To enable the physics research that continues to deepen our understanding of the Universe, future circular colliders will require a critical and unique instrument—magnets that can generate a dipole field of 20 T and above. However, today’s maturing magnet technology for low-temperature superconductors (Nb-Ti and Nb₃Sn) can lead to a maximum dipole field of around 16 T. High-temperature superconductors such as REBCO can, in principle, generate higher dipole fields but significant challenges exist for both conductor and magnet technology. To address these challenges, several critical research needs, including direct needs on instrumentation and measurements, are identified to push for the maximum dipole fields a REBCO accelerator magnet can generate. We discuss the research needs by reviewing the current results and outlining the perspectives for future technology development, followed by a brief update on the status of the technology development at Lawrence Berkeley National Laboratory. We present a roadmap for the next decade to develop 20 T-class REBCO accelerator magnets as an enabling instrument for future energy-frontier accelerator complex.

Detecting local heat-dissipating zones in high-temperature super-conductor (HTS) magnets is a challenging task due to slow propagation of such zones in HTS conductors. For long conductor lengths voltage-based methods may not provide a sufficient sensitivity or redundancy, and therefore non-voltage based detection alternatives are being sought. One of those is the recently pro-posed method of Eigen Frequency Thermometry (EFT), which is an active acoustic technique for a fast and non-intrusive detection of “hot spots”, utilizing temperature dependence of the conductor elastic moduli. In the present work, we demonstrate efficiency of EFT for detecting localized heating in a 1.2 m-long sample of REBCO tape immersed in liquid nitrogen, and benchmark sensitivity of the acoustic detection with respect to voltage, hot spot temperature, and power dissipation in the conductor. Modifying the original technique for differential mode of operation enables a much improved sensitivity, and adds a hot spot localization capability. Furthermore, we adapt this technique to subscale coils wound with REBCO CORC® conductor built in the framework of US Magnet Development Program. A successful thermal-based detection of dissipation onset at the critical current for a two-layer canted CORC® dipole assembly is 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.

After the fabrication and test of HD1, a 16 T Nb ₃ Sn dipole magnet based on flat racetrack coil configuration, the Superconducting Magnet Program at Lawrence Berkeley National Laboratory (LBNL) is developing the Nb ₃ Sn dipole HD2. With a dipole field above 15 T, a 35 mm clear bore, and nominal field harmonics within a fraction of one unit, HD2 represents a further step towards the application of block-type coils to high-field accelerator magnets. The design features tilted racetrack-type ends, to avoid obstructing the beam path, and a 4 mm thick stainless steel tube, to support the coil during the pre-loading operation. The mechanical structure, similar to the one used for HD1, is based on an external aluminum shell pre-tensioned with pressurized bladders. Axial rods and stainless steel plates provide longitudinal support to the coil ends during magnet excitation. A 3D finite element analysis has been performed to evaluate stresses and deformations from assembly to excitation, with particular emphasis on conductor displacements due to Lorentz forces. Numerical results are presented and discussed

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 $T_c$ of 9.2 K) and ${\mathrm{Nb}}_3\mathrm{Sn}$ ($T_c$ of 18.4 K) conductors due to their very high critical field $B_{c2}$ 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 ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_x$ (Bi-2212 with $T_c$ of $\sim 82\mathrm{K}$) 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 ${\mathrm{Nb}}_3\mathrm{Sn}$ 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 $0.54\mathrm{T}/\mathrm{s}$ without quenches. This work validates the canted-cosine-theta Bi-2212 dipole magnet design, illustrates the fabrication scheme, and establishes an initial performance benchmark.

Superconducting magnet technology is continually evolving in order to meet the demanding needs of new accelerators and to provide necessary upgrades for existing machines. A variety of designs are now under development, including high fields and gradients, rapid cycling and novel coil configurations. This paper presents a summary of R&D programs in the EU, Japan and the USA. A performance comparison between NbTi and Nb/sub 3/Sn along with fabrication and cost issues are also discussed.