Cheng-Hsiang Chang

Development of an Algorithm for Magnet Sorting and Field Shimming for TPS Elliptically Polarized Undulators

A spatially periodic magnetic field is essential to cause an electron beam to wiggle and to emit electromagnetic radiation in a synchrotron (SR) source of radiation, and to provide fully coherent light in free electron lasers (FEL). To create this field, permanent magnets (PM) or electromagnets are patterned in a device commonly called an insertion device for SR and a radiator or modulator for FEL. In reality, magnet blocks or iron poles are not identical, in terms of geometry and magnetic properties, even with progressive manufacture. Compensatory methods are thus desired to recover the magnetic field and also to decrease the duration of construction. Magnet sorting is a pre-process that aims to eliminate the effect of manufacturing error. Before assembly of an insertion device, data of each component, especially the magnetic properties of each magnet block and the gap variation of mechanical structure, are organized to optimize the performance of the magnetic field. After that process, there is sometimes an optimization to shim the magnetic field. An effective algorithm of both processes is significant, particularly for a long undulator and an elliptically polarized undulator (EPU).

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.

A Superconducting Magnetization System With Hybrid Superconducting Wire for the Study and Application of Superconductivity

For the study of high-temperature superconductors and for the application of superconductivity, a superconducting high-field magnetization system with a hybrid superconducting coil was designed. This coil is composed of superconducting wires of three kinds-HTS YBCO 2G-wire, high-field and low-field NbTi wires. Three principal purposes of building this system are to inspect the characteristics of disk-shape bulk YBCO, to develop a HTS-bulk undulator, and to magnetize a portable HTS-bulk high-field magnet for application to a resonant X-ray scattering experiment. With four steps of temperature decrease, a two-stage GM-type cryocooler and liquid nitrogen provide cooling for the magnetization system. The hybrid superconducting coil, two cryogenic systems for the magnetization system and the portable HTS-bulk magnet, and the control and monitor systems that were designed are discussed.

Design of a HTS Magnet for Application to Resonant X-Ray Scattering

In material research, the characteristics of novel materials vary greatly with the environment. A magnet with a strong field will be developed for an experimental station for resonant X-ray scattering to investigate the magnetic properties of materials. This magnet will be developed with high-temperature superconductor (HTS) bulk YBa2Cu3O7 and magnetized with a HTS coil magnet wound with 2G HTS wire. HTS (RE) BCO will be selected to construct the coil of this magnet. Both the bulk and coil magnets will be assembled on the same movable system. The bulk HTS magnet will provide flux density greater than 4 T with a gap 34 mm that can accommodate the sample holder of the experimental station. The bulk magnet will be cooled with cryocoolers to 29 K and the coil magnet to 4.2 K; the coil magnet is separable from the bulk magnet after the field is trapped. We describe the concept of the magnetic-field calculation, the overall design of these magnets, and the cooling algorithm for the bulk HTS magnet system.

Field Trimming by Iron Pieces and Coils in a Superconducting Undulator

Trim-iron-pieces and trim-coils were used to correct the magnetic field error in a superconducting undulator with a magnet period of 15 mm. The main-coils were wound with NbTi wires of 0.77 times 0.51 mm2 rectangular cross section. The entire iron pole array was electrically insulated from the main-coils using Teflon to avoid degradation of the superconducting wire when the main-coils were trained to a large current. The trim-coils were wound with NbTi wires of 0.33 mm diameter. The trim-iron-pieces and trim-coils were mounted directly on the iron pole. The electric current of these trim-coils were generated by a single power supply with a constant voltage. A variable resistor and two golden-case resistors were connected in parallel to adjust the current that passed through each trim-coil. In this work, three trim-iron-pieces and one trim-coil were fabricated and mounted on each of the upper and lower arrays, and their effect on correcting the magnetic field error of the undulator is measured and compared to a model simulation.

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.

Design of a 4.7-T Wavelength Shifter With Cryogenic Permanent Magnets at NSRRC

A 4.77-T wavelength shifter is designed for extending the X-ray spectrum beyond 50 keV in the 3-GeV storage ring of the Taiwan Photon Source. The compact 400-mm-long wavelength shifter is designed by using the PrFeB-based cryogenic permanent magnets to generate 4.77 T at a 5-mm magnet gap. The 0.1% field uniformity on the transverse 40 mm is calculated. A cryocooler system that is cryogen free is designed to cool the magnets and to maintain the temperature of the magnets at 77 K in a vacuum chamber. Numerical simulation shows that the cooling system can cool the magnet array from 300 K to 77 K. In this paper, we present the high-magnetic-field performance and the cooling system for the wavelength shifter.

Magnet-Sorting Algorithm for an Elliptically Polarized Undulator at TPS

An effective initial sorting of permanent magnets can decrease the adverse effects of magnetic inhomogeneities and the imperfect geometry of blocks on the performance of an undulator. A sorting algorithm, based on simulated annealing, for the TPS EPU is implemented. The algorithm takes into account the magnetization of the blocks and the interactions between magnets. The field quality, including multipole, phase error, and particle trajectory, was optimized to within the specified tolerance. In this paper, we describe the sorting procedure and demonstrate the optimization results.

A prototype phase shifter for phase matching between undulators at TPS

Phase shifters of various kinds have been studied to match the double undulators that were installed in the same double-mini Betay-function straight section in TPS. A prototype phase shifter, designed to satisfy the requirement for phase matching between two undulators, comprises three C-type dipole magnets for convenient operation and tuning of the magnetic field to cover photon energies over a wide range. The phase shifter, operating at 5 A, provides phase delay 555° for minimal photon energy 300 eV. A trim current at the side coils serves to compensate for the first and second field integrals. The main current is varied to cover photon energies 0.3–20 keV. Our design takes into account an effect of cross talk with nearby magnets.

Twin-Helix Undulator for Round Beam-Related Light Sources

14 Vol. 31, No. 3, 2018, Synchrotron radiation newS Introduction Due to the strong demand for circularly polarized radiation, insertion devices (ID) have been used to provide elliptical fi elds since the late 1980s. The trend is to increase the degree of circular polarization and also the brightness of the radiation. ID design, therefore, evolved early from an asymmetric wiggler [1] to elliptical wigglers [2] and fi nally to helical undulators. For third-generation light sources, a variety of helical undulator designs have been developed and implemented [3– 5]. Among them, the design of the APPLE II [6] provides the highest fi elds and thus has become the workhorse in several facilities. Development of helical undulators was greatly facilitated thanks to improvements in magnetic materials, especially in rare-earth permanent magnet materials. A neodymium-iron-boron (NdFeB) magnet is preferred due to its higher remanence and its robust mechanical properties. Since the development of the NdFeB magnet by Sagawa in 1983 [7], the magnet has been widely used in the construction of IDs. For ultimate storage rings or linac-driven free electron lasers (FELs), a round vacuum pipe allows the placement of more magnetic materials close to the electron beam. A round-gap ID signifi cantly facilitates a design with enhanced magnetic fi eld properties. Bahrdt proposed the APPLE III [8], which chamfers the magnets for a round vacuum pipe; by tilting the magnetization direction to 45°, the magnetic fi eld could be enhanced for the BESSY soft-X-ray FEL. A Delta undulator [9] was proposed for the Cornell energy recovery linac and is implemented in the Linac Coherent Light Source (LCLS) for the afterburner confi guration [10]. Both types of undulators are used as a radiator for full control of the polarization. More recently, permanent magnet planar undulators have been preferred as modulators for the electron bunching process in FELs because their mature technology is fully understood. Nevertheless, there has been recent interest in the use of helical undulators for more compactness and for a more effi cient FEL [11]. Since commissioning of the Taiwan Photon Source (TPS) in December 2014, three APPLE IIs were installed in Phase I [12], and two or more are being constructed to fulfi ll the requirements of the community using radiation from the EUV to soft X-rays. Based on the construction and commissioning experience, an APPLE II can be improved by optimizing the mechanical design of the keeper structure, which holds the magnet blocks, to reduce effects on beam dynamic effects in various operation modes [13]. Moreover, an exotic twin-helix undulator (THU) has been proposed for the VUV FEL project [14]. The THU is a hybrid structure and has a round gap. The poles and magnet blocks are shaped into a helix to produce a purely helical fi eld, as well as improving the strength and homogeneity of the magnetic fi eld.