Elliptically Polarizing Undulator Research Articles

From design and construction to operation of the APPLE undulator at TPS

Abstract The APPLE type of Elliptically Polarized Undulator (EPU) not only serves as a crucial circularly polarized light source in existing accelerator facilities but also acts as a standard light source for soft X-rays at the Taiwan Photon Source (TPS). Up to the present, we have autonomously designed, constructed, and operated various EPU configurations. The experience gleaned from the construction of various designs and the choice of material will be outlined in this paper. We also elaborate upon the actions taken to address the adverse effects on the magnetic fields due to the characteristics of non-identical magnets and mechanical deformation. In particular, a method for correction of discrepancies in the magnetic field of the EPU with operation in numerous phase modes employing iron shims is described. In addition, we also discuss the adverse effects on the storage ring that must be dealt with for stable operation of the EPU at the TPS and optimization of the magnetic field during the laboratory construction phase. This paper will further explain management issues related to the susceptibility of the EPU to radiation damage. The comprehensive experience related to the EPU at the TPS, from design and construction to operation will be presented in this paper.

Development of an APPLE III undulator prototype with three-dimensional force compensation for SHINE

The Shanghai high-repetition-rate XFEL and extreme light facility (SHINE) plans to install several elliptically polarizing undulators (EPUs) as afterburners behind the planar undulator section to obtain nearly saturated circularly polarized free-electron laser (FEL) radiation. Therefore, the SHINE R&D project needs to develop a 4-m-long EPU with an effective magnetic field of 1.5 T, a period of 68 mm, and a minimum gap of 3 mm. A magnetic force compensation EPU prototype has recently been built and tested at the Shanghai Synchrotron Radiation Facility (SSRF) based on the addition of permanent magnets. This prototype can realize magnetic compensation under various polarization modes in all working gaps, thereby reducing the deformation of the girders and maintaining sufficient rigidity of the mechanical structure. A girder deformation monitoring system was established to obtain real force compensation feedback. The final magnetic field measurement results meet the physical requirements, and the proposed scheme can be used as an alternative to the EPU with a high magnetic field and large magnetic force.

Design and measurement of a compact EPU magnetic structure

One research direction for future synchrotron light sources is the development of elliptically polarized undulators (EPUs) with a short period and high magnetic field. The Shanghai Synchrotron Radiation Facility (SSRF) has developed a compact EPU prototype with a 16 mm period. The prototype incorporates a novel magnetic structure consisting of 8 magnet arrays, generating a peak magnetic field reaching approximately 0.8 T. Additionally, the magnetic structure is compact and lightweight, featuring a 5 mm aperture (ϕ5mm). To assess its performance, the magnetic field was measured using the pulsed wire and Hall sensor methods. This paper presents a detailed analysis of the physical design, mechanical structure, and measurement results of the compact EPU prototype.

Phasing Magnet for CSX-2 Beamline at NSLS-II

A phasing magnet has been developed at National Synchrotron Light Source II (NSLS-II) for the Coherent Soft X-ray Scattering (CSX) beamline. The phasing magnet will be located at the center of the straight section in between two identical and independent variably polarized APPLE-II devices. Based on Permanent Magnet technology, the phasing magnet has been designed to achieve the required electron beam delay to properly adjust the phase matching of these two consecutive EPUs (Elliptically Polarizing Undulators) and ensure a positive interference between the photon beam emitted in each device. This paper will describe the mechanical and magnetic design together with the final field measurements and magnetic tuning results. Also, the spectral performance of the two EPUs and the method used to properly set the field strength of the phasing magnet for any given radiation wavelength and polarization mode will be presented as well.

Mechanical characterization of an APPLE undulator at TPS

Because of a strong demand for circularly polarized radiation, elliptically polarized undulators (EPU) are widely used in synchrotron light source nowadays. At Taiwan Photon Source (TPS) with a 3-GeV ring, the EPU plays an important role to fulfill the needs of the user community for EUV to soft X-ray. Up to phase II, in total five EPU have been installed. Because multiple operating modes are necessary, the EPU suffers changes in attractive and repulsive forces in three directions, in contrast to a general ID, which suffers only a vertical attraction. A mechanical structure of the EPU is required that can withstand multiple stresses and eliminate mechanical deformation. In addition, the EPU, a purely magnetic structure, is sensitive to high-permeability material, which makes the overall structure a heterogeneous material, resulting in a problem with uneven thermal expansion and contraction because of changes in ambient temperature. Here we discuss and propose solutions to resist force and temperature variations of EPU mechanical structures, which includes mechanical simulation and inspection.

Development of an EPU vacuum system using lumped NEG-ion combination pumps

A vacuum design based on five lumped ZAO® non-evaporable getter (NEG) pumps (CapaciTorr® Z 200 × 2 and NEXTorr® Z 200 × 3) was developed to construct a vacuum system of elliptically polarized undulator (EPU). The vacuum tests, including measurements of the base pressure and the sorption capacity of CO, and the pressure-rise test on turning off the ion pumps, were performed to evaluate the vacuum performance of this design. Two such EPU vacuum systems were installed in the storage ring of Taiwan Photon Source (TPS) in 2020 and were conditioned with synchrotron radiation, but without baking. The experimental results show that their dynamic pressures achieved less than 3 × 10−12 mbar mA−1 after an accumulated beam dose 100 A h.

Design and Development of a New Control Architecture for Elliptically Polarizing Undulators at the Taiwan Photon Source

During the phase-II beamline project at the Taiwan Photon Source (TPS), elliptically polarized undulators (EPU), called EPU66 and EPU168, will be installed in 2020. The new control system for the EPUs is based on the Experimental Physics and Industrial Control System (EPICS) and Ethernet Control Automation Technology (EtherCAT) framework. The EPU control elements include: motors with absolute optical encoders for gap and phase control, corrector magnets power supply control for trim coils, interlock safety systems to automatically stop motion based on encoder readings, limit switches, tilt sensors and emergency buttons. All control functionalities are coordinated by a fan-less embedded computer with three Ethernet ports. User interface for operators and beamline users are included to support the operation of the system, such as gap/phase control. Reliable operation of the EPU is important to users of beamlines. The most unpredictable fault is due to a soft error in the absolute optical encoders. There are several protection solutions to avoid faults, e.g., by increasing the distance of the encoder from the beam center, by a lead shield cover and finally by adopting an auxiliary position sensing device to cross check the position and perform the necessary procedure. Efforts to improve operational reliability of the EPU controls will be discussed. Features and benefits of the EPU66 and EPU168 control system will be summarized in this report.

Polarization control at the microscopic and electronic structure observatory

The new Microscopic and Electronic Structure Observatory (MAESTRO) at the Advanced Light Source (ALS) in Berkeley provides X-rays of variable polarization, produced by an elliptically polarized undulator (EPU), for angle resolved photoemission (ARPES) and photoemission electron microscopy (PEEM) experiments. The interpretation of photoemission data, in particular of dichroism effects in ARPES, requires the precise knowledge of the exact polarization state. Numerical simulations show that the first harmonics of the EPU at MAESTRO provides soft X-rays of almost 100% on axis polarization. However, the higher harmonics as well as the downstream optical elements of the beamline, have a considerable impact on the polarization of the light delivered to the experimental end-station. Employing a simple reflective polarimeter, the polarization is characterized for variable EPU and beamline settings and the overall degree of polarization in the MAESTRO end-stations is estimated to be on the order of 83%.

Development and fabrication of the vacuum systems for an elliptically polarized undulator at Taiwan Photon Source

Three sets of a vacuum system were developed and fabricated for elliptically polarized undulators (EPU) of a 3-GeV synchrotron facility. These chambers were shaped with low roughness extrusion and oil-free machining; the design combines aluminium and stainless steel. The use of a bimetallic material to connect the EPU to the vacuum system achieves the vacuum sealing and to resolve the leakage issue due to bake process induced thermal expansion difference. The interior of the EPU chamber consists of a non-evaporable-getter strip pump in a narrow space to absorb photon-stimulated desorption and to provide a RF bridge design to decrease impedance effect in the two ends of EPU chamber. To fabricate these chambers and to evaluate the related performance, we performed a computer simulation to optimize the structure. During the machining and welding, the least deformation was achieved, less than 0.1mm near 4m. In the installation, the linear slider can provide a stable and precision moved along parallel the electron beam direction smoothly for the EPU chamber to decrease the twist issue during baking process. The pressure of the EPU chamber attained less than 2×10−8Pa through baking. These vacuum systems of the EPU magnet have been installed in the electron storage ring of Taiwan Photon Source in 2015 May and have normally operated at 300mA continuously since, and to keep beam life time achieved over than 12h.

Active and passive compensation of APPLE II-introduced multipole errors through beam-based measurement

The effect of an APPLE II-type elliptically polarized undulator (EPU) on the beam dynamics were investigated using active and passive methods. To reduce the tune shift and improve the injection efficiency, dynamic multipole errors were compensated using L-shaped iron shims, which resulted in stable top-up operation for a minimum gap. The skew quadrupole error was compensated using a multipole corrector, which was located downstream of the EPU for minimizing betatron coupling, and it ensured the enhancement of the synchrotron radiation brightness. The investigation methods, a numerical simulation algorithm, a multipole error correction method, and the beam-based measurement results are discussed.

Practical method using superposition of individual magnetic fields for initial arrangement of undulator magnets.

We have developed a practical method for determining an excellent initial arrangement of magnetic arrays for a pure-magnet Halbach-type undulator. In this method, the longitudinal magnetic field distribution of each magnet is measured using a moving Hall probe system along the beam axis with a high positional resolution. The initial arrangement of magnetic arrays is optimized and selected by analyzing the superposition of all distribution data in order to achieve adequate spectral quality for the undulator. We applied this method to two elliptically polarizing undulators (EPUs), called U#16-2 and U#02-2, at the Photon Factory storage ring (PF ring) in the High Energy Accelerator Research Organization (KEK). The measured field distribution of the undulator was demonstrated to be excellent for the initial arrangement of the magnet array, and this method saved a great deal of effort in adjusting the magnetic fields of EPUs.

The Latest Status of NSLS-II Insertion Devices

The National Synchrotron Light Source-II (NSLS-II) project is now in the final stage of construction. The Linac, the Booster synchrotron, and the Storage Ring magnets girder assemblies have been installed. The first damping wiggler has been delivered and its field characteristics are carefully measured. A Three Pole Wiggler (3PW) and Apple-II type elliptically polarizing undulators (EPUs) have been fabricated by the vendors. Two 3.0m long in-vacuum undulators (IVUs) and one 1.5m long IVU are almost complete and waiting for factory acceptance tests. One 3.0m long IVU for Inelastic X-ray Scattering beamline is in fabrication by a different vendor. Recently two 2.8m long IVUs for long straight sections (LSSs) have been added to the project for "future beamlines". In addition, two 1.5m long IVUs and one 2.8m long IVU for LSSs have been procured for Advanced Beamlines for Biological Investigations with X-rays (ABBIX) project funded by National Institure of Health (NIH). Further, two 3.5m long EPUs for LSSs are being designed for NSLS-II Experimental Tools (NEXT) -Major Item of Equipment (MIE) project. To succeed these conventional IVUs, PrFeB based cryo-permanent magnet undulator (CPMU) is considered as next generation device of hard X-ray sources. An In-Vacuum Magnetic Measurement System (IVMMS) for cold in-situ Hall probe mapping of CPMUs up to 1.5m in length has been developed. Summary of the current status of each project and future plans for the NSLS-II ring will be discussed.

Operation of a fast polarization-switching source at the Photon Factory

We have been developing a fast polarization-switching source for the vacuum ultraviolet and soft X-ray region at the B15-16 straight section of the 2.5-GeV Photon Factory (PF) storage ring. The source consists of two tandem APPLE-II-type elliptically polarizing undulators (EPUs), namely, U#16-1 and U#16-2, and a fast kicker system. The target frequency of polarization switching is 10 Hz. As the first step, we installed U#16-1 and five identical bump kickers in the PF ring in March 2008. Then, we constructed U#16-2 and installed it in August 2010. The orbit switching operation at 10 Hz, for user experiments, started in January 2012. We describe the details of the operation status of two EPUs and the fast local bump system in this report.

Performance of an Elliptical Polarization Undulator in TPS

The advantages of an APPLE-II elliptically polarized undulator (EPU) are a wide range of tuneable energy and a large polarization rate. National Synchrotron Radiation Research Center (NSRRC) therefore decided to install a 4-m EPU with period length 46 mm at a beamline for soft x-rays in Taiwan Photon Source. Magnetic-field shimming was developed; the final magnetic performances of the phase error, electron trajectory and multipole errors are presented in this paper.

Development of an x-ray beam position monitor for TPS EPU beamline front ends

In order to develop state-of-the-art x-ray beam position monitors (XBPMs) for the Taiwan Photon Source (TPS) project, the National Synchrotron Radiation Research Center (NSRRC) and the Advance Photon Source have established a collaboration project to design and test various XBPMs for the TPS beamline front ends. One of the XBPM prototypes is dedicated to the elliptically polarized undulator (EPU) beamline front ends to be constructed at the TPS. In this paper, we present the design of the XBPM prototype. Preliminary test results for the prototype are also discussed.

Field Correction of an Elliptically Polarized Undulator

The correction of the magnetic field of an elliptically polarized undulator (EPU) decreases the phase errors, the RMS trajectory and multipole magnetic components. The conventional method to correct the field involves tedious work with much trial and error. Based on field-shimming procedures and a field-shimming simulator proposed in National Synchrotron Radiation Research Center, the duration of field correction of EPU46 becomes decreased. The first field integral and the kicker value at each magnet pole determine the amount of adjustment of vertical and horizontal positions of the magnet. The final result of correcting a single pair of a magnet array shows a phase error less than 5 and a variation of the RMS trajectory less than 5 , in both circular and linear polarization modes for single pair of magnet array. Here we describe a standard operating procedure of EPU field correction that is becoming established, which will be beneficial for a future mass-production stage and for training purposes.

Construction and performance of an elliptically polarized undulator of type Apple-II

Ten years ago, an elliptically polarized undulator (EPU) with a periodic length of 56 mm was constructed at the Taiwan Light Source. It was the longest EPU at that time. An electron beam of energy 1.5 GeV and current 300 mA passes through the undulator gap and radiates variously polarized light in the soft X-ray spectral domain. This EPU was extensively used in a wide range of research fields, including inelastic scattering, spin-polarized photoemission spectroscopy, photoelectron emission microscopy, and soft X-ray scattering. In response to increased demand by users with differing experimental requirements and applications, the construction of an EPU with a period of length 46 mm is under way. This investigation describes many aspects of the magnetic design, the structural engineering, and the control system. As magnetic technology is undergoing dramatic advances, advanced mechanical devices, and mechanisms have been included in the new EPU design; this design and its differences from the earlier design are summarized here.

Support structure design of Quantum Material Spectroscopy Center beamline insertion devices

A high-energy elliptically polarizing undulator (EPU) and a low-energy EPU are designed to share a single C style supporting structure. Each of the EPUs is 4 m in length. Finite-element analyses (FEA) are performed to choose the proper girder style and section size and to verify the components' strength and deflection. The most difficult challenges are the huge three dimensional magnetic forces on high-energy EPU and the space limitation for the two EPUs. In total the whole structure has 18 ball screw drive systems. A total of 30 sets of linear bearings are used in the structure.

Delta undulator model: Magnetic field and beam test results

Abstract A novel type of in-vacuum Elliptical Polarization Undulator (EPU) magnet optimized for linac beam (Delta undulator) was developed at the Laboratory for Elementary-Particle Physics (LEPP) at Cornell University as part of insertion device development for the future Cornell 5 GeV Energy Recovery Source of coherent hard X-rays [1,7]. To evaluate mechanical, vacuum and magnetic properties of the magnet, a short 30 cm model with a 5 mm diameter round gap and a 2.4 cm period was built and tested in LEPP. The beam test of the Delta undulator model was conducted at Accelerator Test Facility (ATF) in BNL with ∼60 MeV linac beam. The beam testing results confirmed basic properties of the undulator magnet obtained through the magnetic field measurement. In the paper we describe the magnet design, techniques and setups used for the magnetic field measurement and the beam testing results.

Investigation of a method to calculate spontaneous radiation spectra from relativistic electrons in undulators

Undulators are key devices to produce brilliant synchrotron radiation at the synchrotron radiation facilities. In this paper we present a numerical computing method, including the computing program that has been developed to calculate the spontaneous radiation emitted from relativistic electrons in undulators by simulating the electrons' trajectory. The effects of electron beam emittance and energy spread have also been taken into account. Comparing with other computing methods available at present, this method has a few advantages with respect to several aspects. It can adopt any measured or arbitrarily simulated 3D magnetic field and arbitrary electron beam pattern for the calculation and it's able to analyze undulators of any type of magnetic structure. It's expected to predict precisely the practical radiation spectrum. The calculation results of a short period in-vacuum undulator and an Elliptically Polarized Undulator (EPU) at Shanghai Synchrotron Radiation Facility (SSRF) are presented as examples.