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Short period undulators are an essential component of a compact free electron laser (FEL) and medium energy storage rings for the production of hard X-rays. The use of ReBCO high temperature superconductors (HTS) in a staggered array undulator geometry is expected to yield a magnetic undulator flux density above 2 T for a 10 mm period and a 4 mm magnetic gap, thus substantially increasing the lasing performance of existing FEL and storage ring facilities. The optimization of the design for a staggered array undulator with FEM simulations and a critical analysis of its working principles are discussed in this paper. Specifically, the maximization of the undulator field and the minimization of the required HTS material are the two main objectives of this study, which is performed with realistic material parameters of commercially available bulks materials. Finally, the outcome of the applied optimization technique is discussed and the intrinsic limits of this undulator geometry are highlighted..

A backward computation method has been developed to accelerate modelling of the critical state magnetization current in a staggered-array bulk high-temperature superconducting (HTS) undulator. The key concept is as follows: i) a large magnetization current is first generated on the surface of the HTS bulks after rapid field-cooling (FC) magnetization; ii) the magnetization current then relaxes inwards step-by-step obeying the critical state model; iii) after tens of backward iterations the magnetization current reaches a steady state. The simulation results show excellent agreement with the H-formulation method for both the electromagnetic and electromagnetic-mechanical coupled analyses, but with significantly faster computation speed. Solving the FEA model with 1.8 million degrees of freedom (DOFs), the backward computation method takes less than 1.4 hours, an order of magnitude or higher faster than other state-of-the-art numerical methods. Finally, the models are used to investigate the influence of the mechanical stress on the distribution of the critical state magnetization current and the undulator field along the central axis., 12 pages, 7 figures

We demonstrate the operation of a compact x-ray free-electron laser (FEL) using the optical klystron mechanism. This effect speeds up the FEL process using the dispersion of magnetic chicanes placed between the undulator modules of the FEL beamline. The demonstration was performed at the soft x-ray beamline of SwissFEL, called Athos, which is, as far as we are aware, the only x-ray FEL to date with magnetic chicanes between every two undulator modules. In our measurements, we show that, compared to standard operation without chicanes, the required undulator length to achieve FEL saturation is reduced between 15% and 30% for radiation wavelengths between 1 and 2 nm. Fully exploiting the optical klystron effect represents an important step toward more compact FEL designs, rendering this key technology more affordable and hence accessible to a larger science community.

The development of a new hard x-ray beamline I-TOMCAT equipped with a 1-meter-long short-period bulk high-temperature superconductor undulator (BHTSU) has been scheduled for the upgrade of the Swiss Light Source (SLS 2.0) at the Paul Scherrer Institute (PSI). The very hard x-ray source generated by the BHTSU will increase the brilliance at the beamline by over one order of magnitude in comparison to other state-of-the-art undulator technologies and allow experiments to be carried out with photon energies in excess of 60 keV. One of the key challenges for designing a 1-meter-long (100 periods) BHTSU is the large-scale simulation of the magnetization currents inside 200 staggered-array bulk superconductors. A feasible approach to simplify the electromagnetic model is to retain five periods from both ends of the 1-meter-long BHTSU, reducing the number of degrees of freedom (DOFs) to the scale of millions. In this paper, the theory of the recently-proposed 2D A-V formulation-based backward computation method is extended to calculate the critical state magnetization currents in the ten-period staggered-array BHTSU in 3D. The simulation results of the magnetization currents and the associated undulator field along the electron beam axis are compared with the well-known 3D H-formulation and the highly efficient 3D H-{\phi} formulation method, all methods showing excellent agreement with each other as well as with experimental results. The mixed H-{\phi} formulation avoids computing the eddy currents in the air subdomain and is significantly faster than the full H-formulation method, but is slower in comparison to the A-V formulation-based backward computation. Finally, the fastest and the most efficient A-V formulation in ANSYS 2020R1 Academic is adopted to optimize the integrals of the undulator field along the electron beam axis by optimizing the sizes of the end bulks., Comment: 19 pages, 12 figures

This theory demonstrates the existence of a variable transverse K gradient in Apple-type undulators. It is supported by numerical simulations to assess the limits of this analytical approach., Apple-type undulators are globally recognized as the most flexible devices for the production of variable polarized light in the soft X-ray regime, both at synchrotron and free-electron laser facilities. Recently, the implementation of transverse gradient undulators has been proposed to enhance the performance of new generation light sources. In this paper it is demonstrated that Apple undulators do not only generate linear and elliptical polarized light but also variable transverse gradient under certain conditions. A general theoretical framework is introduced to evaluate the K-value and its transverse gradient for an Apple undulator, and formulas for all regular operational modes and different Apple types (including the most recent Delta type and Apple X) are calculated and critically discussed.

In the framework of the SwissFEL project at the Paul Scherrer Institute (PSI), a Hall probe bench is being developed for the high-precision magnetic characterization of the insertion devices for the ATHOS soft X-ray beamline. For this purpose, a novel three-axis teslameter has been developed, which will be placed between the undulator and its outer shell in a very limited volumetric space of 150 x 50 x 45 mm. Together with a Hall probe at the center of the cross sectional area of the undulator, the setup will traverse along the undulator length on a specifically designed rig with minimal vibrations. This teslameter has all the analog signal conditioning circuitry for the Hall probe and also has on board 24-bit digitization. The instrument also handles an interface to a linear absolute encoder. The old instrumentation used only had analog signal conditioning circuitry whilst digitization was done off board. The new instrument also provides a very accurate magnetic field map in the &micro, T range with simultaneous readings from the position encoder at an accuracy of &plusmn, 3 &micro, m. In this paper, a series of tests are described, which were performed at PSI in order to establish the measuring precision and repeatability of the instrument.

Magnetic measurement benches are used to measure the magnetic field profile of insertion devices such as undulators. A critical feature of these benches is the traversal of a measurement head such as a Hall probe through the undulator whilst keeping it perfectly centered, ensuring that the measurement obtained is not distorted by physical displacements of the probe during measurement. This paper presents a detailed novel design for a new magnetic measurement bench based on the SAFALI Hall probe positioning system, which facilitates the measurement as well as magnetic optimisation of the magnetic field from inside of a vacuum chamber. The paper gives a brief introduction to in-vacuum undulators and describes the new developments in the magnetic measurement bench. The design of the electronics involved in the bench is then explained, including the wireless communication systems used to operate the bench. The reproducibility of magnetic field measurements as well as the precision of the hall probe positioning are also discussed and presented.

A novel three-axis teslameter and other similar machines have been designed and developed for SwissFEL at the Paul Scherrer Institute (PSI). The developed instrument will be used for high fidelity characterisation and optimisation of the undulators for the ATHOS soft X-ray beamline. The teslameter incorporates analogue signal conditioning for the three-axes interface to a SENIS Hall probe, an interface to a Heidenhain linear absolute encoder and an on-board high-resolution 24-bit analogue-to-digital conversion. This is in contrast to the old instrumentation setup used, which only comprises the analogue circuitry with digitization being done externally to the instrument. The new instrument fits in a volumetric space of 150 mm &times, 50 mm &times, 45 mm, being very compact in size and also compatible with the in-vacuum undulators. This paper describes the design and the development of the different components of the teslameter. Performance results are presented that demonstrate offset fluctuation and drift (0.1&ndash, 10 Hz) with a standard deviation of 0.78 &micro, T and a broadband noise (10&ndash, 500 Hz) of 2.05 &micro, T with an acquisition frequency of 2 kHz.

The new APPLE X undulators are novel elliptically-polarised undulators, which have a highly symmetric geometry. Theoretically, all the elliptical polarisation states are expected to have the same deflection parameter if the magnetic and geometrical errors are negligible. Nonetheless, the magnetic measurements of the first ten APPLE X undulators performed at Paul Scherrer Institut (PSI) contradicted this simple statement and numerical simulations were required to understand and validate the results. After the valuation of the magnetic-force induced mechanical deformations of the undulator frame, the central role played by the magnetic susceptibility χ was investigated. The simulation results indicate that the impact of different magnet types in the Halbach array, the anisotropic properties, and the choice of shaped field radial magnets are critical for the achieved magnetic field. The numerical results are compared with the measurement data.

A novel, flexible yet simple method to generate gigawatt X-ray free-electron-laser radiation with unprecedented spectral bandwidth above the 10% level is presented. Such broadband radiation will improve substantially the efficiency of techniques like X-ray crystallography and spectroscopy, paving the way for outstanding progress in fields like biology and material science., A new and simple method to generate X-ray free-electron-laser radiation with unprecedented spectral bandwidth above the 10% level is presented. The broad bandwidth is achieved by sending a transversely tilted beam through a transverse-gradient undulator. The extent of the bandwidth can easily be controlled by variation of the beam tilt or the undulator gradient. Numerical simulations confirm the validity and feasibility of this method.

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., Journal of Synchrotron Radiation, 26 (4), ISSN:0909-0495, ISSN:1600-5775

Within the SwissFEL project at the Paul Scherrer Institute (PSI), the hard X-ray line (Aramis) has been equipped with short-period in-vacuum undulators, known as the U15 series. The undulator design has been developed within the institute itself, while the prototyping and the series production have been implemented through a close collaboration with a Swiss industrial partner, Max Daetwyler AG, and several subcontractors. The magnetic measurement system has been built at PSI, together with all the data analysis tools. The Hall probe has been designed for PSI by the Swiss company SENIS. In this paper the general concepts of both the mechanical and the magnetic properties of the U15 series of undulators are presented. A description of the magnetic measurement equipment is given and the results of the magnetic measurement campaign are reported. Lastly, the data reduction methods and the associated models are presented and their actual implementation in the control system is detailed., peer-reviewed

The SwissFEL X-ray Free Electron Laser (XFEL) facility started construction at the Paul Scherrer Institute (Villigen, Switzerland) in 2013 and will be ready to accept its first users in 2018 on the Aramis hard X-ray branch. In the following sections we will summarize the various aspects of the project, including the design of the soft and hard X-ray branches of the accelerator, the results of SwissFEL performance simulations, details of the photon beamlines and experimental stations, and our first commissioning results.

The wiggler X-ray source of the Materials Science beamline at the Swiss Light Source has been replaced with a 14 mm-period cryogenically cooled in-vacuum undulator. In order to best exploit the increased brilliance of this new source, the entire front-end and optics have been redesigned., The Materials Science beamline at the Swiss Light Source has been operational since 2001. In late 2010, the original wiggler source was replaced with a novel insertion device, which allows unprecedented access to high photon energies from an undulator installed in a medium-energy storage ring. In order to best exploit the increased brilliance of this new source, the entire front-end and optics had to be redesigned. In this work, the upgrade of the beamline is described in detail. The tone is didactic, from which it is hoped the reader can adapt the concepts and ideas to his or her needs.

An optimization of the undulator layout of X-ray free-electron-laser (FEL) facilities based on placing small chicanes between the undulator modules is presented. The installation of magnetic chicanes offers the following benefits with respect to state-of-the-art FEL facilities: reduction of the required undulator length to achieve FEL saturation, improvement of the longitudinal coherence of the FEL pulses, and the ability to produce shorter FEL pulses with higher power levels. Numerical simulations performed for the soft X-ray beamline of the SwissFEL facility show that optimizing the advantages of the layout requires shorter undulator modules than the standard ones. This proposal allows a very compact undulator beamline that produces fully coherent FEL pulses and it makes possible new kinds of experiments that require very short and high-power FEL pulses.

Physical review accelerators and beams 19(10), 100702(2016). doi:10.1103/PhysRevAccelBeams.19.100702, The SwissFEL Injector Test Facility operated at the Paul Scherrer Institute between 2010 and 2014, serving as a pilot plant and test bed for the development and realization of SwissFEL, the x-ray Free-Electron Laser facility under construction at the same institute. The test facility consisted of a laser-driven rf electron gun followed by an S-band booster linac, a magnetic bunch compression chicane and a diagnostic section including a transverse deflecting rf cavity. It delivered electron bunches of up to 200 pC charge and up to 250 MeV beam energy at a repetition rate of 10 Hz. The measurements performed at the test facility not only demonstrated the beam parameters required to drive the first stage of an FEL facility, but also led to significant advances in instrumentation technologies, beam characterization methods and the generation, transport and compression of ultralow-emittance beams. We give a comprehensive overview of the commissioning experience of the principal subsystems and the beam physics measurements performed during the operation of the test facility, including the results of the test of an in-vacuum undulator prototype generating radiation in the vacuum ultraviolet and optical range., Published by American Physical Society, College Park, MD

After 10 years of operation the wiggler-source Materials Science beamline at the Swiss Light Source was the first beamline to undergo a significant upgrade. The replacement of the W61 wiggler by the cryogenic undulator U14 makes the SLS the first wiggler free third generation light source. With the help of the cryogenic technology [1], the period length could be reduced from 19 mm to 14 mm. With a minimum gap of 3.8 mm and the x-ray energy range could be extended to nearly 40 keV. The undulator has been built in cooperation with SPring-8 and Hitachi. PSI designed the liquid-nitrogen-based cryogenic system and made the magnetic measurements under cryogenic conditions before installation. To be cost efficient, the undulator shares the cryogenic refrigeration system with the monochromator. Operational aspects like stability or temporal response to gap changes will be discussed as well as the spectral performance.

The tolerance in the alignment of the quadrupoles in the linacs and in the undulator lines of the Swiss Free Electron Laser (SwissFEL), the next project at the Paul Scherrer Institute (PSI), will be about 1 μm. This accuracy will be reached using beam alignment techniques. To minimize the commissioning time, it is also requested to install the quadrupoles with a precision of ±50 μm. To achieve this goal, the vibrating wire technique will be used in association with other systems to determine the position of the magnetic axis of the quadrupoles with respect to external fiducials. After a general introduction to the project and to the instrumentation principle, the novel approach developed at PSI is presented. The main innovation consists in the use of a phase lock loop (PLL) for maintaining the wire in its resonance condition. This approach simplifies the operation of the system in case the wire is moved or replaced and increases substantially the reliability of the measurement outcomes in the frame of a series test campaign.

The Insertion Device group of the Paul Scherrer Institute has started an R&D program on a high temperature superconducting undulator to reduce the period length and increase the undulator's magnetic field well beyond the present capability. Simulation results for a 10 mm period and 4 mm magnetic gap staggered array of GdBCO bulks predict peak magnetic field above 2 T. Building on the existing working principle of undulator design and simulated performance, the first experimental results of a 5 period 6.0 mm gap short undulator measured in the new test facility available at the University of Cambridge will be presented together with details of the experimental setup and sample preparation.