Development of a high-performance and cost-effective in-vacuum undulator.

In-vacuum undulators (IVUs), which have become an essential tool in synchrotron radiation facilities, have two technical challenges toward further advancement: one is a strong attractive force between top and bottom magnetic arrays, and the other is a stringent requirement on magnetic materials to avoid demagnetization. The former imposes a complicated design on mechanical and vacuum structures, while the latter limits the possibility of using high-performance permanent magnets. To solve these issues, a number of technical developments have been made, such as force cancellation and modularization of magnetic arrays, and enhancement of resistance against demagnetization by means of a special magnetic circuit. The performance of a new IVU built upon these technologies has revealed their effectiveness for constructing high-performance IVUs in a cost-effective manner.

The soft X-ray spectromicroscopy beamline BL08U1A upgrade at SSRF.

Beamline BL08U1A is a soft X-ray spectromicroscopy beamline at Shanghai Synchrotron Radiation Facility (SSRF) that exhibits the capabilities of high spatial resolution (30 nm) and high energy resolving power (over 104). As a first-generation beamline of SSRF, owing to its continuous operation over the last ten years, an urgent upgrade of the equipment including the monochromator was deemed necessary. The upgrade work included the overall construction of the monochromator and replacement of the mirrors upstream and downstream of the monochromator. Based on its original skeleton, two elliptically cylinder mirrors were designed to focus the beam horizontally, which can increase the flux density by about three times on the exit slits. Meanwhile, the application of variable-line-space gratings in the monochromator demonstrates the dual functions of dispersing and focusing on the exit slits which can decrease abberations dramatically. After the upgrade of the main components of the beamline, the energy range is 180-2000 eV, the energy resolving power reaches 16333 @ 244 eV and 12730 @ 401 eV, and the photon flux measured in the experimental station is over 2.45 × 109 photons s-1 (E/ΔE = 6440 @ 244 eV).

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.

Chirality in electromagnetic radiation from relativistic electrons.

Electrons in circular motion emit electromagnetic radiation and lose their energy and angular momentum, both of which are carried away by the radiation field. Electromagnetic radiation from such electrons is not only circularly polarized but also, in general, possessing helical phase structure, the former of which corresponds to spin angular momentum and the latter orbital angular momentum. Based on the classical electrodynamics, we show that the chiral topological property related to the orbital angular momentum arises from deformation of the electromagnetic field due to the relativistic effect.

Development of an insertion device selectively operational as a helical/figure-8 undulator.

An insertion device capable of switching the operation mode between helical and figure-8 undulators, and thus referred to as a helical-8 undulator, has been developed. It has the advantage that the on-axis heat load can be kept low regardless of the polarization state, even when a high K value is required to lower the fundamental photon energy. This is in contrast to conventional undulators in which the on-axis heat load tends to be significantly high to generate linearly polarized radiation with a high K value, and optical elements can be seriously damaged. The principle of operation, specification and light source performance of the developed helical-8 undulator are presented together with further options to enhance its capability.

FOCUS: fast Monte Carlo approach to coherence of undulator sources.

FOCUS (Fast Monte CarlO approach to Coherence of Undulator Sources) is a new GPU-based simulation code to compute the transverse coherence of undulator radiation from ultra-relativistic electrons. The core structure of the code, which is written in the language C++ accelerated with CUDA, combines an analytical description of the emitted electric fields and massively parallel computations on GPUs. The combination is rigorously justified by a statistical description of synchrotron radiation based on a Fourier optics approach. FOCUS is validated by direct comparison with multi-electron Synchrotron Radiation Workshop (SRW) simulations, evidencing a reduction in computation times by up to five orders of magnitude on a consumer laptop. FOCUS is then applied to systematically study the transverse coherence in typical third- and fourth-generation facilities, highlighting peculiar features of undulator sources close to the diffraction limit. FOCUS is aimed at fast evaluation of the transverse coherence of undulator radiation as a function of the electron beam parameters, to support and help prepare more advanced and detailed numerical simulations with traditional codes like SRW.

Improving the magnet alignment of undulator systems by laser interferometer.

The issue of intrinsic-type misalignment errors arising from angular offsets between magnets in an undulator is addressed. A random tilt of the magnets or poles generates undesirable magnetic field components in both transverse and longitudinal directions and gives rise to errors in period lengths and amplitudes. These localized errors are carried to the entire undulator segments and are a cause of concern for precision field integral and phase error estimates. A laser interferometer has been designed to read the offsets and to fix the magnets to minimize the offsets.

A neural network model of a quasiperiodic elliptically polarizing undulator in universal mode.

Machine learning has recently been applied and deployed at several light source facilities in the domain of accelerator physics. Here, an approach based on machine learning to produce a fast-executing model is introduced that predicts the polarization and energy of the radiated light produced at an insertion device. This paper demonstrates how a machine learning model can be trained on simulated data and later calibrated to a smaller, limited measured data set, a technique referred to as transfer learning. This result will enable users to efficiently determine the insertion device settings for achieving arbitrary beam characteristics.

Development and magnetic field measurement of a 0.5-m-long superconducting undulator at IHEP.

Undulators are important devices for accelerator-based light sources whose magnetic field quality determines the photon beam performance. Superconducting-type undulators have been developing rapidly in recent years around the world. The insertion device group at the Institute of High Energy Physics (IHEP) in China started an R&D project to develop prototypes of superconducting undulators (SCUs). A half-meter-long planar SCU has been produced recently. The SCU was designed based on the simulation program OPERA-3D giving a period length of 15 mm and a gap of 7 mm. This prototype was manufactured and fabricated precisely, and was then tested by vertically submerging in liquid helium in a Dewar. After quench training several times, the maximum current in the main coils reached 480 A. The magnetic field was measured by Hall probes mounted on a sledge in the middle of the undulator gap. The peak magnetic field reached 1 T. The measurement results indicate that correction coils with suitable current can not only optimize the magnetic first and second integrals but also reduce the phase error, which is expected by design.

A fast and lightweight tool for partially coherent beamline simulations in fourth-generation storage rings based on coherent mode decomposition.

A new algorithm to perform coherent mode decomposition of undulator radiation is proposed. It is based on separating the horizontal and vertical directions, reducing the problem by working with one-dimension wavefronts. The validity conditions of this approximation are discussed. Simulations require low computer resources and run interactively on a laptop. The focusing with lenses of the radiation emitted by an undulator in a fourth-generation storage ring (EBS-ESRF) is studied. Results are compared against multiple optics packages implementing a variety of methods for dealing with partial coherence: full two-dimension coherent mode decomposition, Monte Carlo combination of wavefronts from electrons entering the undulator with different initial conditions, and hybrid ray-tracing correcting geometrical optics with wave optics.

Development of an undulator with a variable magnetic field profile.

An undulator generating a magnetic field whose longitudinal profile is arbitrarily varied has been developed, which is one of the key components in a number of proposed new concepts in free-electron lasers. The undulator is composed of magnet modules, each of which corresponds to a single undulator period, and is driven by a linear actuator to change the magnetic gap independently. To relax the requirement on the actuator, the mechanical load on each module due to magnetic force acting from opponent and adjacent modules is reduced by means of two kinds of spring systems. The performance of the constructed undulator has been successfully demonstrated by magnetic measurement and characterization of synchrotron radiation.

Spatial-frequency features of radiation produced by a step-wise tapered undulator.

A scheme to generate wide-bandwidth radiation using a step-wise tapered undulator with a segmented structure is proposed. This magnetic field configuration allows to broaden the undulator harmonic spectrum by two orders of magnitude, providing 1 keV bandwidth with spectral flux density exceeding 1016 photons s-1 mm-2 (0.1% bandwidth)-1 at 5 keV on the sample. Such a magnetic setup is applicable to superconducting devices where magnetic tapering cannot be arranged mechanically. The resulting radiation with broadband spectrum and flat-top shape may be exploited at a multipurpose beamline for scanning over the spectrum at time scales of 10-100 ms. The radiation from a segmented undulator is described analytically and derivations with numerical simulations are verified. In addition, a start-to-end simulation of an optical beamline is performed and issues related to the longitudinally distributed radiation source and its image upon focusing on the sample are addressed.

Elliptical plasma-filled waveguide as a new standard short-period undulator.

Undulators as the sources of high-brilliance synchrotron radiation are of widespread interest in new generations of light sources and free-electron lasers. Microwave propagation in a plasma-filled elliptical waveguide can be studied as a standard short-period undulator. This structure as a lucrative insertion device can be installed in the storage ring of third- and fourth-generation light sources to produce high-energy and high-brilliance synchrotron radiation. In this article, the propagation of the transverse electric modes in a plasma-filled waveguide with an elliptical cross-section is investigated, and the field components, the cut-off frequencies and the electron beam trajectory are calculated. With due consideration of the electron beam dynamics and in order to achieve a standard short-period undulator, parameters such as the dimensions of the waveguide elliptical cross-section, the microwave frequency and the plasma density are calculated.

Adaptive feedforward control of closed orbit distortion caused by fast helicity-switching undulators.

A new correction algorithm for closed orbit distortion based on an adaptive feedforward control (AFC) has been developed. At SPring-8, two helicity-switching twin-helical undulators (THUs) had been implemented with conventional feedforward corrections. However, the validity of these corrections turned out to be expiring due to unforeseen variation in the error magnetic fields with time. The developed AFC system has been applied to the THUs dynamically updating the feedforward table without stopping the helicity switching amid user experiments. The error sources in the two THUs are successfully resolved and corrected even while the two THUs are switching simultaneously with the same repetition period. The actual operation of the new AFC system enables us to keep the orbit variations suppressed with an accuracy at the sub-micrometre level in a transparent way for light source users.

Theoretical spectral analysis of FEL radiation from multi-harmonic undulators. Corrigendum.

A correction to an equation in the paper by Zhukovsky [(2020). J. Synchrotron Rad. 27, 1648-1661] is made and, following from this, values of some Bessel coefficients and some harmonic intensities from an elliptic undulator with the third field harmonic.

Numerical study of the second harmonic generation in FELs.

A numerical study of the effect of betatron oscillations on the second harmonic generation in free-electron lasers (FELs) is presented. Analytical expressions for the effective coupling strength factors are derived that clearly distinguish all contributions in subharmonics and each polarization of the radiation. A three-dimensional time-dependent numerical FEL code that takes into account the main FEL effects and the individual contribution of each electron to the second harmonic generation is presented. Also, the X- and Y-polarizations of the second harmonic are analyzed. The second harmonic was detected in experiments at the Advanced Photon Source (APS) Low Energy Undulator Test Line (LEUTL) and Linac Coherent Light Source (LCLS) in the soft X-ray regime. The approach presented in the article can be useful for a comprehensive study and diagnostics of XFELs. In the paper, the LCLS and Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) experiments are modeled. The simulation results are in a good agreement with the experimental data.

Performance of BL07A at NewSUBARU with installation of a new multi-layered-mirror monochromator.

Soft X-rays excite the inner shells of materials more efficiently than any other form of light. The investigation of synchrotron radiation (SR) processes using inner-shell excitation requires the beamline to supply a single-color and high-photon-flux light in the soft X-ray region. A new integrated computing multi-layered-mirror (MLM) monochromator was installed at beamline 07A (BL07A) of NewSUBARU, which has a 3 m undulator as a light source for irradiation experiments with high-photon-flux monochromatic light. The MLM monochromator has a high reflectivity index in the soft X-ray region; it eliminates unnecessary harmonic light from the undulator and lowers the temperature of the irradiated sample surfaces. The monochromator can be operated in a high vacuum, and three different mirror pairs are available for different experimental energy ranges; they can be exchanged without exposing the monochromator to the atmosphere. Measurements of the photon current of a photodiode on the sample stage indicated that the photon flux of the monochromatic beam was more than 1014 photons s-1 cm-2 in the energy range 80-400 eV and 1013 photons s-1 cm-2 in the energy range 400-800 eV. Thus, BL07A is capable of performing SR-stimulated process experiments.

X-SPEC: a 70†eV to 15†keV undulator beamline for X-ray and electron spectroscopies.

X-SPEC is a high-flux spectroscopy beamline at the KIT (Karlsruhe Institute of Technology) Synchrotron for electron and X-ray spectroscopy featuring a wide photon energy range. The beamline is equipped with a permanent magnet undulator with two magnetic structures of different period lengths, a focusing variable-line-space plane-grating monochromator, a double-crystal monochromator and three Kirkpatrick-Baez mirror pairs. By selectively moving these elements in or out of the beam, X-SPEC is capable of covering an energy range from 70 eV up to 15 keV. The flux of the beamline is maximized by optimizing the magnetic design of the undulator, minimizing the number of optical elements and optimizing their parameters. The beam can be focused into two experimental stations while maintaining the same spot position throughout the entire energy range. The first experimental station is optimized for measuring solid samples under ultra-high-vacuum conditions, while the second experimental station allows in situ and operando studies under ambient conditions. Measurement techniques include X-ray absorption spectroscopy (XAS), extended X-ray absorption fine structure (EXAFS), photoelectron spectroscopy (PES) and hard X-ray PES (HAXPES), as well as X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS).

Practical design and performance of a new merged APPLE-Knot undulator.

Constructing vacuum-ultraviolet beamlines at synchrotron radiation facilities with giga-electron volt storage ring results in serious heat load on the beamlines which can reduce their performance. To solve this problem, an APPLE-Knot undulator with eight magnet rows has been built at the Shanghai Synchrotron Radiation Facility and has achieved very good performance. However, its performance in vertical polarization mode is imperfect. Here, a new configuration of a magnet-merged APPLE-Knot undulator that has achieved a better performance is reported.

Measurement of the horizontal beam emittance of undulator radiation by tandem-double-slit optical system.

A tandem-double-slit optical system was constructed to evaluate the practical beam emittance of undulator radiation. The optical system was a combination of an upstream slit (S1) and downstream slit (S2) aligned on the optical axis with an appropriate separation. The intensity distribution after the double slits, I(x1, x2), was measured by scanning S1 and S2 in the horizontal direction. Coordinates having 1/\sqrt e intensity were extracted from I(x1, x2), whose contour provided the standard deviation ellipse in the x1-x2 space. I(x1, x2) was converted to the corresponding distribution in the phase space, I(x1, x1'). The horizontal beam emittance was evaluated to be 3.1 nm rad, which was larger than the value of 2.4 nm rad estimated by using ray-tracing. It was found that the increase was mainly due to an increase in beam divergence rather than size.