Diego Arbelaez

Superconducting magnets enable energy-frontier accelerators by generating strong magnetic fields to steer and focus the particles. Although high-temperature superconductors such as REBa 2 Cu 3 O x (, RE = rare earth) hold a strong potential for generating a higher magnetic field than Nb-Ti and Nb 3 Sn , the associated magnet and conductor technology for accelerator applications is still in its infancy. The U.S. Magnet Development Program is developing magnet technology in collaboration with industry. Here we report an experiment of making a dipole magnet called C3 using commercial high-temperature superconducting wires. The magnet, following a canted cos θ design, generated a dipole field of 5.99 T at 4.2 K in its clear aperture of 65 mm at 6.795 kA when a resistive voltage of 105 μ V appeared across one of the coils in the magnet. The stored energy was 53 kJ at the peak field. The magnet showed no degradation in the current-carrying capability at 4.2 K after the thermal cycle. We report on the detailed design, fabrication, and performance of the C3 magnet that can be of interest to potential users of this emerging technology. We also discuss issues and research needs to inform future magnet development. The experiment represented another step to addressing if the high-temperature superconducting accelerator magnet technology can increase the discovery capability of future particle accelerators.

Due to an error in setting the orientation of the conductor groove, Fig. 3 and Fig. 4 in [1] were not consistent with the description in Section II-C “the conductor wide surface is oriented close to the normal direction of the elliptic surface.” Figs. 3 and 4 in [1] should be replaced by Figs. 1 and 2, respectively.

Magnets with noncircular aperture are desired for fixed-field accelerators to accommodate horizontal orbit offset between beams of different energy. For muon colliders, an elliptic bore allows for concentration of radiation shielding on the midplane and combined-function fields in the same aperture are required in the arc magnets. In this article, we present the design of an Nb-Ti combined-function canted–cosine–theta (CCT) magnet with an elliptic aperture. We first review the basic principles to construct the CCT coil path for dipole and quadrupole fields on an elliptic surface. We conducted an optimization study considering windability and field quality, resulting in a magnet with short-sample current of $\text{12.1} \,\mathrm{k}\mathrm{A}$ at $\text{4.2} \,\mathrm{K}$, generating $\text{4.38} \,\text{T}$ dipole field and 31.6$\text{T}$$\mathrm{/}$$\mathrm{m}$ quadrupole gradient. Finally, stress analysis with full 3-D models was carried out to ensure mechanical robustness.

We present results from testing a high-temperature superconducting (HTS) magnet prototype for proton therapy. This magnet is specifically designed for a novel rotating gantry capable of delivering the entire proton beam energy range (70–225 MeV) while maintaining a fixed magnetic field in the superconducting magnets. The gantry layout simplifies the magnet design, enabling the use of straight, flat racetrack Bi-2223 (DI-BSCCO) coil technology and operation at higher temperatures (10–15 K). The magnet has a non-linear field distribution for bending and focusing the proton beams. To validate this feature, we developed a system for measuring the magnetic field distribution in the magnet aperture. We present the design of this hall probe array and experimental results from two different magnet tests at 4.2 K in a liquid helium bath. These results are compared with the simulated field distribution and discussed in the context of the required field quality for the application.

rebco coated conductors have a strong potential for high-field magnet applications. The rebco technology, however, is still in its infancy for accelerator magnet applications. As part of the U.S. Magnet Development Program, we developed a six-layer canted $\cos \theta$ dipole magnet, C3a, using corc wires developed by Advanced Conductor Technologies LLC. All the layers were wound using a semiautomated winding machine. Three layers of the magnet used corc wires containing the SuperPower “AP” rebco tapes and the remaining layers used the wires containing the “HM” tapes. At 77 K, both kinds of corc wires showed 5% to 10% degradation, after bending to a minimum bend radius of 30 or 35 mm, with respect to the self-field critical current measured before winding. At 4.2 K, the magnet reached 9.5 kA at a ramp rate of 9 A s$^{-1}$ and generated a dipole field of 1.4 T. The critical current of one layer degraded by 4% after a current transient up to 10.5 kA ramped in an averaged rate of 175 kA s$^{-1}$ or 20 T s$^{-1}$. We confirmed the HM corc wire can carry a higher current than the AP corc wire at 4.2 K. The test results of the C3a magnet showed that the fabrication and assembly procedure can be used for the upcoming full-scale C3 magnet.

Third-generation electron cyclotron resonance ion sources (ECRISs) have relied on a combination of sextupole and solenoid fields from Nb-Ti superconductors to produce the magnetic fields required for 28 GHz operation. The VENUS ion source at Lawrence Berkeley National Laboratory (LBNL) was the first third-generation ion source optimized for 28 GHz operation, and since that time a similar design has been adopted at other heavy-ion facilities such as RIKEN, GANIL and FRIB. These sources utilize a sextupole-in-solenoid design and represent the high-field limit of this design using Nb-Ti superconductor. To get past this limit without moving to a more difficult-to-use superconductor material, LBNL is developing a fourth-generation, 45 GHz ECRIS, called MARS-D, with a closed-loop coil structure. Since both the sextupole and solenoid fields are efficiently produced by the closed-looped coil, the 45 GHz ECRIS can be realized using the Nb-Ti conductor. In the MARS-D design, the maximum field on the conductor is 8.3 T at 4.2 K, and the operation point is about 90% of the short sample limit. As this tight load line margin may leave the source susceptible to quench, we investigate and compare the design parameters with the most recent VENUS-type superconducting ECRIS which is already under operation at FRIB in this paper. Additionally, we investigate the advantages of the MARS-D-type structure by keeping the coil design constant and only replacing the conductor with the one used in FRIB and VENUS.

Transient release of heat due to cracking, interfacial debonding and conductor motion can lead to premature quenching and training in high-field superconducting accelerator magnets. Understanding the physical mechanisms behind the heat release and quantifying it for various impregnation materials and mechanical stress conditions is essential for eliminating the quench training phenomenon in future magnets. We have developed a system to perform simultaneous cryogenic measurements of stress-induced acoustic emissions (AE) and local temperature variations for samples of copper wire embedded in commonly used magnet impregnation materials, and we present initial test results for the epoxy CTD-101 K. The samples, monitored simultaneously by a shear-piezo transducer and a miniature temperature sensor, were installed in a variable-temperature cryogenic probe while mechanical stress was gradually applied using an external driver. We have found coincident AE and temperature spikes corresponding to cracking/debonding and slip-stick motion. We have estimated the heat released during the debonding event and reported initial efforts towards AE/thermal energy correlation.

Lawrence Berkeley National Laboratory is pursuing stress-managed Nb₃Sn Canted–Cosine–Theta (CCT) magnet technology for high field accelerator magnets. Although promising results have been reported, improvements in the training performance are desired. This work describes the fabrication and testing campaigns of two subscale Nb₃Sn CCT magnets; a baseline impregnated with National High Magnetic Field Laboratory Mix-61 and a magnet impregnated with paraffin wax. The paraffin magnet reached the short sample limit of the conductor, to within the measurement uncertainty, with no training quenches inside the magnet. In contrast, the baseline magnet reached 80% of the short sample limit after approximately 20 quenches and exhibits some loss of memory after thermal cycles. Inter-layer flexible quench antennas combined with voltage tap data show that all quenches appear identical and originate from a region corresponding to the location of peak field in the cable. Although this success should be replicated at higher fields, these first test results demonstrate the potential for training free Nb₃Sn accelerator magnets operating near the short-sample limit.

Impregnation plays a crucial role in the performance and protection of superconducting magnets. To investigate the impregnation status during quench training, a vector network analyzer (VNA)-based time domain reflectometry (TDR) is introduced. The proposed method and analyses focus on demonstrating their applicability within canted-cosine-theta (CCT) magnets, covering both artificially induced quenches using spot heaters and naturally occurring quenches. To verify the performance of the proposed method, VNA-based TDR is applied to CCT Subscale magnets developed in the US Magnet Development Program. The findings contribute to a deeper understanding of CCT superconducting magnet behavior and inform strategies for improving their performance.

A large aperture dipole magnet for testing inserts and cables at high field is under development at LBNL. Its design targets a 15 T field in a 144 mm by 94 mm rectangular aperture, and is based on block coils with flared ends. The coils are inserted in an aluminum shell based structure and prestressed using the bladder and key technology. The quench protection relies on energy extraction. Measurements and computations on cos( $\theta$ ) magnets have shown that the temperature rise after quench events and its gradient within the windings can significantly increase the mechanical stresses in the superconducting coils. In this study, we couple STEAM-LEDET 2-dimensional (2D) quench simulations to the 2D electro-thermo-mechanical ANSYS model of the magnet, predicting the stress acting on the coils during a quench discharge after activating the energy extraction system. The model is then used to optimize the quench protection system, in terms of hot-spot temperature, peak voltage, and limiting the peak stress reached during an energy discharge below the cooldown and powering one.