Short-period Undulator Research Articles

Green upgrading of SPring-8 to produce stable, ultrabrilliant hard X-ray beams.

SPring-8-II is a major upgrade project of SPring-8 that was inaugurated in October 1997 as a third-generation synchrotron radiation light source. This upgrade project aims to achieve three goals simultaneously: achievement of excellent light source performance, refurbishment of aged systems, and significant reduction in power consumption for the entire facility. A small emittance of 50 pm rad will be achieved by (1) replacing the existing double-bend lattice structure with a five-bend achromat one, (2) lowering the stored beam energy from 8 to 6 GeV, (3) increasing the horizontal damping partition number from 1to 1.3, and (4) enhancing horizontal radiation damping by installing damping wigglers in long straight sections. The use of short-period in-vacuum undulators allows ultrabrilliant X-rays to be provided while keeping a high-energy spectral range even at the reduced electron-beam energy of 6 GeV. To reduce power consumption, the dedicated, aged injector system has been shut down and the high-performance linear accelerator of SACLA, a compact X-ray free-electron laser (XFEL) facility, is used as the injector of the ring in a time-shared manner. This allows the simultaneous operation of XFEL experiments at SACLA and full/top-up injection of the electron beam into the ring. This paper overviews the concept of the SPring-8-II project, the system design of the light source and the details of the accelerator component design.

High-Temperature Superconducting Undulator Prototype Coils for Compact Free-Electron Lasers

Short-period and high-field undulators are crucial for the production of coherent light up to X-rays in compact free-electron lasers (FELs). Besides, future colliders like CLIC or FCC-ee demand high-field damping wigglers to reach a low beam emittance. Both applications may benefit from the use of high-temperature superconductors (HTS), e.g. by providing high magnetic field amplitudes or higher operating temperatures. This paper presents and summarizes our HTS undulator prototype coils, designed and manufactured at CERN, made from coated REBCO tape superconductor. The coil design, described and already powered at 77K in previous works, is based on no-insulation (NI) vertical racetracks with a period length of 13 mm, assembled with iron poles. Powering tests of several HTS undulator prototype coils at 4.2 K, performed at KIT, revealed safe operations at high engineering current densities of 2.3 kA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with produced magnetic fields of up to 1.5 T at 3.5mm distance from the magnetic iron poles. Reasonable effective time constants of around 100 s were obtained and further powering tests in regions beyond <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> c with 3.6 kA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> showed the stability of our NI design and created magnetic fields in the region of 2 T.

Status and powering test results of HTS undulator coils at 77 K for compact FEL designs

The production of low emittance positron beams for future linear and circular lepton colliders, like CLIC or FCC-ee, requires high-field damping wigglers. Just as compact free-electron lasers (FELs) require high-field but as well short-period undulators to emit high energetic, coherent photons. Using high-temperature superconductors (HTS) in the form of coated ReBCO tape superconductors allows higher magnetic field amplitudes at 4 K and larger operating margins as compared to low-temperature superconductors, like Nb-Ti.This contribution discusses the development work on superconducting vertical racetrack (VR) undulator coils, wound from coated ReBCO tape superconductors. The presented VR coils were modularly designed with a period length of 13 mm. Powering tests in liquid nitrogen of multiple vertical racetrack coils were performed at CERN. The results from the measurements are presented for three VR coils and compared with electromagnetic simulations.

Requirements and limitations of the pulsed wire technique for measuring short-period long undulators’ magnetic field

The pulsed wire method is an attractive option to measure the magnetic field in insertion devices, mainly for those with restricted access (e.g., small gaps, in-vacuum/cryogenic environments, etc.). Besides first and second field integrals, experiments have proved the feasibility of reconstructing the magnetic field profile. Undulators with a small gap and short period are — and are planned to be — used at diffraction-limited storage rings and free-electron lasers. This contribution outlines the pulsed wire system’s requirements to perform magnetic field reconstruction in such undulators. We examine the main expected limitations, particularly the dispersive, finite pulse-width, discretization error, and sag effects. Furthermore, we present the current status of developing the pulsed wire system at the European XFEL.

Progress on HTS Undulator Prototype Coils for Compact FEL Designs

Compact free-electron lasers require short accelerator structures followed by high peak field and short-period undulators to produce coherent radiation up to hard X-rays. Likewise, future lepton colliders, like CLIC or FCC-ee, require high-field damping wigglers for the creation of low emittance beams. The application of high-temperature superconductor (HTS) in the form of coated REBCO tape to undulators and wigglers enables higher magnetic fields and larger operating margins. This paper discusses the potential of HTS for a superconducting vertical racetrack (VR) undulator coil geometry as well as results of the development work done on a prototype coil, with a period length of 13 mm, wound from coated REBCO tape conductor. Design and assembly aspects as well as first results from powering tests at 77 K are presented and discussed.

Magnetic field reconstruction using the pulsed wire method: An accuracy analysis

Learning how to reconstruct the magnetic field profile from the pulsed wire method (PWM) is of fundamental importance to characterize undulators with limited magnetic gap accessibility. This paper presents a new data processing approach to reconstruct undulator magnetic field profiles from the PWM. We study via numerical simulations the accuracy and precision of the novel approach to recover the local undulator parameter and the root-mean-square (RMS) phase error. Due to the wire parameters uncertainties, the PWM can, in the best case, reach an accuracy of approximately 10−3 to reconstruct the local undulator parameter, and a precision (needed to determine the magnetic field quality) of about 10−5. Furthermore, our results show that the RMS phase error can be determined with a precision better than 0.2°. This contribution opens the possibility to apply the PWM to perform magnetic field reconstruction in small-gap short-period long undulators for advanced light sources.

Simulation and experimental study of short-period strong magnetic field undulator

Further reducing the period length will be a very attractive direction for the future development of undulators, and it is one of the effective ways to realize the miniaturization of free-electron laser facilities and obtain short wavelengths. In this paper, a novel planar hybrid permanent magnet undulator is proposed which can obtain a higher peak field with a short period length. The dimensions of the magnet blocks and the poles are numerically optimized for a sample undulator with a period length of 16 mm and a gap of 5 mm. Compared to the conventional hybrid undulator, the effective peak field can be enhanced by about 48%. An eight-period model is built and the result of the magnetic field measurement by a pulsed wire system shows a good consistent with the simulation result.

Fully-staggered-array bulk Re-Ba-Cu-O short-period undulator: large-scale 3D electromagnetic modelling and design optimization using A-V and H-formulation methods

The development of a new hard x-ray beamline I-TOMCAT equipped with a 1 m long short-period bulk high-temperature superconductor undulator (BHTSU) has been scheduled for the upgrade of the Swiss Light Source at the Paul Scherrer Institute. 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 m 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 m long BHTSU, reducing the number of degrees of freedom 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 -ϕ formulation method, all methods showing excellent agreement with each other as well as with experimental results. The mixed H -ϕ 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, implemented 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.

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.

Standard variable short period microwave-plasma undulator

Microwave-plasma undulator (MPU), as a standard short-period length undulator, is a desirable source of hard X-ray with continuous wide spectrum. In this article, the electron wiggling motion under the effect of transverse electric mode, propagating in cold plasma-filled waveguide, is investigated. The deflection parameter and period length of MPU in counter-propagating and co-propagating modes are calculated. Advantages of MPU over conventional and microwave undulators are investigated. It is shown that MPUs generate higher photon flux with overlapping harmonics compared to conventional standard undulators and cover wider energy range of synchrotron radiation. A rectangular MPU with period length of 2.98 mm and deflection parameter of 2.23 and a circular MPU with period length of 2.99 mm and deflection parameter of 2.23 are introduced. Also, a short variable period MPU, with the period length varying from 2.62 mm to 4.12 mm, and a helical MPU is introduced.

Effects of Staggered Undulator Magnetic Field Contributions on Inverse Free Electron Laser Accelerator

In this paper, we include the important effects of sextupolar field contributions on saturation gain and length of a linear polarized undulator driven inverse free electron laser (IFEL) that arise when the electrons are injected off axis. The modified IFEL equations are derived and analyzed. Both the accelerating gradient and the saturation length of the IFEL are influenced by the imperfect beam trajectory. For a fixed laser wavelength, the effects are more pronounced in the case of short period undulator in comparison with a larger period undulator IFEL. A useful empirical fit is prescribed to estimate the tolerance of the IFEL for imperfect beam trajectory.

THz-driven split ring resonator undulator

We propose a short period undulator based on the electromagnetic field pattern in a THz-driven split ring resonator structure. An analytical model is developed that allows us to assess the key performance parameters of the undulator and to estimate the emitted radiation spectrum. Different geometric configurations are compared in detail using numerical simulations. A 100 MeV electron bunch with 5 pC charge is shown to emit narrow band 83 eV photon pulses with a peak brightness of approximately ${10}^{19}\text{ }\mathrm{photons}/(\mathrm{s}\text{ }{\mathrm{mrad}}^{2}\text{ }{\mathrm{mm}}^{2}\text{ }\text{ }0.1%\text{ }\text{ }\mathrm{BW})$ when passing through the 100 mm long undulator with a 1 mm period.

Single-block measurement for the cryogenic permanent magnet undulator sorting

The magnet sorting is a standard step in the undulator fabrication procedure. The shortest undulators period used in the High Energy Photon Source (HEPS) is only 12 mm. To the short period undulator, the sorting may be more important than the long-period undulator. Normally, the Helmholtz measurement is used as the input for the work. It is the averaged orthogonal magnetization of each block. In order to investigate whether the Helmholtz coil measurement is enough for the sorting, a careful study has been made. Firstly, a magnetic camera was used to scan the 3D surface field of a magnet. Afterward, its field integral is measured by the stretched wire. In the measurement, the magnet was placed at different statuses. The results were checked to see whether they are consistent with expected features supposing a homogeneous magnetized block. Finally, the results measured by the Helmholtz coil and the stretched wire were compared. The surface field scan demonstrates that the field over a magnet block is inhomogeneous. Moreover, the field integral measurement by the stretched wire also shows big difference when the different magnet sides toward the wire. The comparison between the stretched wire and the Helmholtz coil measurement shows no correlation. The study presented in this paper reveals that the homogeneity of the magnetization is imperfect. Therefore, the Helmholtz coil data are insufficient to the short-period undulators sorting.

A compact and cost-effective hard X-ray free-electron laser driven by a high-brightness and low-energy electron beam

We present the first lasing results of SwissFEL, a hard X-ray free-electron laser (FEL) that recently came into operation at the Paul Scherrer Institute in Switzerland. SwissFEL is a very stable, compact and cost-effective X-ray FEL facility driven by a low-energy and ultra-low-emittance electron beam travelling through short-period undulators. It delivers stable hard X-ray FEL radiation at 1-A wavelength with pulse energies of more than 500 μJ, pulse durations of ~30 fs (root mean square) and spectral bandwidth below the per-mil level. Using special configurations, we have produced pulses shorter than 1 fs and, in a different set-up, broadband radiation with an unprecedented bandwidth of ~2%. The extremely small emittance demonstrated at SwissFEL paves the way for even more compact and affordable hard X-ray FELs, potentially boosting the number of facilities worldwide and thereby expanding the population of the scientific community that has access to X-ray FEL radiation. The first lasing results at SwissFEL, an X-ray free-electron laser, are presented, highlighting the facility’s unique capabilities. A general comparison to other major facilities is also provided.

An ultra-compact x-ray free-electron laser

In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV m−1. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 m. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by 50 nm-rad normalized emittances. The emittance, we note, is the effective area in transverse phase space (x, p x /m e c) or (y, p y /m e c) occupied by the beam distribution, and it is relevant to achievable beam sizes as well as setting a limit on FEL wavelength. These beams, when injected into innovative, short-period (1–10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine—including the imaging of virus particles—which may profit from this new model of performing XFEL science.

Numerical Design Optimization of Short-Period HTS Staggered Array Undulators

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..

Photocathode study of the CompactLight Collaboration for a novel XFEL development

The latest generation of light sources, the Free Electron Lasers that are capable of delivering high-intensity photon beams of unprecedented brilliance and quality, provide a substantially novel way to probe matter with a very high, largely unexplored potential for science and innovation. The CompactLight Collaboration intends to design a hard X-ray FEL facility beyond today’s state of the art using the latest concepts for bright electron photoinjectors, very high-gradient X-band structures at 12 GHz, and innovative compact short-period undulators. When compared to existing facilities, the proposed facility will benefit from a lower electron beam energy, due to the enhanced undulator performance, will be significantly more compact with a smaller footprint, as a consequence of both the lower energy and the high-gradient X-band structures, and will have a much lower electrical power demand. This paper gives an overview of different candidate photocathode materials for use in conjunction with a laser and presents the proposed design of the photocathode and the related laser. Also presented are the e-gun with solenoid selection and design, together with relevant simulation results.

Short-period high-strength helical undulator by laser-driven bifilar capacitor coil.

Laser wakefield accelerators have emerged as a promising candidate for compact synchrotron radiation and even x-ray free electron lasers. Today, to make the electrons emit electromagnetic radiation, the trajectories of laser wakefield accelerated electrons are deflected by transverse wakefield, counter-propagating laser field or external permanent magnet insertion device. Here, we propose a novel type of undulator that has a period of a few hundred microns and a magnetic field of tens of Tesla. The undulator consists of a bifilar capacitor-coil target that sustains a strong discharge current that generates a helical magnetic field around the coil axis when irradiated by a high-energy laser. Coupling this undulator with state-of-the-art laser wakefield accelerators can, simultaneously, produce ultra-bright quasi-monochromatic x-rays with tunable energy ranging 5-250 keV and optimize the free electron laser parameter and gain length compared with a permanent magnet-based undulator. This concept may pave a path toward ultra-compact synchrotron radiation and even x-ray free electron lasers.

THz-driven surface plasmon undulator as a compact highly directional narrow band incoherent x-ray source

We propose a short period undulator which is based on the alternating electromagnetic field pattern of THz-driven surface plasmons in a thin conductive layer on a dielectric grating. An approximate analytical model allows to assess the key performance parameters of the undulator and to estimate the emitted radiation spectrum. The specific example of a graphene based undulator is simulated in detail. For a moderate electron beam energy of 100 MeV and a bunch charge of 0.5 pC the 40 mm long undulator is shown to emit narrow band 1 keV x-ray pulses with a peak brightness of approximately $1{0}^{16}\text{ }\text{ }\mathrm{photons}/(\mathrm{s}\text{ }{\mathrm{mrad}}^{2}\text{ }{\mathrm{mm}}^{2}\text{ }0.1%\text{ }\text{ }\mathrm{BW})$. It therefore has potential for a compact and low cost x-ray source.

Demonstration of high-performance pole pieces made of monocrystalline dysprosium for short-period undulators.

Reported here are the results of experiments carried out to demonstrate the magnetic performance of dysprosium (Dy) to enhance the capability of undulators. Tiny pieces of monocrystalline Dy surrounded by permanent magnets (PMs) work as pole pieces (PPs) to concentrate the magnetic flux, when cooled down below the Curie temperature of 85 K. A PP made of Dy is much more attractive than one made of a conventional material, because its saturation magnetization is much higher. Furthermore, it also allows for a more flexible selection of PM material, potentially leading to further enhancement of the performance of short-period undulators. Besides these advantages, practical issues related to using Dy PPs and countermeasures against them are discussed.