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.

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.

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 (MDP), 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 semi-automated 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.

The US Physics community completed the Snowmass planning process in 2022, culminating in the HEPAP Particle Physics Project Prioritization Panel (P5) publishing its summary report at the end of 2023. Building on this, the US Magnet Development Program, a national accelerator magnet R D program established by DOE-OHEP in 2016, has updated its strategic plan to align with the 2023 P5 report, resulting in this roadmap document.

Stress managed magnet designs allow to limit the strain and stresses applied to the conductor during assembly and operation. In canted cos(θ) (CCT) designs, the conductor is wound around a mandrel: the impregnation process creates a bonding between the two, that can fail during magnet powering. The energy releases due to debonding are considered a potential cause of training quenches. In this study, we investigate these events modeling the mandrel-conductor interfaces by means of cohesive zone material models. The material properties were calibrated by means of measurements performed on representative interfaces, and the models were validated comparing the results with strain gauge measurements. A thermal model was used to compute the local temperature increase in the strands as a function of the energy released by debonding and frictional sliding across the newly formed interfaces. The result was then used to define a quench condition for the model, allowing to simulate the full training process of the CCT magnet. The obtained training curve is in reasonable agreement with the experimental results.

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.

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.

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.

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.

Within the U.S. Magnet Development Program, LBNL is planning to fabricate and test hybrid magnets combining Nb3Sn and Bi-2212 canted-cosine theta (CCT) magnets. The first planned hybrid magnet test integrates a 1 m long, 90 mm bore Nb3Sn dipole magnet (CCT5), and a 39 cm long, 30.8 mm bore Bi-2212 dipole magnet (BIN5c), which are already fabricated and were tested individually. The second planned hybrid magnet test integrates a 1.5 m long, 120 mm bore Nb3Sn dipole magnet (CCT6), still under design, and a 1 m long, 40 mm bore Bi-2212 dipole magnet (BiCCT1), which is under fabrication. This work gives an update of the status of the Bi-2212 CCT magnet program, and focuses on the first hybrid magnet (CCT5/BIN5c). The electromagnetic and mechanical analysis of the CCT5/BIN5c hybrid magnet is presented, as well as the proposed mechanical assembly process. The quench protection strategy associated with the hybrid magnet test is also presented.

A test facility dipole is being developed at LBNL, targeting a 16 T field in a 144 mm wide aperture. The magnet uses a block design, with two double-pancake coils. In order to minimize motion under the large Lorentz forces, the coils are preloaded against a thick aluminum shell and iron yoke using bladder and key technology. It is then crucial to verify that the performance of the magnet is not degraded due to strain induced on the Nb3Sn conductor during assembly, cool-down and powering. The critical current of extracted strands was measured in a varying background magnetic field and as a function of the applied longitudinal strain. Finite element analysis was used to extract the strain state inside the superconducting strands during magnet assembly and operation. This strain was then compared to the measurements to evaluate potential reversible and irreversible effects on the magnet performances. The results suggest that the magnet can reach 16 T with sufficient margin, with no irreversible degradation in the high field region.

High-temperature superconducting REBa $_{2}$ Cu $_{3}$ O $_{7-x}$ ( rebco ) conductors have the potential to generate a high magnetic field over a broad temperature range. The corresponding accelerator magnet technology, still in its infancy, can be attractive for future energy-frontier particle colliders such as a multi-TeV muon collider. To help develop the technology, we explore the requirements and potential characteristics of a rebco magnet, operating at 4.2 or 20 K, with a dipole field of 8–10 T in a clear aperture of 150 mm. We use the canted $\cos \theta$ magnet configuration to reduce the electromagnetic stresses on the conductors. We present the resulting dipole fields, field gradients for combined-function cases, conductor stresses, magnet dimensions and conductor lengths. We also discuss the conductor performance that is required to achieve the target dipole field at 4.2 and 20 K. The information can provide useful input to the development of rebco magnet and conductor technology for collider-ring magnets in a muon collider.

As part of the US Magnet Development Program, Lawrence Berkeley National Laboratory (LBNL) is working on the development of high field stress-managed Nb$_{3}$Sn dipole magnets using canted-cosine-theta (CCT) technology. As part of this program, a series of two layer magnets, CCT3/4/5, with short sample bore field of approximately 10 T and a 90 mm diameter open aperture have been designed, fabricated, and tested. The first magnet in the series, CCT3, was limited to less than 70% of the short sample current, this limitation is believed to be due to conductor damage. The second magnet in the series, CCT4, reached 86% of the short sample current, after changes to the groove geometry were made to accommodate dimensional changes in the cable during heat treatment. The third and final magnet of this series, CCT5, reached 88% of the short sample current with improved training relative to CCT4, after changes were made to the impregnation and assembly methods. While this two layer series was used to successfully demonstrate Nb$_{3}$Sn CCT magnet technology, improvements in the training behavior of these magnets is desirable. For this purpose, a subscale program has been devised in order to probe the causes and explore reductions / improvements to training in stress-managed magnet technology. The subscale nature of the magnets allows for faster turnaround in the fabrication and testing process. In this paper, we present the design and test results for the first (baseline) subscale Nb$_{3}$Sn CCT magnet and demonstrate that the subscale platform and the larger two-layer magnets produce similar training results.

The most effective way to achieve very high collision energies in a circular particle accelerator is to maximize the field strength of the main bending dipoles. In dipole magnets using Nb-Ti superconductor the practical field limit is considered to be 8-9 T. When Nb3Sn superconductor material is utilized, a field level of 15-16 T can be achieved. To further push the magnetic field beyond the Nb3Sn limits, High Temperature Superconductors (HTS) need to be considered in the magnet design. The most promising HTS materials for particle accelerator magnets are Bi2212 and REBCO. However, their outstanding performance comes with a significantly higher cost. Therefore, an economically viable option towards 20 T dipole magnets could consist in an hybrid solution, where both HTS and Nb3Sn materials are used. We discuss in this paper preliminary conceptual designs of various 20 T hybrid magnet concepts. After the definition of the overall design criteria, the coil dimensions and parameters are investigated with finite element models based on simple sector coils. Preliminary 2D cross-section computation results are then presented and three main layouts compared: cos-theta, block, and common-coil. Both traditional designs and more advanced stress-management options are considered.

The Nb$_{3}$Sn based test facility dipole magnet (TFD), with a rectangular aperture of 100 × 150 mm, and an operation target bore field of 15 T at 1.9 K, is designed to provide a background dipole field for cables and inserts. The design of the magnet is based on four double-layer coils and an aluminum shell-based structure, using key-and-bladder technology, with axial pre-load. The status of the magnet design, and optimization analyses, are here presented. The results of the initial prototyping, initial winding tests, characterization of cable hard-way bend curvature, and layer jump prototype test are also discussed.

As part of the US Magnet Development Program, Lawrence Berkeley National Laboratory (LBNL) is working on the development of high field stress-managed Nb$_{3}$Sn dipole magnets using canted-cosine-theta (CCT) technology. As part of this program, a series of two layer magnets, CCT3/4/5, with short sample bore field of approximately 10 T and a 90 mm diameter open aperture have been designed, fabricated, and tested. The first magnet in the series, CCT3, was limited to less than 70% of the short sample current, this limitation is believed to be due to conductor damage. The second magnet in the series, CCT4, reached 86% of the short sample current, after changes to the groove geometry were made to accommodate dimensional changes in the cable during heat treatment. The third and final magnet of this series, CCT5, reached 88% of the short sample current with improved training relative to CCT4, after changes were made to the impregnation and assembly methods. While this two layer series was used to successfully demonstrate Nb$_{3}$Sn CCT magnet technology, improvements in the training behavior of these magnets is desirable. For this purpose, a subscale program has been devised in order to probe the causes and explore reductions / improvements to training in stress-managed magnet technology. The subscale nature of the magnets allows for faster turnaround in the fabrication and testing process. In this paper, we present the design and test results for the first (baseline) subscale Nb$_{3}$Sn CCT magnet and demonstrate that the subscale platform and the larger two-layer magnets produce similar training results.

The use of high-field superconducting magnets has furthered the development of medical diagnosis, fusion research, accelerators, and particle physics. High-temperature superconductors enable magnets more powerful than those possible with Nb-Ti (superconducting transition temperature Tc of 9.2 K) and Nb3Sn (Tc of 18.4 K) conductors due to their very high critical field Bc2 of greater than 100 T near 4.2 K. However, the development of high-field accelerator magnets using high-temperature superconductors is still at its early stage. We report the construction of the world's first high-temperature superconducting Bi2Sr2CaCu2Ox (Bi-2212 with Tc of similar to 82 K) accelerator dipole magnet. The magnet is based on a canted-cosine-theta design with Bi-2212 Rutherford cables. A high critical current was achieved by an overpressure processing heat treatment. The magnet was constructed from a nine-strand Rutherford cable made from industrial 0.8 mm wires. At 4.2 K, it reached a quench current of 3600 A and a dipole field of 1.64 T in a bore of 31 mm. The magnet did not exhibit the undesirable quench training common in Nb-Ti and Nb3Sn accelerator magnets. It quenched a dozen times without degradation. The magnet exhibited low magnetic field hysteresis (<0.1%) as measured by a cryogenic Hall sensor. It was fast cycled to 1.47 T at 0.54 T/s without quenches. This work validates the canted-cosine-theta Bi-2212 dipole magnet design, illustrates the fabrication scheme, and establishes an initial performance benchmark.