The Current Status and Future Direction of High-Magnetic-Field Science and Technology in the United States (2024)
Chapter: 5 Accelerator Magnets
5
Accelerator Magnets
INTRODUCTION
The U.S. national laboratories and universities, along with industrial partners, have a long history of leadership and have made critical contributions to the development of accelerator magnets for high-energy physics (HEP) and other accelerator facilities. Nb-Ti superconducting magnets, benefiting from investments by the Department of Energy Office of High Energy Physics (DOE-OHEP), have been the workhorse for colliders, for example: the Tevatron, Hadron–Electron Ring Accelerator (HERA), the Relativistic Heavy Ion Collider, and the Large Hadron Collider (LHC) facilities. For reference, the LHC Nb-Ti dipole magnets operate up to a field of 8.3 Tesla (T) in super-fluid helium at 1.9 K. The investment from DOE-OHEP in the development Nb-Ti conductors for accelerator magnets has had wide reaching influence in high-field magnet science in general,1 as it led to significant gains in conductor performance that have been leveraged extensively. Nb-Ti is a ductile superconductor, which allows for simpler coil and magnet fabrication methods. However, the maximum field of a Nb-Ti accelerator dipole is limited to less than approximately 9 T, owing to critical field limitations of the superconductor. To overcome these limitations, brittle superconductors such as the low temperature superconductor (LTS), Nb3Sn, or high-temperature superconductors (HTS),
___________________
1 P.J. Lee and B. Strauss, 2011, 100 Years of Superconductivity, H. Rogalla, and P.H. Kes, eds., Boca Raton, FL: CRC Press, https://fs.magnet.fsu.edu/~lee/superconductor-history_files/Centennial_Supplemental/11_2_Nb-Ti_from_beginnings_to_perfection-fullreferences.pdf.
such as rare-earth barium copper oxide (REBCO) or cuprate superconductor Bi2Sr2CaCu2Oy (BSCCO) are used. However, magnets using these brittle superconductors require more complicated design and fabrication methods.
When new superconducting materials reach sufficient maturity, they are introduced into magnet technologies. Initial phases of research and development (R&D) are focused on developing fabrication methods and understanding basic behavior and limitations of new superconducting technologies. Fabrication methods can pose many challenges because the conductors tend to be brittle, and depending on the technology, may require high temperature and sometimes high-pressure reactions to form the superconductor. Basic R&D programs have been essential in developing the methods and tools required to introduce new superconducting materials into magnet technology. These programs are associated with fabrication of model magnets whose tests lead to early understanding of issues and challenges that need to be overcome for the application of new technologies in real accelerators. Beyond the advances in a basic R&D phase, technology-readiness or directed R&D programs are required to further advance a magnet’s technology. These programs will deal with technical issues in the magnet’s performance and reliability as well as the scale-up (typically in length and quantity) and the preparation for a production run. In cases where tens of magnets are needed, this production can be maintained within the national laboratory environment. Large production would typically require technology transfer to industry for full industrialization of the process.
As described above, the development of these magnet technologies has a long arc. It can take decades in going from the early development of conductors to the final implementation of a magnet in a facility. Over the past several decades, there has been a substantial focus on developing Nb3Sn magnet technology for accelerators. Nb3Sn is a brittle superconductor, which requires special design considerations to avoid damaging the conductor during magnet fabrication and operation. Furthermore, owing to its brittle nature, this superconductor typically requires a wind-and-react process. In this process, the coil winding is done with the wire in a pre-reacted ductile state, and the coil is then reacted at a temperature typically above 650°C in an inert atmosphere to form the superconductor. Today, the DOE Accelerator Upgrade Program (AUP) is well under way in the fabrication of Nb3Sn interaction region quadrupoles for the High Luminosity Upgrade of the LHC2 (HL-LHC). This is a prime example of a successful development path, including basic and directed R&D programs in preparation for a large project. These magnets have a coil aperture of 150 mm, a length of 4.2 m, and a peak field on the superconductor of 11.4 T. The project is executed by a collaboration of DOE National Laboratories: Fermilab (FNAL), Brookhaven National Laboratory (BNL), and Lawrence
___________________
2 O. Brüning and L. Rossi, 2020, “High-Luminosity Large Hadron Collider,” CERN Yellow Reports: Monographs 10:1.
Berkeley National Laboratory (LBNL). This effort required strong DOE-OHEP led investments in basic magnet R&D, conductor development, and technology-readiness R&D toward the final implementation of the technology. In the 1990s, nondirected R&D efforts led to the development of 10–13 T laboratory dipoles. In 1998, a DOE-HEP conductor development program investment began with an objective to improve the current density of the conductor. In 2003, a 16 T (LBNL HD1) laboratory dipole (without an open bore) was demonstrated, which was a significant accomplishment that took advantage of the conductor advancements. To move the technology from a laboratory test environment toward operation in real facilities, directed R&D efforts are required. A directed R&D program aimed at quadrupole focusing magnets for HL-LHC was established in 2004 and ran until 2019. Figure 5-1 (top) shows the magnet development path for the large aperture quadrupoles as part of the LHC accelerator research program (LARP) program. The completion of the LARP program led directly to the start of the Accelerator Upgrade Program (AUP) project that is projected to end in 2028. Figure 5-1 (bottom) shows the magnet assembly area, showing the coils and magnet structure, for the AUP Nb3Sn IR quadrupole magnets that are currently being delivered to the HL-LHC project. It is notable that the projected end date of the project is 30 years after the start of the DOE-HEP focused investment on conductor development and 24 years after the start of the directed R&D phase.
Finding: The development time for accelerator magnet technologies can be several decades long and requires basic R&D, conductor development investment, and directed (technology-readiness) R&D efforts. The recent experience with the HL-LHC interaction region quadrupoles, which are currently under production, is a prime example of this process. The several decades-long process can be accelerated by 30–40 percent, but not more, if appropriate funding is provided.
As mentioned above, Nb3Sn accelerator magnets have been under development over the last few decades and are now, in the case of the HL-LHC, sufficiently mature to be integrated into a facility. Another successful example of Nb3Sn development involves their application in DOE Office of Basic Energy Science (BES) light sources. A Nb3Sn undulator was recently installed and operated at the Advanced Photon Source (APS) in the Argonne National Laboratory (ANL). This success also highlights the synergy between different accelerator applications and the benefits from the investment by DOE-OHEP in Nb3Sn technology.
For typical high-field accelerator magnets (i.e., dipoles and quadrupoles), Nb3Sn accelerator magnets have a strong case for applicability in the range of 8–16 T, which is approximately beyond the limits of Nb-Ti on the low end and at the limits of the Nb3Sn superconducting properties on the high end. For magnets
SOURCE: Courtesy of S. Prestemon, Lawrence Berkeley National Laboratory, presentation to the committee, August 17, 2023.
producing fields higher than 16 T, the use of HTS will be essential owing to their high-upper-critical fields. Their use at lower fields than 16 T may also be desirable as the technology advances and under the assumption that the cost will decrease in the future. Their possible operation at higher temperatures is also a key advantage. Two types of HTS are primarily being pursued in the United States at this time for accelerator magnets, namely REBCO and Bi-2212 conductors. Recent advances in the current carrying capacity of these conductors can be leveraged for accelerator magnet applications. However, these conductors pose some unique challenges. For example, REBCO has a clear advantage in that it does not require a reaction step. However, there are several challenges associated with the implementation of these conductors in accelerator magnets:
- The conductor is produced in the form of a tape which exhibits anisotropy in the superconducting, mechanical, and thermal properties.
- High-current cables are desirable for accelerator magnets; for REBCO these can be produced as stacks of tapes or by wrapping tapes around a core (e.g., Conductor on Round Core [CORC]3 conductors developed by Advanced Conductor Technologies LLC and Symmetric Tape Round [STAR]4 wires developed by AMPeers LLC).
- The bending radius of the tapes and cables is limited, owing to the brittle nature of the conductor.
- There are challenges for quench detection, owing to the slow normal zone propagation.
- The tape geometry of the conductor is susceptible to magnetization and screening current effects.
Bi-2212 conductors have a key advantage that they come in the form of multifilamentary wire forms that can be used to produce Rutherford cables. Some of the challenges associated with Bi-2212 conductors for accelerator magnets are:
- The material has relatively low strength which is driven by the Ag matrix and brittle superconductor that make up the composite wire.
- A wind-and-react approach is typically required with a high temperature reaction at ~900°C in a partial O2 environment, leading to challenges related
___________________
3 D.C. Van Der Laan, J.D. Weiss, U.P. Trociewitz, et al., 2020, “A CORC® Cable Insert Solenoid: The First High-Temperature Superconducting Insert Magnet Tested at Currents Exceeding 4 kA in 14 T Background Magnetic Field,” Superconductor Science and Technology 33(5):05LT03.
4 S. Kar, W. Luo, A.B. Yahia, X. Li, G. Majkic, and V. Selvamanickam, 2018, “Symmetric Tape Round REBCO Wire with Je (4.2 K, 15 T) Beyond 450 A mm−2 at 15 mm Bend Radius: A Viable Candidate for Future Compact Accelerator Magnet Applications,” Superconductor Science and Technology 31(4):04LT01.
- to both the high temperature response of materials and the chemical compatibility with the cable insulation.
- Reaction needs to be performed at high pressure (~50 bar) to obtain the high current density.
Finding: Nb3Sn accelerator magnet technology, with a range of interest of approximately 8–16 T, is sufficiently mature to be implemented in project environments requiring tens of magnets. Further investment and development are needed for full-scale deployment of Nb3Sn magnets in an accelerator requiring hundreds or thousands of magnets. HTS materials (namely REBCO and BSCCO) have advanced sufficiently to be introduced into accelerator magnet R&D and are essential for technology beyond 16 T and/or operating at higher temperatures.
CURRENT RESEARCH AND DEVELOPMENT STATUS AND EFFORTS
The Particle Physics Project Prioritization Panel (P5) is an advisory group tasked with producing recommendations for U.S. prioritization of projects and initiatives in particle physics. This prioritization process, starting with the “Snowmass” community input and followed by the P5 outcome, occurs approximately every ten years. The 2014 P5 report and subsequent High Energy Physics Advisory Panel (HEPAP) Accelerator R&D subpanel report identified a critical need for high-field magnet R&D focused on substantially increasing performance and decreasing cost per T-m5. In response to these recommendations, the DOE-OHEP initiated the U.S. Magnet Development Program (US-MDP) in 2016 to aggressively pursue development of accelerator magnets operating near the fundamental limits of the superconducting materials.6 The US-MDP is a collaboration between LBNL, FNAL, BNL, and NHMFL Applied Superconductivity Center (ASC), with a major objective to integrate magnet research across the partner labs and universities. The vision of the US-MDP is stated as7:
- Maintain and strengthen U.S. leadership in high-field accelerator magnet technology for future colliders.
___________________
5 Department of Energy, Office of Science, 2015, Accelerating Discovery: A Strategic Plan for Accelerator R&D in the U.S., Report of the Accelerator Research and Development, Washington, DC, https://science.osti.gov/-/media/hep/hepap/pdf/Reports/Accelerator_RD_Subpanel_Report.pdf.
6 S.A. Gourlay, S.O. Prestemon, A.V. Zlobin, L. Cooley, and D. Larbalestier, 2016, The U.S. Magnet Development Program Plan, Washington, DC: Department of Energy High Energy Physics, https://www2.lbl.gov/LBL-Programs/atap/MagnetDevelopmentProgramPlan.pdf.
7 Presentation by Ken Marken on July 21, 2023, slide 14.
- Further develop and integrate magnet research teams across the partner laboratories and U.S. universities for maximum value and effectiveness to US-MDP.
- Identify and nurture cross-cutting and synergistic activities with other programs to more rapidly advance progress toward our goals.
The US-MDP is composed of four major areas: Nb3Sn Magnets, HTS Magnets, Technology Development, Conductor Procurement, and R&D. Aligned with these areas are the following goals, as defined in DOE strategic plan:6
- Explore the performance limits of Nb3Sn accelerator magnets with a focus on minimizing the required operating margin and significantly reducing or eliminating training.
- Develop and demonstrate an HTS accelerator magnet with a self-field of 5 T or greater compatible with operation in a hybrid LTS/HTS magnet for fields beyond 16 T.
- Investigate fundamental aspects of magnet design, technology, and performance that could lead to substantial performance improvements and reduction of magnet cost.
- Pursue Nb3Sn and HTS conductor R&D with clear targets to increase performance and reduce the cost of accelerator magnets.
The research tasks and detailed goals of the different areas of US-MDP are adjusted in approximately 5-year intervals through an updated roadmap process. The most recent roadmaps were updated in 2020.8 In the next paragraphs, the current status of the US-MDP areas is summarized.
Figure 5-2 shows the historical progression of Nb3Sn dipole magnets. The progress from the early 1980s to the early 2000s closely tracks with increases Nb3Sn critical current density. The development effort in the 2000s to the mid-2010s was substantially directed toward technology-readiness through the LARP program. The maximum bore field reached today for accelerator type dipole magnets with an open bore is approximately 14.5 T (represented as solid markers in Figure 5-2), demonstrated with the FRESCA2 magnet at CERN and more recently with the MDPCT19 accelerator type dipole magnet in the United States (as described below). For demonstrator magnets without an open bore (represented by open markers) record fields of approximately 16 T have been demonstrated, first in the United
___________________
8 S. Prestemon, K. Amm, L. Cooley, et al., 2020, “The 2020 Updated Roadmaps for the US Magnet Development Program,” U.S. Magnet Development Program, https://arxiv.org/ftp/arxiv/papers/2011/2011.09539.pdf.
SOURCE: L. Bottura, S. Prestemon, L. Rossi, and A.V. Zlobin, 2022, “Superconducting Magnets and Technologies for Future Colliders,” Frontiers in Physics 10:935196, https://doi.org/10.3389/fphy.2022.935196. Copyright © 2022 Bottura, Prestemon, Rossi, and Zlobin. CC BY.
States in the early 2000s and recently at the European Organization for Nuclear Research (CERN).
The Nb3Sn accelerator magnet area of the US-MDP focuses on exploring the performance limits, minimizing the required operating margin, and reducing or eliminating training. In 2020, a demonstrator magnet, MDPCT1—designed and built at Fermilab as part of the US-MDP—achieved a record dipole field of 14.5 T for an accelerator magnet.9 To mitigate conductor stresses at higher fields, the primary focus in this area is on the development of stress-managed concepts. Two different methods of stress-management are being pursued in parallel: stress-managed cosine theta (SMCT) and canted-cosine-theta (CCT) approaches. Within this area, studies are also ongoing on training reduction and reduction in required operating margin by combining stress-managed concepts with novel coil impregnation mediums. A primary focus of this area is also on the development of large-aperture outsert structure for hybrid dipole operation (LTS ousters with HTS insert). In
___________________
9 A.V. Zlobin, I. Novitski, E. Barzi, et al., 2020, “Development and First Test of the 15 T Nb 3 Sn Dipole Demonstrator MDPCT1,” IEEE Transactions on Applied Superconductivity 30(4):1–5.
the short-term, this is the most efficient means, owing to the high cost of the HTS conductor, of testing HTS dipoles at high field and high stress. This will be a major focus of the US-MDP for the coming years.
The HTS magnet development area focuses on development of REBCO and Bi-2212 accelerator magnet technology. These efforts leverage and support recent increases in the engineering current density of these conductors.10 Specifically for REBCO, developments in conductor architectures that is suitable for accelerator magnet winding are also being leveraged.3,4 The current focus is on development of small-scale stand-alone dipole magnets with maximum field strength on the order of 5 T. Figure 5-3 shows the maximum field obtained for a subset of recent HTS magnets both in the United States and in Europe. Within the US-MDP, magnet geometries being pursued are consistent with the stress-management approaches developed in the Nb3Sn magnet area owing to the strain-sensitive nature of the HTS conductors. For hybrid testing, a facility at BNL is available with an outsert
SOURCE: L. Bottura, S. Prestemon, L. Rossi, and A.V. Zlobin, 2022, “Superconducting Magnets and Technologies for Future Colliders,” Frontiers in Physics 10:935196, https://doi.org/10.3389/fphy.2022.935196. Copyright © 2022 Bottura, Prestemon, Rossi and Zlobin. CC BY.
___________________
10 T. Shen and L. Garcia Fajardo, 2020, “Superconducting Accelerator Magnets Based on High-Temperature Superconducting Bi-2212 Round Wires,” Instruments 4(2):17, https://doi.org/10.3390/instruments4020017.
common coil Nb3Sn magnet.11 Efforts for hybrid testing are being accelerated in conjunction with the development of the Nb3Sn large aperture outserts mentioned above, to accommodate larger-insert coils. These efforts will allow for probing the HTS magnet performance at higher field and stress in the material. Other major areas of focus for the HTS magnets focus on quench detection and protection methods, field quality, and probing of mechanical limits.
The third area of the US-MDP focuses on investigating fundamental aspects of magnet technology. This includes investigation of structural materials and fabrication techniques, development of novel diagnostic concepts, development of analysis methods, development of training reduction studies, and development of advanced modeling techniques. Some of the diagnostic tools under investigation include various geometries and types of quench antennas, optical fibers for strain and temperature measurement, acoustic sensors, hall probe arrays for current re-distribution measurements, and time domain reflectometry techniques. These diagnostics, combined with novel analysis methods, have the potential to be used for novel quench detection methods (especially for HTS conductors) and will lead to a better understanding of the physical phenomena leading to current redistribution and magnet quenching events. Topics in structural materials studies include the development of novel impregnation systems to increase strength and reduce training, as well as development of novel fabrication methods, for example using 3D printing techniques for fabrication of winding formers with complex geometries. Another critical topic within the magnet technology area is the comparative analysis for a 20 T hybrid magnet design. The goal of this task is to address challenges for a 20 T design related to stress-management and understanding the cost and engineering trade-offs between the HTS and LTS portions of the hybrid magnets.
The US-MDP also includes a Conductor Procurement and Research and Development portion focused on (1) maintaining inventory of conductor magnet R&D, (2) investing in developmental magnet conductors, and (3) defining needs and opportunities for university, small business initiatives (e.g., SBIR programs), and Laboratory Directed R&D (e.g., LDRD) grants.12
Finding: The US-MDP is developing Nb3Sn and HTS accelerator magnet technology that includes conductor stress management with a goal toward reaching the fundamental limits of the conductor material. It leverages the expertise and infrastructure at various institutions in a coordinated manner for efficient use
___________________
11 R. Gupta, K. Amm, J. Avronsart, et al., 2021, “Common Coil Dipole for High Field Magnet Design and R&D,” White paper contribution for Snowmass 2021, arXiv:2203.08750, https://doi.org/10.48550/arXiv.2203.08750.
12 Department of Energy, 2018, “Laboratory Directed Research and Development (LDRD),” https://science.osti.gov/lp/Laboratory-Directed-Research-and-Development.
of personnel and resources. The program has a well-defined management and oversight structure. The program updates key goals and roadmaps with updates in approximately 5-year intervals.
Finding: The US-MDP is investigating designs for 20 T magnet technology with a longer-term objective of demonstrating this goal. This requires HTS materials with magnet configuration either in all-HTS configurations or in hybrid configurations (HTS and LTS). In the near term, hybrid test platforms (HTS insert in LTS outsert) are essential to demonstrating HTS magnet technology, owing to the high cost of HTS materials today.
For applications to light sources, the main recent R&D efforts for high-field magnets have been in the area of superconducting undulators. This effort has primarily taken place at Argonne National Laboratory (ANL) with contributions from other national laboratories such as LBNL, FNAL, and SLAC. Initial work focused on the development of the technology using Nb-Ti conductors, and the demonstration of reliable operation in a synchrotron facility. Recent efforts have focused on increasing the magnetic field strength by using Nb3Sn and REBCO conductors. The recent operation of a Nb3Sn undulator at the APS in ANL was a significant achievement for the advancement of this technology.13
FUTURE DIRECTIONS
The recently released 2023 P5 report recommends dedicated R&D to promising future projects, including a 10 TeV parton center-of-momentum (pCM) collider.14 Two possible approaches that require significant investment in high-field magnet R&D are a 100 TeV proton-proton collider and a 10 TeV muon collider. Future proton-proton colliders, the Future Circular Collider (FCC-hh), and the Super Proton-Proton Collider (SPPC), are being considered in Europe and China respectively. There is no preliminary decision on the site selection for a muon collider,
___________________
13 I. Kesgin, S. MacDonald, M. Kasa, et al., 2024, “Quench Behavior of 18-mm-period, 1.1-m-long Nb3Sn Undulator Magnets,” IEEE Transactions on Applied Superconductivity.
14 See U.S. Particle Physics, 2023, “Pathyways to Innovation and Discovery in Particle Physics,” Report of the 2023 Particle Physics Project Prioritization Panel, https://www.usparticlephysics.org/2023-p5-report; Department of Energy, 2023, “Charge from Office of Science to HEPAP to Form a Committee of Visitors to Assess HEP Facilities Division,” https://science.osti.gov/-/media/hep/hepap/pdf/Reports/2024/2023-514---FY-24-HEP-Facilities-Division-COV-Charge_AA-Berhe-Signed.pdf; H. Murayama, S. Asai, K. Heeger, et al., 2023 , Exploring the Quantum Universe: Pathways to Innovation and Discovery in Particle Physics, Washington, DC: Department of Energy, Office of Scientific and Technical Information, https://www.osti.gov/biblio/2368847.
but P5 in its recent report strongly advocated for targeted R&D to build a 10 TeV muon collider on U.S. soil.
These colliders all require beyond state-of-the-art magnet technology. For the proton-proton colliders, the cost is dominated by the high-field dipoles. These require thousands of high-field dipoles in a ring with circumference of approximately 100 km. To make such a facility feasible, magnet R&D is required for both significant technological advancements as well as cost reduction. The 100 TeV FCC-hh has proposed to use 16 T Nb3Sn dipole magnets with length of ~15 m and a 50 mm aperture. Quadrupoles with high-field gradient and large aperture are also required. The dipoles specifically require significant development in magnet design and technology as well as conductor development. Significant effort is required to develop reliable accelerator magnet technology for >16 T operation with respect to mechanical design to support the magnetic forces, magnet protection, reliable construction methods, and scale-up for industrial production. With respect to the superconductors, the target critical current density for the FCC-hh Nb3Sn conductor is 1500 A/mm2 at 16 T and 4.2 K. This target is beyond state-of-the-art for conductors that can be produced in significant quantities today. HTS conductors could also provide significant advantages in the future with the ability to reach higher magnetic fields. These areas also require critical R&D efforts to improve the conductor properties and reduce their cost.
Because the muon is a fundamental particle, muon colliders can have a similar discovery reach as hadron colliders at a fraction of the energy. Like hadrons, muons can also be accelerated in rings because they don’t suffer from large synchrotron radiation losses of electron-positron colliders. This is a strong motivation for pursuing this approach for a future 10 TeV pCM collider. Muon colliders require several types of superconducting magnets for muon production, cooling, acceleration, ring storage, and final focus. Muon production (target) and muon cooling regions require solenoids with different field strengths up to ~20 T and different aperture sizes up to ~1.2 m.15 The final cooling solenoid has a target field of 40–60 T with a 50 mm bore. Fast ramping resistive or superconducting magnets are required in the acceleration section. The superconducting magnet option needs additional R&D, but it is an attractive path to decrease the energy consumption of the acceleration section. The storage ring magnets require large bores (approximately 150 mm) owing to the radiation flux and peak conductor fields of approximately 16 T.15
Finding: Future colliders require beyond state-of-the-art magnet technologies with higher magnetic fields and apertures than currently achieved. Beyond the
___________________
15 S. Gourlay, “Magnet Technology to Enable the Next Generation of Facilities for Particle Physics,” Presentation at the study meeting, August 19, 2023, Washington, DC: National Academies of Sciences, Engineering, and Medicine.
typical high-field dipole and quadrupole accelerator magnets, muon colliders require significant investment in high-field large aperture solenoids, fast ramping magnets, and large aperture dipole and quadrupole magnets.
For advances of future light sources, one of the most significant impacts could be made through the developments of superconducting undulators—SCUs. An undulator is an assembly of many magnets that forces electrons to undulate or wiggle periodically along the straight line. On each wiggle electrons generate radiation and the coherent sum of radiation from all wiggles produces powerful undulator radiation. Typical undulator has about 100–150 wiggle periods with the length of 2 to 5 cm and with the peak magnetic field on undulator axis around 1 T. Superconducting undulators could shorten the undulator period length down to 1 cm and at the same time increase the peak magnetic field up to 2 T. To achieve such a goal the undulator magnet must operate at extremely high current density: >5,000 A/mm2. This would be possible only with HTS wire or tape. Short-period advanced SCUs would double or triple the brightness of existing light sources and enable the future generation of compact X-ray FELs. SCU magnets would also enable the technology of next generation of storage ring facilities that would require compact dipole, multi-pole, and combined function accelerator magnets with high gradients to further reduce storage ring emittance.
SYNERGIES
There are numerous synergies that exist in the high-field solenoid space between high-energy physics, high-field science, fusion energy, high-field NMR and MRI. The emergence of practical HTS superconductors is allowing the development of higher field solenoids across these applications. Although the applications have a variety of HTS conductor requirements, there are many potential areas that could benefit from similar areas of research and development. For example, insulation magnets and cables are being developed for fusion magnets that could also be utilized in the high-field solenoids needed for Axion searches and Muon colliders.
During the Snowmass process,16 the development of large bore, high-field solenoids was identified as a critical technology area for both the cosmic and energy frontiers. As noted in the white paper “High field HTS Solenoid for the Future of Particle Physics,”17 future searches for Axion Dark matter, Charged Lepton Flavor
___________________
16 J.N. Butler, R.S. Chivukula, A. de Gouvea, et al., 2023, “Report of the 2021 US Community Study on the Future of Particle Physics (Snowmass 2021) Summary Chapter,” arXiv preprint arXiv:2301.06581, https://doi.org/10.2172/1922503.
17 R. Bernstein, T. Bose, A. Chou, et al., 2023, “High-Field HTS Solenoids for the Future of Particle Physics,” preprint Elementary Particle Physics December 21.
Violation, beam dump experiments, as well as the proposed Muon Collider all require large bore solenoids with fields above 10 T ranging up to 40 T. This will require the development of robust, high-field HTS magnets. NHMFL has been at the forefront of high-field HTS magnet development with the development of the 32 T HTS magnet and now the 40 T HTS magnet. The knowledge that has been developed at NHMFL can serve as the foundation for future large bore solenoids that are needed for the future HEP experiments. In addition, the emerging fusion industry is working on developing robust large bore solenoids for their central solenoids. Many of the compact fusion companies are driving significant development and improvement in the HTS supply chain and are beginning to drive the volume of available conductors up and the cost of REBCO conductors down. This will potentially enable better availability and economics of HTS conductors for the HEP, NMR, and MRI communities.
Finding: While researchers in the HEP, high-magnetic-field science, and fusion communities have begun to self-organize, including DOE program managers in the FES and HEP spaces, to co-sponsor synergistic research that can benefit both fusion and HEP, there has yet to be a broad, multi-agency approach between NSF, DOE, and NIH to realize the benefits of co-developing HTS magnet technology that could benefit applications requiring high-field solenoids.
Key Recommendation 2: The National Science Foundation, Department of Energy, and National Institutes of Health should develop collaborative programs to accelerate the development of high-temperature superconductor (HTS) magnet technology to support development in high-field magnetic resonance imaging, nuclear magnetic resonance spectroscopy, fusion, and accelerator magnets. For example, a large bore solenoid (900 mm+), high-field (14 T+) magnet demonstrator employing HTS technologies, ideally with ramp capability (5–10 T/s), should be commissioned to develop the foundational design method and wire technology that has potential applications across high-magnetic-field science.
FOREIGN HIGH-FIELD ACCELERATOR MAGNET RESEARCH AND DEVELOPMENT EFFORTS
Europe, China, and Japan all have extensive accelerator magnet research programs. The status and goals of these programs are described below.
Europe
In 2021, the European Laboratory Directors Group was mandated by CERN council to oversee the development of a high-energy physics accelerator R&D roadmap. High-field magnets (HFMs) is one of five key R&D areas highlighted as an enabling technology for the search of new physics at the energy frontier. Two primary long-term R&D goals are defined.18 The first goal is to explore the maximum practical operating field for Nb3Sn magnets, which is divided into two sub-goals. The first sub-goal is to develop the conductor and magnet technology toward ultimate Nb3Sn performance. This is projected at a target of 16 T dipole magnets (i.e., FCC-hh dipoles). The second sub-goal is to develop Nb3Sn magnet technology for large-scale deployment, focusing on reducing manufacturing complexity and cost. This work is projected to be performed on magnets in field range near 12 T. The second goal of the European HFM R&D program is to develop HTS magnet technologies for accelerators. The target dipole field is set at 20 T, beyond the reach of Nb3Sn. The possibility of all-HTS magnets operating at 14–16 T at higher temperature (15–20 K) is also to be explored.
The European HFM program is organized in to three focus areas: Nb3Sn magnets, HTS magnets, and Nb3Sn and HTS conductors. These are supported by three cross-cutting R&D technology activities: structural materials cryogenics and modeling, magnet powering and protection, and infrastructure and instruments.
Japan
The focus is on three areas: The first area is high precision 3D magnetic field technologies. This primarily involves magnets for muon experiments with a requirement for high homogeneity (+/– 0.1 ppm). The second area involves the development of radiation-hard superconducting magnets. Future muon experiments will require HTS magnets with high radiation resistance. Radiation-hard HTS magnet technology R&D programs are currently ongoing within a U.S.–Japan cooperation framework. Within this program, irradiation tests are being performed on various types of REBCO conductors. Tests on insulation and impregnation materials are also being performed. The third research area involves the development of superconducting magnet technology for future accelerators. A primary target of this work is the beam separation dipole (D1) for FCC-hh. This is a 12 T dipole with a large aperture (100 mm). Because the large aperture results in higher stress, stress management structure as well as mechanically strengthened Nb3Sn
___________________
18 L. Bottura, S. Prestemon, L. Rossi, and A.V. Zlobin, 2022, “Superconducting Magnets and Technologies for Future Colliders,” Frontiers in Physics 10:935196, https://doi.org/10.3389/fphy.2022.935196.
(see conductor section) approaches are a major R&D target. The goal of this third area is to construct and test a Nb3Sn model magnet. Beyond this target, there are plans for combining the large aperture dipoles with HTS or Nb3Sn insert coils to target the 16–20 T range.19
China
The SPPC, under consideration as a future facility in China, will require thousands of 12–24 T accelerator magnets. Long term superconducting accelerator magnet R&D for future high-energy accelerators is taking place at the Institute of High Energy Physics of Chinese Academy of Science (IHEP-CAS) in partnership with research organizations working on superconductivity and advanced HTS materials. The goals of the program are listed as:20
- Explore new methods and related mechanism for HTS materials with superior performance for applications. Reveal key factors in current-carrying capacities through studying microstructures and vortex dynamics. Develop advanced technologies of HTS wires for high-field applications with high critical current density (Jc) and high mechanical strength.
- Development of novel high-current-density HTS superconducting cables, and significant reduction of their costs. Exploration of novel structures and fabrication process of high-field superconducting magnets, based on advanced superconducting materials and helium-free cooling methods.
- Exploration of novel stress management and quench protection methods for high-field superconducting magnets, especially for high-field insert coils with HTS conductors. Complete the prototype development with high field and field quality, lay the foundation for the applications of advanced HTS technology in high-energy particle accelerators.
Recent advances include the fabrication of a common coil NbTi + Nb3Sn dipole (>12 T). There are near term plans of testing HTS inserts within this common coil as a hybrid HTS / LTS configuration to demonstrate 16 T operation. Beyond this, there are plans to target 20 T and higher magnets. Furthermore, there is a significant focus on the development of Iron Based Superconductors (IBS). Recent
___________________
19 T. Ogitsu, T. Nakamoto, K.-i. Sasaki, et al., 2022, “R&D Works for Superconducting Magnet for Future Accelerator Applications in Japan,” https://arxiv.org/abs/2203.12118.
20 S. Izquierdo Bermudez, G. Sabbi, and A.V. Zlobin, 2022, “Accelerator Technology Magnets,” https://arxiv.org/abs/2208.13349.
results have been published on demonstration of IBS racetrack coils operating in 10 T background field.21
Finding: The primary efforts on accelerator magnet R&D outside of the United States are in Europe, Japan, and China. All three programs, along with the U.S. program, have a common long-term goal of developing 20 T or greater accelerator magnets. Japan has specific goals related to developing radiation-hard accelerator magnet technology and large aperture magnets. China has a strong focus on development of iron based superconductors for future accelerators. The European program has a similar structure as the U.S. program on the long-term R&D activities, but it also includes a component that focuses on development of magnet technology for large-scale production.
SUMMARY AND RECOMMENDATIONS
The development path for high-field accelerator magnets requires early R&D investment to develop the tools and methods required to reach sufficient maturity to demonstrate high fields on model magnets. Beyond model demonstrators, technology-readiness R&D programs are necessary to scale-up (usually both in length and quantity) the basic technology in order to be ready to be considered for a facility project. Nb3Sn magnets are now at a sufficient maturity level to be implemented in the LHC upgrade project, in a production of tens of magnets. Further development is required to drive Nb3Sn magnet performance toward to the material limits (16 T) and for larger-scale application (hundreds to thousands of magnets). HTS materials are sufficiently mature to be implemented in R&D demonstrator magnets. Hybrid magnet testing (HTS and LTS) is essential to operate HTS magnets closer to their ultimate limits at an early phase of the development process. Designs of future hadron colliders will require hundreds to thousands of dipole and quadrupole magnets with conductor fields near 16 T. Muon collider designs require both large aperture dipole and quadrupole magnets, as well as high-field solenoids that require HTS materials. There is strong synergy in this area with MRI, NMR, and fusion magnets. Magnet programs in the United States, Europe, Japan, and China have medium to longer-term goals to develop magnet technology at 20 T and beyond.
Conclusion: Investment in high-field accelerator magnet R&D is essential for the advancement of high-energy physics experiments. Past experience has shown that the development of novel technologies can take decades and require both
___________________
21 Z. Zhang, D. Wang, S. Wei, et al., 2021, “First Performance Test of the Iron-Based Superconducting Racetrack Coils at 10 T,” Superconductor Science and Technology 34(3):035021.
general R&D as well as directed R&D programs in the later phases to bring the technologies to a sufficiently mature level (e.g., the quadrupoles for the HL-LHC). Increased support in these areas can keep the United States at the forefront of high-energy physics magnet technology.
Conclusion: Compact and precise high-field accelerator magnets and insertion devices are an enabling technology for the next generation of light sources.
Recommendation 5-1: The Department of Energy Office of Science, through the Office of High Energy Physics, should continue its support for the U.S. Magnet Development Program and establish new directed research and development (toward technology-readiness) magnet development programs within the next 3 years, in order to advance the enabling magnet technology that is required for future high-energy physics facilities.
Recommendation 5-2: In order to enable the next generation of light sources, resources must be made available from the Department of Energy’s Basic Energy Sciences program for early development of high-field, short-period undulators and compact combined function accelerator magnets for application in the next generation of light sources.
