with a subharmonic of the wavelength of interest, the coherence of the produced radiation is improved, the undulator beamline becomes more compact, and the monochromator realization is simplified. Numerical simulations for SwissFEL are presented showing that the method can be employed, within a given space, to increase the spectral brightness by one order of magnitude or more with respect to standard self-seeding. This coherence enhancement will be fundamental for many photon science applications and techniques such as resonant inelastic X-ray scattering.

Perspective: Opportunities for ultrafast science at SwissFEL

We present the main specifications of the newly constructed Swiss Free Electron Laser, SwissFEL, and explore its potential impact on ultrafast science. In light of recent achievements at current X-ray free electron lasers, we discuss the potential territory for new scientific breakthroughs offered by SwissFEL in Chemistry, Biology, and Materials Science, as well as nonlinear X-ray science.