Scientific Achievement

High-temperature superconductor (HTS) magnets made from rare-earth barium copper oxide (ReBCO) have the potential to produce magnetic fields of 20 tesla (T) or greater. These magnets are crucial for advancing compact fusion reactors and future circular colliders and have broad societal applications. Leveraging the multidisciplinary expertise at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), researchers from the Accelerator Technology & Applied Physics (ATAP) Division and Energy Geosciences Division (EGD) at Berkeley Lab have employed distributed fiber-optic sensing (DFOS) technology to investigate the behavior of subscale HTS magnets wound with ReBCO cables at cryogenic temperatures.

The work demonstrates that DFOS can effectively identify locations of strain and temperature variations along the superconducting wires. This provides new insights into magnet performance and facilitates the development of more powerful magnets for fusion and particle accelerator applications.

Significance and Impact

The 2023 P5 Report emphasizes the significance of high-field superconducting magnet technology for future proton and muon colliders, incorporating HTS wires for operation at elevated fields and temperatures. Primarily driven by the private sector, the fusion community is advancing magnetic confinement technology that employs HTS magnets.

Advanced instrumentation and diagnostic capabilities are key to advancing our understanding of HTS magnet behavior and improving HTS magnet performance. They are also crucial for the reliable and robust operation of these magnets in future colliders and fusion power plants. However, for emerging HTS magnet technologies fabricated with multi-tape conductors, traditional instrumentation, such as voltage taps and strain gauges, has become inadequate, hindering magnet development.

This work represents one of the first practical steps toward effectively applying DFOS in large-scale HTS magnets for fusion and high-energy physics. It demonstrates that DFOS can successfully monitor mechanical deformations in HTS superconducting cables wound with ReBCO tape, providing experimental data for modeling and simulations critical to advancing magnet technology. Furthermore, the research has identified several open questions that must be addressed to develop DFOS and HTS magnet technologies.

Research Details

The research was motivated by the following questions:

1. What is the required DFOS performance to measure the distributed strain and temperature inside superconducting magnets at cryogenic temperatures? (This includes several aspects such as fiber architecture, interrogator hardware, computing platform, and integration method); and
2. What types of strain and temperature profiles can be measured in the magnets, and what insights can be gained from them? (One specific challenge is separating the temperature and strain response for Rayleigh-scattering technology. Modeling and simulating magnet performance could help address this.)

To address these questions, the team wound optical fibers into each canted cosine-theta dipole magnet layer fabricated using HTS superconducting cable on-round-core (CORC) wires. They then compared various commercial fibers and mold-release agents to minimize power attenuation in the fibers. Using DFOS, the researchers measured mechanical deformation and temperature with a spatial resolution of 2.6 mm along the HTS conductor during tests at temperatures of 77K and 4.2 K. During the 77 K test, a temperature gradient of 1 K along the conductors was observed, which can influence the critical current distribution along the CORC wire at 77 K and the resultant magnet performance.

Strain distribution along the circumference of the magnet measured by the fiber at 4.2 K. (Credit: Linqing Luo)

They measured the strain distribution along the magnet shell and found that this strain matched a finite-element mechanical model, validating the simulation assumptions. Their experimental results indicate that DFOS can effectively identify locations of strain and temperature changes, which will significantly advance our understanding of the behavior and performance of HTS magnets.

Next steps

Based on their results, the researchers have implemented DFOS in a dipole magnet with a field strength of 5 T—a significant milestone for the U.S. Magnet Development Program. This advancement will enhance their understanding of the performance of HTS magnets and DFOS under more challenging conditions. They also plan to integrate the optical fibers into fusion cable samples and compare them with other sensing technologies developed at ATAP and other institutions.

Additionally, they are developing open-source interrogator technology in collaboration with ATAP’s Berkeley Accelerator Controls & Instrumentation Program. Compared to commercial solutions, this in-house development will offer greater flexibility and options for customizing hardware and software performance, thereby enhancing the use of DFOS in high-field magnets at cryogenic temperatures.

The ultimate vision of this collaboration is to deploy DFOS as part of a comprehensive diagnostic system to provide online and real-time temperature and strain profiles in HTS magnets for human and artificial intelligence. This will enable degradation-free magnet fabrication and robust magnet operation and maintenance in future particle colliders and fusion power plants. To achieve this vision, the researchers systematically implement the evolving DFOS technology into HTS magnets, identifying and addressing the performance-limiting factors. The current work is a first step toward this goal.

Contact: Soren Prestemon

Researchers: Paolo Ferracin, Hugh Higley, Maxim Marchevsky, Soren Prestemon, José Luis Rudeiros Fernández, Reed Teyber, Marcos Turqueti, Giorgio Vallone, and Xiaorong Wang (ATAP); and Linqing Luo and Yuxin Wu (EGD).

Funding: The Department of Energy (DOE) Office of Fusion Energy Sciences and the U.S. Magnet Development Program, funded by the DOE Office of High Energy Physics.

Publication: Linqing Luo, Paolo Ferracin, Hugh Higley, Maxim Marchevsky, Soren Prestemon, José Luis Rudeiros Fernández, Reed Teyber, Marcos Turqueti, Giorgio Vallone, Xiaorong Wang, and Yuxin Wu. “Distributed Fiber-Optic Sensing in a Subscale High-Temperature Superconducting Dipole Magnet,” Superconductor Science & Technology 38 (3) 035029, 2025. https://doi.org/10.1088/1361-6668/adba98.

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