Berkeley Lab

Toward a Transposed and Flexible Magnet Conductor Using High-Temperature Superconducting Wires for High-Field Magnet Applications

(from left): Paolo Ferracin, senior scientist and deputy of ATAP’s Superconducting Magnet Program, Hugh Higley, contractor to the program, and Xiaorong Wang, a staff scientist at SMP, analyze a sample of 6-around-1 superconducting REBCO cable. (Credit: Michele Pixa/Berkeley Lab)

By Xiaorong Wang and Carl A. Williams, April 4, 2024


Researchers from the Accelerator Technology & Applied Physics (ATAP) Division at Berkeley Lab, in collaboration with AMPeers LLC, a technology company based in Houston, Texas, and the University of Houston, have developed and tested prototype samples of 6-around-1 cable using high-temperature superconducting STAR® wires provided by AMPeers. Supported by a Department of Energy (DOE) Office of High-Energy Physics Small Business Innovation Research (SBIR) award, the research was led by AMPeers and leveraged the Lab’s extensive expertise and experience in superconducting cables and its state-of-the-art infrastructure. Cables with six STAR® wires were 1.5 m long, 5.7 mm in diameter, with a pitch length of 52 mm. The critical current of the cable before bending was 1448 A at 77 K, retaining at least 77% of the total critical current from individual STAR® wires. The cable had a low n value of around 4.5. At a bend radius of 30 mm, the critical current and n value remained the same as before bending. The work also demonstrated a method to install electrical terminations to the cable samples. The total resistance of electrical terminations was 61 nΩ at 77 K.


The recently released P5 Report emphasizes the importance of high-field superconducting magnet technology for future proton and muon colliders, including high-temperature superconductors (HTS) suitable for operation at higher fields and temperatures. In addition, the report stresses that “Accelerator technologies play a key role in sustainability …, high-temperature superconducting materials, operated at temperatures higher than superfluid or liquid helium, can play a key role in reducing energy consumption, for example, for future colliders including muons or FCC-hh.”

The work on 6-around-1 cable using STAR® wires could provide a route toward commercial HTS conductors for high-field magnets, extending the capabilities and enhancing the sustainability of future circular colliders. Furthermore, the 6-around-1 cable concept using STAR® wires can benefit fusion applications requiring 3D magnet configurations.

Thanks to the SBIR award, this successful proof-of-principle study paves the way for further leveraging Berkeley Lab’s expertise and resources to support the U.S. manufacturer in developing high-performance, cost-effective conductors for HTS magnet technology. It also helps strengthen the intellectual leadership of the ATAP’s Superconducting Magnet Program (SMP) in high-field superconducting cable and magnet technology innovation.

Research Details

Developed by AMPeers, the STAR® superconducting wire is emerging as an HTS magnet conductor. Although it features a small bend radius of 15 mm, the total current that a STAR® wire carries can be limited. A high-current transposed and flexible conductor is needed for high-field magnet applications, especially those generating changing magnetic fields. To address this need, the researchers adopted a classic 6-around-1 architecture suitable for STAR® wires.

Leveraging expertise and state-of-the-art facilities 

Leveraging the Lab’s unique infrastructure, expertise, and decades-long experience in accelerator and magnet technologies, especially that of Hugh Higley, a former senior scientific engineering associate in SMP and now a contractor to the program, the researchers developed several prototype cables using STAR® wires. Higley also developed a method to make cables from the wires and install the electrical terminations to the cable samples, enabling the testing of cable samples.

The experimental setup to measure the current-carrying capability of the solenoid coil wound using the 6-around-1 cable. (Credit: Hugh Higley/Berkeley Lab)

Several questions drive the research. One is the current-carrying capability of the prototype cables at different bend radii, which is essential for accelerator and fusion magnet applications. Cable samples were bent to smaller radii, and the team measured their current-carrying capability at each bend radius. These cable samples contained six STAR® wires and were found to be surprisingly flexible and maintained the same current-carrying capability even at a bend radius of 30 mm.

The second question is the impact of the cabling process on the brittle superconducting tapes in the STAR® wires. After cabling, the prototype samples showed around 80% retention in the current-carrying capability. There is, however, room for improvement, and the SMP team plans to pursue several optimization strategies with their collaborators.

The last practical question concerns the electrical termination for the 6-around-1 STAR® cables. A low-resistance electrical termination is essential for successful cable R&D and, eventually, applications of high-current cable in magnets. Higley developed a new flute-type termination that performs satisfactorily for the cables.

Next steps

The team plans to make and test a 10-m-long 6-around-1 STAR® cable in the next several months and wind it into a subscale canted cosine-theta dipole magnet. They also plan to test the magnet’s performance to provide effective feedback on the cable’s performance and identify goals for further R&D. The impact of stress transverse to the longitudinal axis of the cable is another important subject to be studied in the future.

The 6-around-1 STAR® cable research is part of the U.S. Magnet Development Program’s strategy to collaborate closely and effectively with conductor vendors to develop the HTS magnet and conductor technology. Ongoing research in SMP will address the high-field magnet needs, as laid out in the P5 Report, within the next decade. 

Contact: Soren Prestemon

Researchers: Paolo Ferracin, Hugh Higley, Soren Prestemon, and Xiaorong Wang from ATAP; Nathaly Castaneda, Sri Ram Korupolu, Goran Majkic, Venkat Selvamanickam, and Huy Truong from the University of Houston; and Cyrus Funkhouser, Eduard Galstyan, and Hoang Nguyen from AMPeers LLC

Funding: The research described in this article was supported by a DOE Office of Science High-Energy Physics Small Business Innovation Research (SBIR) award and the DOE High-Energy Physics U.S. Magnet Development Program, led by Berkeley Lab.

Publication: N. Castaneda, P. Ferracin, C. Funkhouser, E. Galstyan, H. Higley, S. R. Korupolu, G. Majkic, et al. 2024. “A 6-around-1 Cable Using High-Temperature Superconducting STAR® Wires for Magnet Applications,” Supercond. Sci. and Technol. 37, 2024,


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