Jeroen van Nugteren

The European I.FAST project’s Work Package 8 (WP8) initiative focuses on advancing Canted Cosine Theta (CCT) magnet technology using High-Temperature Superconductors (HTS). These magnets aim to revolutionize synchrotron and gantry designs by reducing their size and cost, offering transformative potential for research and hadron therapy. HTS materials enable higher magnetic fields and smaller cryogenic systems compared to Low-Temperature Superconductors (LTS). However, challenges such as cable production, magnet design, and cost-efficiency must be addressed. This work presents the design of a straight HTS CCT magnet capable of achieving a central dipole field of 4 T at an operating temperature of 20 K, using a cable configuration composed of two HTS tapes. A ramp rate of 0.4 T/s is targeted, with a lower initial ramp rate of 0.15–0.2 T/s deemed acceptable for early-stage development. The magnetic design was optimized to ensure high field quality under operational conditions. Heat extraction is managed through a closed-loop cooling system, with 20 K-helium gas flowing through channels digged directly in the body of the magnet. Thermal simulations demonstrate the effectiveness of this cooling approach, ensuring operational stability. Additionally, the paper highlights key assembly procedures and experimental tests conducted for qualifying critical fabrication steps, including cabling and winding. Results from these tests confirm the reliability and robustness of the developed methods, flooring the way for future HTS CCT magnet advancements. The progress presented here represents a significant step toward compact and efficient superconducting magnets, aligning with the project’s goal of fostering innovative solutions for next-generation synchrotrons and gantries.

This paper presents the design and optimization of a low-weight helium-free Nb-Ti CCT superconducting magnet (MAGDEM – MAGent DEMonstrator) for the R&D phase of the ISOLDE Superconducting Recoil Separator (ISRS) at CERN (Geneva, Switzerland). The system can be expanded to other accelerator applications demanding compact lightweight superconducting magnets, such as medical hadron therapy systems. The innovative design incorporates a dipole coil within a quadrupole coil, enhancing field integrals and operational efficiency within its concise 590 mm length. This configuration effectively achieves a 36-degree maximum bend of the particle beam, with a concentrated effort on minimizing field errors on a tightly curved path to below one unit at a peak field of nearly 4 Tesla. Additionally, the magnet includes a slightly tapered design in its straight tubes, which aids in two key areas: simplifying the assembly of the tube sets and enabling the application of radial pre-stress to the assembly. Operating at approximately 100 A, The MAGDEM design is tailored to efficiently reduce heat leakage, allowing for a single cryocooler to maintain 4.5 K. The particle storage ring of the ISRS spectrometer will incorporate a set of MAGDEM units to guide and bend the trajectory of radioactive beams using a Fixed Field Alternating Gradient (FFAG) configuration. The paper emphasizes the novel design elements, such as the tightly curved field error optimization in a straight yet tapered coil set of formers.

The European project IFAST's WP8 Innovative Superconducting Magnets aims to develop the technology of Canted Cosine Theta (CCT) magnets wound with High-Temperature Superconductors (HTS). Superconducting magnets could lower the size and cost of synchrotrons and gantries for research and hadron therapy. HTS materials can lead to higher magnetic fields with smaller cryogenic systems than LTS. However, they pose challenges in cable production schemes, magnet design, and cost. The project's final goal is to design a straight CCT-layout magnet with a central dipole field of 4 T, and a ramp rate of 0.4 T/s, but a lower ramp rate of 0.15–0.2 T/s is acceptable for the first step. The paper presents a preliminary design, discussing in particular the protection scheme with the magnet protection solutions for the conductor: one with two and the other with four HTS tapes. Both options generate 4 T of dipole field without an Iron shell, with at least 10 K of margin at an operational temperature of 20 K. To meet the time and budget constraints of the project, a simple cable based on a co-winding of commercial REBCO tapes is proposed. Protection is the most challenging aspect of the design and an adiabatic quench analysis has been used to determine the required thickness of copper stabilizer tapes to mitigate the risk of damaging the magnet during a quench. Finally, the paper evaluates ac and radial current losses during the magnet cycles, discussing the heat distribution and possible solutions for heat extraction.

Accelerators magnets must have minimal magnetic field imperfections to reduce particle-beam instabilities. In the case of coils made of high-temperature superconducting (HTS) tapes, the magnetization due to persistent currents adds an undesired field contribution, potentially degrading the magnetic field quality. In this paper we study the use of superconducting screens based on HTS tapes for reducing the magnetic field imperfections in accelerator magnets. The screens exploit the magnetization by persistent currents to cancel out the magnetic field error. The screens are aligned with the main field component, such that only the undesired field components are compensated. The screens are self-regulating, and do not require any externally applied source of energy. Measurements in liquid nitrogen at 77   K show for dipole-field configurations a significant reduction of the magnetic field error up to a factor of four. The residual error is explained via numerical simulations accounting for the geometric defects in the HTS screens, achieving satisfactory agreement with experimental results. Simulations show that if screens are increased in width and thickness, and operated at 4.5   K , field errors may be eliminated almost entirely for the typical excitation cycles of accelerator magnets.

In 2018, FRESCA2 magnet, developed within a collaboration be-tween CEA and CERN to provide a background field for tests of cables and small coils, reached a field of 14.6 T. This represents a new record field for dipole magnets with a clear aperture. As a continuity of the European project EUCARD-2, CERN aims at exploring accelerator magnet technology up to a 20 T operating field level. For that aim, Feather magnet 2, coils 3 and 4 (FM2.3-4) utilizing REBCO Roebel cable is planned to be inserted in FRESCA2 to reach a higher field. A new structure must be de-signed to accommodate the insert coils inside FRESCA2. The in-tegrated design has to be capable to deal with several challenges. The magnetic forces in the coils of the insert magnet are high. FRESCA2 is pre-loaded by the so-called bladder-and-key con-cept, and a shrinking Al-alloy shell. The preload, however, de-creases the diameter of the aperture at cryogenic temperature. The stiff external tube of FM2.3-4 is specifically designed for the background field operation. This component, made of 3D printed Inconel-based alloy, has a different thermal contraction with re-spect to the inner pole of FRESCA2. As a result, the thickness of the shell of FM2.3-4 needs to be maximized to guarantee mechan-ical support for the FM2.3-4, while simultaneously enabling the integration with FRESCA2.

In the framework of the FP7 EuCARD2 project, CEA Saclay developed a cos-theta insert magnet, wound with REBCO high temperature superconductor Roebel cable. Each dipole coil consists of a single arched layer of conductors, insulated with a glass fiber sleeve and impregnated with epoxy resin for mechanical reinforcement. Tooling has been developed to carefully guide the Roebel cable during the winding phase, transfer the coil to the impregnation mold and impregnate it. The magnet is made of two coils assembled around a Nitronic’ 40 hollow core to mechanically limit its ovalization when operating in a 13 T background field. The structure is completed by the addition of an external stainless steel tube surrounding the coils. This paper details manufacturing tools and fabrication processes validated with stainless steel dummy cable and relates about the fabrication of two superconducting coils. The magnet assembly and preparation for the tests in standalone configuration in INFN-LASA test facility are foreseen for the end of this year.

This paper discusses the simulated and experimentally observed quench behavior of the first HL-LHC Twin Aperture Orbit Corrector Prototype, also known as the MCBRDp1 magnet. This superconducting magnet features two independently powered apertures. Each aperture comprises two concentric canted-cosine-theta-type Nb-Ti/Cu coils that together generate a dipolar magnetic field over the bore. These coils are held in place by conductive aluminum-alloy formers. The circuit is protected by a combination of energy extraction and quench-back in the coils. When the coils are discharged over an energy extractor, eddy currents are generated in the formers, and the resulting heat quickly and efficiently brings the Nb-Ti/Cu strands above their current sharing temperature, provided that the resistive voltage over the energy extractor is sufficiently large. This paper compares simulations and experimental observations. It is shown that with the BBQ tool, the initial voltage development after a training quench is correctly reproduced. The ProteCCT simulation tool is shown to be consistent with experimentally observed discharges of the MCBRDp1 prototype for different bath temperatures, energy extractor types, and initial operating currents. The baseline energy extractor resistor value of 1.5Ω and the non-linear varistor option both give worst-case hotspot temperatures below the 200 K hotspot temperature limit. At ultimate current, the resulting hotspot temperatures are 143 and 167 K, and the peak voltages-to-ground are 590 and 440 V, respectively.

The application of high-temperature superconductors to accelerator magnets for future particle colliders is under study. Numerical methods are crucial for an accurate evaluation of the complex dynamical behavior of the magnets, especially concerning the magnetic field quality and thermal behavior. We present a coupled A-H field formulation for the analysis of magneto-thermal transients in accelerator magnets. The magnetic field strength H accounts for the eddy current problem in the source regions containing the superconducting domains, while the magnetic vector potential A represents the magnetoquasistatic problem in the normal and non-conducting domains. Furthermore, we include a thin shell approximation for the source regions, making the formulation suitable for large scale models composed of thousands of tapes. In this work, the relevant equations are derived and discussed, with emphasis on the coupling conditions. The weak formulation is derived, and numerical results are provided in order to both, verify the formulation and scale it to the size of an accelerator magnet.