Jose Luis Rudeiros Fernandez

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.

Detection and localization of quench events are essential to protect superconducting magnets for particle accelerators. Voltage taps are widely adopted for low-temperature superconductors (LTS), but they do not ensure reliable quench detection for high temperature superconducting (HTS) magnets. The propagation velocity of the normal conducting zone is in the order of m/s for LTS, compared to cm/s for HTS, leading to a higher risk of irreversible conductor degradation before any voltage can be detected. As an alternative solution, non-leaky ultrasonic waveguides have been proposed as a diagnostic option to monitor hot-spots by tracking thermally induced sound velocity variations. A first practical implementation of this concept has been developed and tested for a Uni-layer winding prototype at Lawrence Berkeley National Laboratory (LBNL). In this work, we present the practical design of a non-leaky acoustic waveguide tailored for this magnet. We also show the results of an experimental campaign conducted to assess hot-spot detection and localization for the ultrasonic waveguide sensor, both in a straight and a configuration representing the magnet real geometry. Finally, the application of this technique is presented over a Uni-Layer winding prototype copper mock-up. The results of the tests at room temperature and liquid nitrogen temperature are discussed.

High-temperature REBa$_{2}$Cu$_{3}$O$_{7-x}$ (REBCO) superconductors are a route to increase the field of accelerator magnets beyond the 15-16T practical limit of Nb$_{3}$Sn. REBCO cabled in a CORC geometry is a promising fit for this application, enabling the design of low inductance magnets with conductor transposition. However, the use of CORC in coils with the tight conductor bending radii typical of accelerator magnets remains a key challenge, with on-going research both on the conductor development and coil design fronts seeking to address this issue. In this work we explore the tradeoffs between circular and elliptic aperture canted-cosine-theta (CCT) coils when the conductor minimum bending radius is a key consideration, concluding that aperture ellipticity is an important free parameter in the optimization of magnetically efficient CORC dipole magnets. In particular, we show that elliptic bore designs enable a regime of smaller aperture HTS coils, an important result for LTS-HTS hybrid magnets and more generally for accelerator applications benefiting from non-circular magnet apertures. To this end we present an elliptic aperture dipole design optimized for hybrid testing within the US Magnet Development Program (US-MDP) and share 77 K test results of a prototype coil which provides first experimental confirmation of the advantages.

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.

High-temperature superconductors, such as REBa₂Cu₃O_7−x (REBCO, RE = rare earth), are becoming pivotal for high-field magnet technology for future circular colliders and compact fusion reactors. The U.S. Magnet Development Program, in collaboration with industry, is developing REBCO magnet technology using round conductors consisting of multiple REBCO tapes. For these multi-tape cables, traditional instrumentation, such as voltage taps and resistive strain gauges, become insufficient to help measure and understand the performance-limiting factors in these model magnets. Distributed fiber-optic sensing (DFOS) is a potential solution to address this challenge. Although DFOS is well established for various applications, measuring temperature and strain in high-temperature superconducting magnets is in its infancy. Here we report the detailed implementation and test results of DFOS based on Rayleigh scattering in a subscale canted cosθ (CCT) dipole magnet using high-temperature superconducting CORC^® wires. We co-wound optical fibers in each layer of the CCT magnet and compared different types of commercial fibers and mold-release agents to reduce the power attenuation in the fibers. The DFOS allowed us to measure mechanical deformation and temperature along the conductor during tests at 77 and 4.2 K. The measured strain agreed quantitively with a finite-element mechanical model of the subscale magnet. Our results indicate that DFOS can effectively identify locations of strain and temperature changes, offering unique insight into magnet performance that can advance our understanding and development of the REBCO magnet technology for high-energy physics and fusion applications.

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.

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.

The nature of the resin used in the impregnation of superconducting magnets plays a crucial role on the magnet's performance. The interfacial region between magnet constituents has been postulated as the potential region for training origin. Its characterization and analysis are therefore crucial to understand the phenomena involved in magnet training. In this paper, we focus on the development of an experimental method for the characterization of the interfacial bond strength between individual constituents normally found in superconducting magnets. The sample preparation developed for this method as well as the required hardware for testing are presented and discussed. The initial studies focus on a copper strand and different resins normally used in superconducting magnets, such as CTD-101 K, NHMFL Mix61, paraffin wax and Stycast 2850. The initial results are reported and discussed for experiments at room temperature and 77 K, analyzed based on a shear stress failure criteria, and demonstrating the low energy dissipation of the paraffin wax system that might explain the performance improvement of superconducting magnets based on this resin in terms of training.