Jean-Francois Croteau

REBCO coated conductors are of significant interest for high-field superconducting applications owing to their exceptional critical current retention under high magnetic fields. However, depending on the fabrication route, microstructural inhomogeneities such as porosity, Cu x O precipitates, and a -axis oriented grains can emerge at various length scales, disrupting current transport and limiting the critical current density. This study investigates and characterizes such defects in commercial REBCO conductors of varying specifications. Top-view scanning electron microscope (SEM) images of the REBCO layer were acquired following chemical etching of Cu and Ag layers to expose the microstructure for analysis. Conventional image analysis and segmentation techniques prove insufficient for reliably quantifying these defects, while manual identification remains prohibitively labor-intensive. To overcome these limitations, a machine learning approach is explored to enable rapid, automated, and accurate defect detection. Specifically, an open-source computer vision model, Mask region-based convolutional neural network, is fine-tuned on domain-specific SEM image data. The fine-tuned model achieved a validation mean average precision of 45.23% and enabled quantitative defect analysis across 63 tapes. Partial correlation analysis, controlling for confounding between defect types, revealed independent associations with critical current density that varied in strength and sign across defect types and measurement conditions. These findings motivate further targeted characterization to establish the microstructural origins of these relationships and inform conductor optimization.

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.

Niobium monocristallin et polycristallin et cuivre OFE pour les cavités SRF : caractérisation mécanique de basse à haute vitesse de déformation et investigation de la microstructure: La fabrication de cavités SRF à hautes performances est essentielle pour augmenter l’énergie de collision dans de nouveaux accélérateurs de particules. L’utilisation de procédés de fabrication à haute vitesse, comme l’électro-hydro formage, peut être bénéfique, mais requiert une compréhension détaillée des propriétés mécaniques des matériaux déformés à haute vitesse et de l’impact sur leurs microstructures. Les objectifs de cette thèse sont d’étudier les propriétés mécaniques de monocristaux de niobium et de tôles polycristallines de niobium à haute pureté et de cuivre OFE déformés à des taux de déformation d’environ 10−4 s−1 à 103 s−1. Les résultats de cette étude sont séparés en deux parties selon le matériau étudié. En partie I, la caractérisation de monocristaux de niobium se concentre sur les propriétés mécaniques en traction et en compression à des taux de déformation d’environ 10−4 s−1 à 103 s−1 et sur la microstructure (MEB, EBSD, MET et nanoindentation) d’éprouvettes déformées. Les effets de l’orientation des cristaux, la vitesse de déformation et de la direction de chargement sur les propriétés mécaniques, la rotation des cristaux et la structure de dislocations sont présentés. En partie II, la formabilité de tôles polycristallines de niobium et de cuivre OFE sont présentées à l’aide de courbes limites de formage (CLF) obtenues à un taux de déformation quasi-statique. Les CLFs de ces matériaux offrent des données importantes pour les techniques de formage conventionnel, tels que l’emboutissage et le repoussage. Cette seconde partie présente aussi les propriétés mécaniques d’éprouvettes de cuivre OFE recuites et de tôles de niobium polycristallines soudées par faisceau d’électron et déformées en traction et en compression à des taux de déformation de 10−3 à 103 s−1.

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.

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.

Bi-2212 Rutherford cables have been fabricated into flat racetrack coils and canted-cosine-theta dipole magnets. The performance gap between the magnets made with Rutherford cables and the “short-sample-limit” is about 30%. To better understand the influence of Rutherford cable processing on the strand performance, we studied three Bi-2212 wires with filament architectures of 37 × 18 and 55 × 18 and diameters of 0.8 and 1.0 mm. To simulate the deformation caused by cabling process, the three wires were rolled with thickness reductions ranging from 10% to 30%. The aspect ratios of rolled strands are between 1.29 and 2.05. The low aspect-ratio wire is also an interesting form for fabricating solenoid coils with higher packing density. The round and rolled strands were heat-treated under 50 bar and with maximum heat treatment temperatures of 885.5 °C and 890.5 °C. The rolling deformation reduced filament size uniformity, resulting in filament merging in fully heat-treated wires. It was found that rolling reduction reduced wire critical current density (J_E) by 16 to 18%, but the J_E decrease saturated at 15 to 20% of the thickness reduction. It is believed that the reduced J_E results from the filament merging caused by rolling and non-uniform shrinking during overpressure heat treatment.

An investigation of instance segmentation of cracks in Nb₃Sn 4-stack 40-strand Rutherford cables using machine learning is presented. Three samples were uniaxially and biaxially loaded before metallographic inspections were performed. The Mask R-CNN model was used in the Detectron2 framework with pre-trained weights but fine-tuned to detect and segment cracks. The model detected cracks with bounding box and mask average precisions (AP) of 42.8 and 27.9, respectively, and was used for instance segmentation of all cracks in the three samples. More cracks were found in the sample pre-loaded along the z-axis (i.e., along the cable length). Pre-loading along the x-axis (i.e., on the cables’ edges) reduced the number of cracks and changed the crack orientation distribution, away from being highly aligned with the y-axis (i.e., normal to the cables’ broad faces), i.e., the direction with the highest applied load. Fine-tuning of the Segment Anything Model (SAM) was also studied but performed poorly without human-provided prompts. However, the zero-shot capability of SAM showed high promises to accelerate the image annotation process for applications beyond this study.