Longitudinally variable bend prototype fabrication

I.FAST Innovation Fostering in Accelerator Science and Technology Horizon 2020 Research Infrastructures GA n° 101004730 DELIVERABLE REPORT Longitudinally variable bend prototype fabrication DELIVERABLE: D7.3 Document identifier: IFAST-D7.3 Due date of deliverable: End of Month 46 (February 2025) Report release date: 07/11/2025 Work package: WP7: High brightness accelerators for light sources Lead beneficiary: CERN Document status: Final ABSTRACT The VADER magnet is a permanent magnet combined-function dipole designed for synchrotron applications, specifically developed for its integration into the Elettra Synchrotron Light Source in Trieste, Italy. This advanced magnet is an evolution of a previous design: the CLIC Damping Rings (DRs) Longitudinally Variable Field Dipole (LVFD) [1], featuring improvements in magnetic performance, structural integrity and operational efficiency. LONGITUDINALLY VARIABLE BEND PROTOTYPE FABRICATION Deliverable: D7.3 Date: 07/11/2025 Grant Agreement 101004730 PUBLIC 2 / 16 I.FAST Consortium, 2025 For more information on IFAST, its partners and contributors please see https://ifast-project.eu/ This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No 101004730. IFAST began in May 2021 and will run for 4 years. Delivery Slip Name Partner Date Authored by Y. Papaphilippou CERN 07/11/2025 Reviewed by M. Vretenar and L. Celona [on behalf of Steering Committee] CERN 07/11/2025 Approved by Steering Committee 07/11/2025 LONGITUDINALLY VARIABLE BEND PROTOTYPE FABRICATION Deliverable: D7.3 Date: 07/11/2025 Grant Agreement 101004730 PUBLIC 3 / 16 TABLE OF CONTENTS 1 INTRODUCTION ......................................................................................................................................... 4 1.1 BACKGROUND AND PURPOSE .................................................................................................................................... 4 1.2 ROLE IN ELETTRA SYNCHROTRON ............................................................................................................................ 4 1.3 DESIGN EVOLUTION AND OPTIMIZATION .................................................................................................................. 5 2 VADER MAGNET TECHNICAL SPECIFICATIONS ............................................................................ 5 2.1 MAGNETIC FIELD STRENGTH ............................................................................................................................. 6 2.2 FIELD GRADIENT (TRANSVERSE) ....................................................................................................................... 6 2.3 MATERIAL COMPOSITION .................................................................................................................................. 6 2.4 OPERATING CONDITIONS ................................................................................................................................... 6 2.5 TECHNICAL SPECIFICATIONS TABLE .................................................................................................................. 6 3 MAGNETIC DESIGN .................................................................................................................................. 7 3.1 PRELIMINARY CONCEPTS, DESIGN AND OPTIMIZATION OF THE VADER MAGNET ................................................... 7 3.1.1 Magnet Geometry and Structural Considerations .............................................................................. 7 3.1.2 Material Selection ................................................................................................................................... 7 3.1.3 Beam Sagitta Compensation ................................................................................................................. 8 3.1.4 Pole Design and Optimization ............................................................................................................... 8 3.1.5 Permanent Magnet Selection and Configuration ............................................................................... 8 3.1.6 Magnet Block Sizing and Optimization ................................................................................................ 9 3.2 3D FEM SIMULATIONS AND FIELD OPTIMIZATION ............................................................................................ 9 3.3 FIELD TRIMMING .............................................................................................................................................. 10 3.4 FINAL MAGNETIC DESIGN OVERVIEW ............................................................................................................. 12 4 MECHANICAL DESIGN .......................................................................................................................... 12 4.1 MECHANICAL SIMULATIONS ............................................................................................................................ 14 5 ASSEMBLY................................................................................................................................................. 15 6 CONCLUSIONS ......................................................................................................................................... 15 7 REFERENCES ............................................................................................................................................ 16 LONGITUDINALLY VARIABLE BEND PROTOTYPE FABRICATION Deliverable: D7.3 Date: 07/11/2025 Grant Agreement 101004730 PUBLIC 4 / 16 1 Introduction The VADER magnet is a permanent magnet combined-function dipole designed for synchrotron applications, specifically developed for its integration into the Elettra Synchrotron Light Source in Trieste, Italy. This advanced magnet is an evolution of a previous design: the CLIC Damping Rings (DRs) Longitudinally Variable Field Dipole (LVFD) [1], featuring improvements in magnetic performance, structural integrity and operational efficiency. 1.1 BACKGROUND AND PURPOSE Synchrotrons like Elettra play a crucial role in scientific research, providing high-intensity and highly collimated synchrotron radiation for a wide range of applications, from material science and chemistry to biomedical imaging and condensed matter physics. The VADER magnet is designed to function as a longitudinal variable bending field dipole, essential for controlling electron beam trajectories with enhanced precision and stability. In contrast to conventional electromagnets, which rely on external power supplies and cooling systems, VADER employs permanent magnet technology, eliminating the need for continuous electrical input while maintaining a highly stable and predictable magnetic field. This results in a compact, cost-efficient and energy-saving solution, aligning with modern trends in accelerator technology to minimize operational costs and environmental impact. 1.2 ROLE IN ELETTRA SYNCHROTRON The Elettra synchrotron is a 2–2.4 GeV third-generation light source, serving a broad user community with its array of beamlines dedicated to advanced scientific research. The integration of VADER within this infrastructure provides: • High-field magnetic stability, ensuring precise beam steering within the accelerator lattice. • Compact and efficient design, allowing for optimal space utilization within the accelerator tunnel. • Minimal maintenance requirements, as permanent magnets do not require active power supplies or complex cooling systems. • Field homogeneity and tunability, ensuring consistent beam dynamics across operational energy ranges. The implementation of VADER within Elettra aligns with broader efforts to modernize and enhance the efficiency of synchrotron radiation facilities worldwide. Its design allows for seamless integration into the accelerator's lattice configuration, providing robust field quality with precise control over the beam emittance and trajectory. LONGITUDINALLY VARIABLE BEND PROTOTYPE FABRICATION Deliverable: D7.3 Date: 07/11/2025 Grant Agreement 101004730 PUBLIC 5 / 16 1.3 DESIGN EVOLUTION AND OPTIMIZATION VADER represents a significant step forward in the development of synchrotron magnet technology. It builds upon lessons learned from previous designs such as the LVFD dipole used in the CLIC Damping Rings [1]. Key areas of improvement include: • Enhanced material selection, utilizing NdFeB permanent magnets for improved radiation resistance and long-term field stability. • Optimized magnetic circuit design, ensuring minimal field leakage and superior homogeneity within the good field region. • Advanced mechanical structuring, leveraging high-purity iron and cobalt-iron alloys to maximize magnetic efficiency while maintaining mechanical robustness. • Improved assembly and alignment procedures, incorporating precision tooling and non-magnetic supports to ensure consistent field quality across multiple units. The transition from electromagnetic to permanent magnet dipoles reflects a broader industry trend toward sustainable and low-energy accelerator technologies. VADER serves as a prototype for future implementations in compact synchrotron light sources, medical accelerators and free-electron lasers (FELs). 2 VADER Magnet Technical Specifications The magnetic field of a longitudinal variable field dipole varies along its length. This variable field can provide a reduction of the horizontal beam emittance in comparison with a fixed field dipole of the same bending angle. The maximum field should be at the centre of the magnet and it should decrease towards both sides. Based on [1], [2], [3] and [4] two main field profiles have been selected: step and trapezoidal shape (Fig. 1). Theoretically, the trapezium profile would yield the best results with respect to the lowest beam emittance as the bending radius would change linearly along the magnet length. At the same time, it is the most complicated design due to the shortest high field region length and the exponential field decay towards the edges. This trapezoidal profile was achieved for the first time in the CLIC Drs LVFD prototype. The VADER design continues and further elaborates on the same philosophy. Fig. 1: (a) Step and (b) trapezium profiles showing L1 and L2. LONGITUDINALLY VARIABLE BEND PROTOTYPE FABRICATION Deliverable: D7.3 Date: 07/11/2025 Grant Agreement 101004730 PUBLIC 6 / 16 2.1 MAGNETIC FIELD STRENGTH The VADER dipole magnet is a permanent-magnet combined-function dipole with a peak magnetic field (Bₘₐₓ) of 2.3 Tesla in the centre of the gap. The field follows a trapezoidal longitudinal profile – highest at the mid-span and decreasing towards the ends – to achieve the desired longitudinal variable field distribution. This field shape ensures optimized beam emittance and bending strength. 2.2 FIELD GRADIENT (TRANSVERSE) VADER also provides a quadrupole field component (transverse gradient) as part of its combinedfunction design. The demanded integrated transverse gradient is about 17 T/m – much higher than that of the earlier CLIC DR prototype, which was 11 T/m [1]. This enables effective beam focusing while maintaining good field homogeneity (~10⁴ in the GFR). 2.3 MATERIAL COMPOSITION The yoke is composed of low-carbon steel (AISI 1010) for structural support and flux return. Pole tips use Armco iron (low and mid-field regions) and Fe-Co Vacoflux (high-field regions) for high saturation field efficiency. The magnet employs NdFeB permanent magnet blocks for high remanent field and strong coercivity. 2.4 OPERATING CONDITIONS The magnet operates in air at ambient temperature (20°C) with no power or active cooling required. NdFeB blocks are positioned to minimize radiation exposure and maintain long-term field stability [6]. 2.5 TECHNICAL SPECIFICATIONS TABLE Parameter Unit VADER Magnet Good field region radius mm 5 Field quality (harmonics) 10⁴ (units) ~5 Magnet gap (aperture) mm 18 Magnet length m 0.80 Transverse gradient (quadrupole) T/m 17 Dipole peak field (Bₘₐₓ) T 2.3 Dipole field (integrated) T⋅m 0.91 LONGITUDINALLY VARIABLE BEND PROTOTYPE FABRICATION Deliverable: D7.3 Date: 07/11/2025 Grant Agreement 101004730 PUBLIC 7 / 16 Yoke/Pole material – Low-carbon steel 1010 (yoke); Armco iron (low & midfield poles); Fe–Co Vacoflux (high-field poles) Magnet material – NdFeB permanent magnets (all modules) Assembly/Alignment features – Guided assembly, shims, calibration plates (0.02 mm accuracy) Operating conditions – 2.3 T at room temp., in air; field trimming via adjustable yoke Table 1: VADER Technical Specifications 3 Magnetic Design 3.1 PRELIMINARY CONCEPTS, DESIGN AND OPTIMIZATION OF THE VADER MAGNET 3.1.1 Magnet Geometry and Structural Considerations The design of the VADER magnet required an extensive evaluation of possible configurations to achieve the required magnetic field characteristics while maintaining mechanical robustness and ease of integration. Two main geometries were initially considered: C-shaped and H-shaped magnet yokes [1]. While an H-shaped yoke provides superior mechanical rigidity and symmetric field distribution, the C-shape offers significant advantages in terms of accessibility and synchrotron radiation evacuation. Given VADER’s role in the Elettra synchrotron, the C-shaped design was selected as it allows easier installation and better radiation handling when the magnet gap is oriented outward within the synchrotron ring. 3.1.2 Material Selection The selection of materials was driven by the need to achieve high field strength, minimize saturation effects and ensure long-term field stability. The yoke and pole pieces were designed to operate below saturation while maintaining high permeability to maximize field efficiency. • The yoke is constructed from low-carbon steel (AISI 1010), ensuring a low reluctance path while avoiding saturation in normal operating conditions. The yoke was designed to maintain an average field below 1 T, ensuring efficient magnetic flux return. • The pole pieces are divided into two different materials depending on the magnetic flux they have to handle: o Low and mid-field regions: Made of Armco iron, which provides excellent permeability and minimizes field distortions. o High-field region: Using cobalt-iron alloy (Vacoflux) due to its higher saturation, allowing for a peak field of 2.3 T while reducing unwanted field harmonics. LONGITUDINALLY VARIABLE BEND PROTOTYPE FABRICATION Deliverable: D7.3 Date: 07/11/2025 Grant Agreement 101004730 PUBLIC 8 / 16 • The support blocks of the split yokes are made of aluminium 7075-T6, providing enough robustness to the assembly. 3.1.3 Beam Sagitta Compensation Since the beam trajectory follows a curved path, while the magnet structure itself remains straight for manufacturing and alignment precision, the sagitta (transverse displacement of the beam) must be considered. As in this magnet the longitudinal bending field varies continuously along the particles path, the sagitta is not constant and varies accordingly. Due to this, using the typical formulae to obtain the sagitta is not recommended. In this case, the real sagitta was empirically obtained from the simulations in Opera. This displacement is incorporated into field simulations to ensure field homogeneity across the good field region (GFR). The calculated sagitta ensures accurate beam transport and minimizes deviations from the theoretical trajectory. 3.1.4 Pole Design and Optimization The poles are essential for enhancing field uniformity by concentrating and shaping the magnetic flux. In VADER, the poles are designed with a narrower tip than the base, provoking efficient flux concentration and allowing for higher peak fields in the magnet gap. Since the magnet operates as a combined-function dipole-quadrupole, the pole tip profile follows a hyperbolic shape, generating the required transverse field gradient (~17 T/m). Pole dimensions and shapes were optimized based on [2], ensuring efficient flux distribution. 3.1.5 Permanent Magnet Selection and Configuration Given that the required field in VADER is fixed, the magnet utilizes permanent magnets, eliminating the need for power supplies or cooling infrastructure. This reduces operational costs, size and weight, making it a highly efficient solution for modern synchrotron applications. Due to radiation exposure and operational stability considerations, both Neodymium-Iron-Boron (NdFeB) and Samarium-Cobalt (SmCo) were evaluated [2], [6]. The selection criteria included: • Radiation resistance: SmCo shows superior radiation tolerance, making it preferable in exposed regions. • Magnetic performance: NdFeB provides higher remanent magnetization (Br) and flux density, allowing stronger fields. • Weight and cost: NdFeB is lighter and less expensive than SmCo. Since the expected radiation in the most exposed areas is extremely low, this time all the modules will use NdFeB permanent magnet blocks. The temperature coefficients (Tc) of this type of magnets is 0.12% per ºC (Tc of Br) and 0.6% per ºC (Tc of Hc) and the expected maximum temperature variation in the Elettra 2.0 tunnel is as low as ±0.1 ºC [7]. Having these numbers in mind, the effects of the temperature variation in the field provided can be neglected. Based on this, the use of temperature compensation systems, like Ni-Fe LONGITUDINALLY VARIABLE BEND PROTOTYPE FABRICATION Deliverable: D7.3 Date: 07/11/2025 Grant Agreement 101004730 PUBLIC 9 / 16 shunts, is dismissed in this prototype. In the case of a future series production, these systems could be studied and implemented if necessary. 3.1.6 Magnet Block Sizing and Optimization To make sure that the permanent magnets operate at their optimal working point, their dimensions were determined using design equations outlined in [2]. The goal is to maximize energy density (BH_max) while minimizing volume and weight. An Excel-based computational tool was also developed to automate magnet block sizing calculations based on required field strength and magnet gap dimensions. This tool was used in the preliminary design to approach the permanent magnet geometry, trying to make sure that each magnet block operates near its optimal working point. 3.2 3D FEM SIMULATIONS AND FIELD OPTIMIZATION Finite Element Method (FEM) simulations were conducted using Opera 3D to fine-tune the magnetic field distribution. The simulations focused on achieving the target peak field of 2.3 T while ensuring gradual field decay at the magnet edges to minimize beam emittance growth. The optimized design consists of: • A central high-field region (2.3 T peak) for strong bending. • A transition region ensuring a smooth field drop. • Outer low-field sections preventing abrupt field changes. This configuration maximizes beam stability while maintaining the required bending angle. The dipolar field profile in VADER was optimized to approximate an ideal trapezoidal distribution, as shown in red in Fig. 2. The field starts from a low value at the edges, increases to 2.3 T in the central region and decays gradually. LONGITUDINALLY VARIABLE BEND PROTOTYPE FABRICATION Deliverable: D7.3 Date: 07/11/2025 Grant Agreement 101004730 PUBLIC 16 / 16 Comprehensive simulations and experimental validation confirm that VADER achieves the expected field quality, minimizing unwanted multipoles and preserving beam stability, contributing in this way to lower the emittance in the ring. The modular assembly approach, together with advanced alignment techniques, guarantees reproducibility and facilitates integration into existing accelerator infrastructures. As a prototype, VADER not only fulfills its intended role in Elettra but also serves as a benchmark for future developments in energy-efficient, high-performance magnet technology. The successful implementation of its design principles could ease the way for similar solutions in next-generation storage rings and free-electron laser facilities. 7 References [1] M. Domínguez, F. Toral, H. Ghasem, S. Papadopoulou & Y. Papaphilippou. (2018). Longitudinally Variable Field Dipole Design Using Permanent Magnets for CLIC Damping Rings. IEEE Transactions on Applied Superconductivity. PP. 1-1. 10.1109/TASC.2018.2795551. [2] J. T. Tanabe, Iron Dominated Electromagnets: Design, Fabrication, Assembly and Measurements. World Scientific, 2005 [3] P. S. Papadopoulou, F. Antoniou, y Y. Papaphilippou, «Alternative Optics Design of the CLIC Damping Rings with Variable Dipole Bends and High-field Wigglers», p. TUPTY022, jun. 2015. [4] G. Le Bec et al., Magnets for the ESRF Diffraction-Limited Light Source Project, vol. 26. 2015. [5] X. R Resende, R. Basilio, L. Liu, P. P Sanchez, y G. Tosin, Modeling of Bending Magnets for Sirius. 2017. [6] S. Trout, Material selection of permanent magnets, considering thermal properties correctly. 2001. [7] Elettra 2.0 TDR, https://www.elettra.eu/images/Documents/ELETTRA%20Machine/Elettra2/TDR-MachineInfrastructures-Final-compresso.pdf.