Undulator Coils Research Articles

Novel correction procedure for compensating thermal contraction errors in the measurement of the magnetic field of superconducting undulator coils in a liquid helium cryostat.

Superconducting undulators (SCUs) can offer a much higher on-axis undulator field than state-of-the-art cryogenic permanent-magnet undulators with the same period and vacuum gap. The development of shorter-period and high-field SCUs would allow the free-electron laser and synchrotron radiation source community to reduce both the length of undulators and the dimensions of the accelerator. Magnetic measurements are essential for characterizing the magnetic field quality of undulators for operation in a modern light source. Hall probe scanning is so far the most mature technique for local field characterization of undulators. This article focuses on the systematic error caused by thermal contraction that influences Hall probe measurements carried out in a liquid helium cryostat. A novel procedure, based on the redundant measurement of the magnetic field using multiple Hall probes at known relative distance, is introduced for the correction of such systematic error.

Effect of SCU long range errors on the FEL performance

The FEL performance strongly correlates with the undulator field quality. The definition of mechanical tolerances for the undulator magnets allows us to achieve the wished field quality. These mechanical tolerances should be defined both on short and long-range errors. With long-range errors, we address problems like deformations of the yoke caused by the support structures or unwanted tapering, which can arise in the positioning procedure of the ideally parallel undulator coils. In this contribution, we quantify the effect and set tolerances of a few types of long-range errors on the FEL radiation generated specifically from superconducting undulator coils.

Design Study for HybriSCU: A HYBRID NbTi/ReBCO Superconducting Undulator

At the European X-ray Free-Electron Laser (XFEL), the undulator systems group has started in 2020 an R&D project dedicated to the development of innovative superconducting undulator (SCU) coils. SCUs are of great interest for storage rings and free electron lasers (FEL) facilities as they enable to widen the tunability range of the generated photon energy for the same electron beam energy compared with the established technology of the permanent magnet undulators. In this contribution, we present the characterization study and the achievable performance of HybriSCU a graded SCU coil combining two different superconductors: NbTi and high temperature superconductor (HTS) based on Rare-Earth Barium Copper Oxide (ReBCO) tape.

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.

Superconducting Phase shifter design for the afterburner at the European XFEL.

At the European XFEL, we are designing a superconducting afterburner for the SASE2 hard X-ray beamline. It will consist of a series of five undulator modules plus a pre-series one called S-PRESSO. One module corresponds to two superconducting undulator (SCU) coils of 2m length plus one phase shifter. Such an afterburner will enable photon energies above 30 keV. We foresee to install superconducting (SC) phase shifters in each undulator module to keep the correct phase delay between the electron beam and photon beam. In this contribution, we present the required SC phase shifter parameters to enable operation in the electron beam energy range 11.5-17.5 GeV. We also analyze different magnetic designs satisfying the calculated specifications.

Superconducting undulator coils with period length doubling

Only since few years it has been demonstrated experimentally that NbTi based superconducting undulators (SCUs) have a higher peak field on axis for the same gap and period length in operation with electron beam with respect to permanent magnet undulators (even the ones in vacuum and cooled to cryogenic temperatures). Another advantage of NbTi based SCUs with respect to permanent magnet devices is radiation hardness, widely demonstrated for NbTi magnets, which is and will become an increasingly important issue with the small gaps in the newest machines as round beam storage rings and FELs. Moreover, SCU technology allows switching of the period length by changing the current direction in one of separately powered subset of winding packages of the superconducting coils. This feature further broadens the energy range of the emitted photons, required by the different beamlines. To this end a 0.41 m long superconducting undulator coil with switchable period length between 17 mm and 34 mm has been developed. In this contribution we describe the design and report on the quench tests, as well as on the magnetic field measurementsThe first section in your paper.

Short-Period Superconducting Undulator Coils With Neutral Poles: Test Results

The success in creating new bright magnetic structures of synchrotron radiation (SR) sources allows generation of subnanometric-emittance electron beams. This has motivated many SR centers to upgrade the old structures to increase the brightness of the SR sources. Smaller emittance imposes higher requirements on the generators of undulator radiation. With the advent of new materials, the production of undulators on permanent magnets has progressed greatly. Superconducting undulators are less common, but as sources of undulator radiation they are much more promising than undulators with permanent magnets. This paper presents test results of a prototype superconducting undulator magnet using active and neutral poles with a period of 15.6 mm and an operational field of ~1.2 T. A pole gap of 8 mm provides a vertical aperture of 6 mm for an electron beam. In contrast to the traditional type of undulator with vertical racetrack coils, in this type of undulator the coils are of the horizontal racetrack type. The magnet design consists of the aluminum-alloy frame, into which individual poles are inserted. The design can significantly improve the manufacturing accuracy and, ultimately, reduce the phase errors. The coils are made of NbTi/Cu superconducting wires, which will be indirectly cooled by cryocoolers in own cryostat to the temperature of liquid helium.

Full-Scale Conduction-Cooled Superconducting Undulator Coils—Training, Stability, and Thermal Behavior

For several years, the Institute for Beam Physics and Technology (IBPT) at the Karlsruhe Institute of Technology (KIT) has collaborated with its industrial partner Babcock Noell GmbH (BNG) on the development of superconducting undulators (SCUs) for the electron storage ring ANgstromquelle KArlsruhe (ANKA), the test facility and synchrotron radiation source run by the IBPT, as well as low-emittance synchrotron radiation sources. After the successful test of the first full scale, 1.5-m-long, superconducting undulator with 15-mm period length in the ANKA storage ring, the collaboration is at present primarily working on an SCU with 20-mm period length (SCU20), foreseen to be installed at ANKA. The 1.5-m-long undulator coils have been tested in a horizontal, conduction-cooled measurement setup developed at the IBPT. This paper describes the training, stability, and thermal behavior of the coils.

Magnetic Field Measurements of Full-Scale Conduction-Cooled Superconducting-Undulator-Coils

The Institute for Beam Physics and Technology (IBPT) of the Karlsruhe Institute of Technology (KIT), and the Babcock Noell GmbH are collaborating to develop superconducting undulators for ANKA and low-emittance light sources. The collaboration is now focusing on a superconducting undulator with a period length of 20 mm (SCU20) planned to be the source of the NANO beamline at ANKA. The local magnetic field and the field integrals of the SCU20 1.5-m-long-coils have been characterized in a conduction-cooled horizontal test facility developed at KIT IBPT. Here we complement the main results reported in a previous work with a more detailed analysis, including the measurements of the geometrical deviations from the ideal case of the undulator coils at room temperature used to simulate the magnetic field profile with the program Radia and a quantitative explanation of the magnetic field measurements in cold conditions. A comparison of the photon spectrum calculated from the measured field with the one from the field simulated with Radia considering the geometry measured at room temperature is also presented.

Field quality of 1.5 m long conduction cooled superconducting undulator coils with 20 mm period length

The Institute for Beam Physics and Technology (IBPT) at the Karlsruhe Institute of Technology (KIT) and the industrial partner Babcock Noell GmbH (BNG) are collaborating since 2007 on the development of superconducting undulators both for ANKA and low emittance light sources. The first full length device with 15 mm period length has been successfully tested in the ANKA storage ring for one year. The next superconducting undulator has 20 mm period length (SCU20) and is also planned to be installed in the accelerator test facility and synchrotron light source ANKA. The SCU20 1.5 m long coils have been characterized in a conduction cooled horizontal test facility developed at KIT IBPT. Here we present the local magnetic field and field integral measurements, as well as their analysis including the expected photon spectrum.

Performance of 2G-HTS REBCO Undulator Coils Impregnated Epoxies Mixed With Different Fillers

The use of second-generation high-temperature superconducting-coated conductors enables an enhancement of the performance of undulator magnets. However, preventing the motion of the wire and providing sufficient conduction cooling to the winding stacks have remained challenges. In this study, we have evaluated epoxy impregnation techniques to address these issues. Epoxy resin was prepared with different nanopowders and the effect on the performance of the undulator coil pack was investigated. All epoxy impregnated coils showed smaller n values and some degree of deterioration of the critical current I c . The I c degradation was most pronounced for epoxy mixed with high aspect ratio multiwalled carbon nanotubes (MWCNTs). It has been found that the crack formation in the epoxy results in plastic deformation of the copper stabilizer layer, which causes the underlying ceramic REBCO superconducting layer to crack resulting in degradation of the superconducting tape performance. Careful adjustment of epoxy thickness surrounding the superconductor and the powder ratio in the epoxy eliminate the performance degradation.

Characterization of 30-cm-Long Superconducting Undulator Coils With the Magnetic Measurement System CASPER II

The performance of superconducting insertion devices (IDs) in an electron storage ring strongly depends on their magnetic field quality. Therefore, it is essential to characterize the local field distribution and field integrals of the superconducting coils before installation in the final ID cryostat. The synchrotron radiation facility ANKA of the Karlsruhe Institute of Technology and Babcock Noell GmbH are collaborating on an R&D program on superconducting undulators (SCUs), which foresees the design, manufacturing, and characterization of an SCU with a 20-mm-period length (SCU20). Following this program, a magnetic measurement setup for conduction-cooled superconducting coils up to 2-m length (CASPER II) was built and commissioned. The system allows for performing training to maximum coil currents, measurements of the local field quality, and the first and second field integrals as well. In this contribution, we describe the measurement setup and, shortly, the magnet design and focus on the tests and measurement results of a 30-cm-long SCU20 coil package to highlight the potentiality of the test facility.

Test of Short Mockups for Optimization of Superconducting Undulator Coils

ANKA and Babcock Noell GmbH (BNG) collaborate to develop planar superconducting undulators for ANKA and low emittance light sources. We report here on the results of the training and magnetic field measurements on short mockup coils with 15- and 20-mm period length performed in the test facility CASPER I, that have qualified the wire and different winding schemes as promising to be applied to longer undulator coils.

Cryogen—Free Setup for Local and Integral Magnetic Field Measurements of Superconducting Undulator Coils

Since in an undulator the photons add up coherently over the whole undulator length even small magnetic field errors can disturb the superposition of photons and reduce the intensity of the generated photon beam. Therefore, as in any other undulator, the magnetic field has to be measured with high accuracy. Precise measurements of the magnetic properties of conventional, i.e., permanent magnet based insertion devices (IDs) has undergone huge improvements over the past years and initiated a new era in synchrotron light sources worldwide. A similar breakthrough is now necessary in the field of superconducting insertion devices. Therefore, a part of our R&D program for superconducting insertion devices is to perform quality assessment of their magnetic field properties. In this contribution we describe the equipment to perform magnetic measurements of the local field and of the field integrals of superconducting undulator coils up to 2 m length in a cold in vacuum (cryogen free) environment with a focus on the results of the factory acceptance test.

Instrumentation for Local and Integral Field Measurements of Superconducting Undulator Coils

Accurate magnetic field measurements are a prerequisite for the characterization and optimization of undulators. The precision for measuring the magnetic properties of conventional, i.e., permanent magnet based insertion devices, has undergone tremendous improvements over the past 10 to 15 years initiating a new era in synchrotron light sources worldwide; a similar breakthrough is now necessary in the field of superconducting insertion devices.

Helium-Free Field Measurement System for Superconducting Undulator Coils

Superconducting undulators generate, for a given period length and a given gap, higher fields than permanent magnet undulators. Since in an undulator the photons add up coherently over the whole undulator length, even small magnetic field errors can disturb the superposition of photons and reduce the intensity of the generated photon beam. Therefore, as in any other undulator, the magnetic field has to be measured with high accuracy.

Magnetic Field Test Facility for Superconductive Undulator Coils

Superconducting undulators and wigglers are developed for synchrotron light sources, damping rings for linear colliders and polarized positron sources. In an undulator the emitted photons along the trajectory have to interfere. In order to do so the magnetic field in all periods has to be almost identical. The field strength over one or several hundred periods is not allowed to deviate by more than 1%. Translated into mechanical accuracy the position of the wire and the poles has to be more accurate than about 5 over 1 to 2 m. High quality measurement of the field is an essential requirement. In this paper we present two field measuring systems, one is under construction and another one is under design phase at the Forschungszentrum Karlsruhe.