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.

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} (rebco, 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 rebco magnet technology in collaboration with industry. Here we report an experiment of making a dipole magnet called C3 using commercial high-temperature superconducting corc® wires. The magnet, following a canted cosθ 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 μV 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 rebco 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.

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.

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.

High-temperature superconductor (HTS) based high-field magnet systems are essential for particle accelerators and fusion energy applications. Quench protection of such magnets is difficult owing to a slow quench propagation velocity in HTS. While in conventional NbTi and Nb3Sn-based magnets, a normal zone expands typically quickly, and the stored energy is dissipated across a large volume of the windings, a normal zone in an HTS magnet propagates slowly and, thus, can heat up quickly to high temperatures destroying the conductor. At the same time, growing experimental evidence suggests that HTS conductors can operate in a stable dissipative flux flow regime for a substantial range of operational currents before entering an irreversible thermal runaway. Therefore, a new protection paradigm for HTS magnets has emerged, aiming to prevent quenching, using advanced diagnostics to detect the dissipative regime onset. In the present paper, we propose a simple criterion for the thermal runaway in HTS conductors and calculate allowable temperature margins within which an HTS magnet can be operated safely. Outside of those temperature margins, a common quench integral approach may be used to estimate the upper boundary of the time margin for activating the protection system. We verify the applicability of our approach by comparing the calculated runaway conditions for a Bi-2223 conductor with the experimentally measured values. The thermal and time margins can define the quench protection system's requirements for implementing the quench-avoiding protection paradigm.

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.

Electronic systems capable of operating cold at deep cryogenic temperatures, sometimes close to absolute zero are in growing demand. New developments in high performance instrumentation, quantum computing, experimental physics, and spaceborne systems are increasing the need for this kind of electronics. Field Programmable Gate Arrays (FPGAs) are a good option for providing the circuitry solution to support these applications. In this work we describe requirements for the reliable operation of digital circuits and analog to digital converters utilizing commercially available deep submicron FPGAs. We will analyze and characterize devices based on 28 nm HPL process offered by TSMC and widely available on commercially available as off-the-shelf (COTS) devices. The characterization of these devices is focused mainly on power dissipation, speed performance, and non-uniform heat distribution at temperatures ranging from 4.2K to 77K.

Superconducting magnets of future fusion reactors are expected to rely on composite high-temperature superconductor (HTS) cable conductors. In presently used HTS cables, current sharing between components is limited due to poorly defined contact resistances between superconducting tapes or by design. The interplay between contact and termination resistances is the defining factor for power dissipation in these cables and ultimately defines their safe operational margins. However, the current distribution between components along the composite conductor and inside its terminations is a priori unknown, and presently, no means are available to actively tune current flow distribution in real-time to improve margins of quench protection. Also, the lack of ability to electrically probe individual components makes it impossible to identify conductor damage locations within the cable. In this work, we address both problems by introducing active current control of current distribution between components using cryogenically operated metal-oxide-semiconductor-field-effect transistors (MOSFETs). We demonstrate through simulation and experiments how real-time current controls can help to drastically reduce heat dissipation in a developing hot spot in a two-conductor model system and help identify critical current degradation of individual cable components. Prospects of other potential uses of MOSFET devices for improved voltage detection, AC loss-driven active quench protection, and remnant magnetization reduction in HTS magnets are also discussed.

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.

The US HL-LHC Accelerator Upgrade Project (AUP) is fabricating the MQXFA magnets to be used in the Q1 and Q3 Inner Triplet elements of the High Luminosity LHC (HL-LHC). This is the first production of Nb3Sn magnets for a particle accelerator, together with the MQXFB magnets for Q2a and Q2b. Here we show status and some results of MQXFA magnets fabrication and vertical test.

Quench localization is one of the most important aspects in identifying the performance limitations of high-temperature superconductor (HTS) devices and applications. In order to localize the quench and improve spatial resolution, an acoustic-based quench detection technique using shear-horizontal waves and chirplet transform is proposed in this paper. To verify the performance of the proposed method, acoustic signals are collected by the shear piezoelectric transducers mounted on the REBCO tape, and the heating points are localized via the time-frequency cross-correlation value and baseline subtraction method. This work proves heating points can be detected and localized with a resolution of better than 1%. It is expected that the proposed method can improve the quench detection and localization for accelerators and fusion power applications.

Operation of high-field superconducting magnets relies on diagnostic instrumentation for measuring strain, temperature, and magnetic field variations, detecting quenching, and identifying performance problems. Discrete sensor implementation is costly, requires multi-channel data acquisition systems, and sensor density is often insufficient to resolve problematic locations spatially. Distributed sensing is a viable alternative approach providing location-specific diagnostic information over single terminal output. In particular, it can be the key to quench protection of high-temperature superconductor (HTS)-based magnets where hot spots are known to form and persist before a quench. Fiber-optic technology offers distributed sensing solution for magnets but suffers from drawbacks such as fiber fragility, high costs of optical interrogators, and difficulty in differentiating between physical quantities such as temperature and strain. We propose an alternative, robust, and easily integrable way of implementing distributed sensing in magnets using radio-frequency (RF) technologies. RF Time Domain Reflectometry (TDR) technique has been around for nearly 60 years, and it is an essential diagnostic tool used in multiple areas of technology and applied research. We discuss operational principles and practical implementation of RF TDR sensors capable of detecting local variations of strain, temperature, and magnetic field through changes in RF impedance and wave propagation time. Results of cryogenic testing of our TDR sensors with HTS tape conductors are presented. The perspective of enabling a new diagnostics paradigm for high-energy physics and fusion energy applications based on distributed RF sensing is discussed.

By the end of October 2022, the US HL-LHC Accelerator Upgrade Project (AUP) had completed fabrication of ten MQXFA magnets and tested eight of them. The MQXFA magnets are the low-beta quadrupole magnets to be used in the Q1 and Q3 Inner Triplet elements of the High Luminosity LHC. This AUP effort is shared by BNL, Fermilab, and LBNL, with strand verification tests at NHMFL. An important step of the AUP QA plan is the testing of MQXFA magnets in a vertical cryostat at BNL. The acceptance criteria that could be tested at BNL were all met by the first four production magnets (MQXFA03-MQXFA06). Subsequently, two magnets (MQXFA07 and MQXFA08) did not meet some of the criteria and were disassembled. Lessons learned during the disassembly of MQXFA07 caused a revision to the assembly specifications that were used for MQXFA10 and subsequent magnets. In this article, we present a summary of: 1) the fabrication and test data for all the MQXFA magnets; 2) the analysis of MQXFA07/A08 test results with characterization of the limiting mechanism; 3) the outcome of the investigation, including the lessons learned during MQXFA07 disassembly; and 4) the finite element analysis correlating observations with test performance.

Distributed temperature sensing is the preferred approach for detecting and localizing normal zones in the high-field magnets of particle accelerators and fusion energy systems based on high-temperature superconductors. Optical fibers show promise in realizing this approach but suffer from known drawbacks, such as fiber fragility and cross-sensitivity to strain. Guided acoustic wave-based thermometry is a viable alternative to fiber optics; however, its application is currently limited by the leaky nature of wave propagation in acoustic waveguides. We propose the novel concept of a cladded acoustic waveguide in which, due to the elimination of the adhesion between the core and cladding, propagation of longitudinal acoustic excitations is sustained over long distances without leaking wave energy to the environment. These acoustic fibers can be structurally integrated into superconducting magnets and enable the distributed detection of local heating via thermally driven variations in the sound velocity. We present the practical design of acoustic fibers and the results of the experimental detection and localization of heat sources using our technique under ambient and cryogenic conditions. The prospects of using this technique for superconducting magnet quench diagnostics are discussed.

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.

A new quench detection and diagnostic system is required for the reliable operation of high-field superconducting magnets for fusion which is fabricated with high-temperature superconductor (HTS) cables. The conventional voltage-based diagnostic methods have a limitation in detecting local temperature changes in large-scale HTS magnets. To address the limitation, an acoustic-based diagnostic method using torsional waves applied by piezoelectric transducers is proposed. In addition, from the sensor data collected by the piezoelectric transducers, a new monitoring index that can measure the temperature change is extracted via chirplet transform (CT), which is one of the time–frequency analyses. The proposed quench detection method using torsional wave guiding is implemented in a vacuum pressure-impregnated, insulated, partial dislocation, extruded, and roll-formed (VIPER) cable, and the performance of the new index, time–frequency-based phase delay, is validated. This work proves that the propagation characteristics of torsional acoustic waves depend on the strength and location of the heat and that the change can be monitored via the proposed time–frequency-based phase delay. The preliminary development and results of a new quench detection method are discussed to validate future use in complex environments such as fusion power plants.

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.

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.