Advanced superconducting magnets capable of generating magnet fields of 20 tesla or more promise to extend the capabilities of particle accelerators and colliders, opening new research areas in high-energy and nuclear physics, applications in medical diagnosis and treatments, and advances in fusion.
However, these magnets must be fabricated using cables made from materials that can maintain the high critical current (the maximum current they can carry before becoming a normal conductor) needed to produce high magnetic field strengths while also withstanding high thermal and mechanical loads.
Now, researchers from the Accelerator Technology & Applied Physics (ATAP) Division at Berkeley Lab have built a numerical model to simulate the behavior and characteristics of cables made from rare-earth barium copper oxide, or ReBCO, a promising candidate for making superconducting cables because of its high critical temperature—making it a so-called high-temperature superconductor (HTS)—and high critical field.
While HTS magnets made from ReBCO promise high magnetic fields, making them into superconducting magnets for accelerators and colliders is “very challenging,” says Sofia Viarengo, a doctoral student from the Politecnico di Torino in Italy, who is working with Senior Scientist and Deputy Program Head of ATAP’s Superconducting Magnet Program Paolo Ferracin, and is leading the development of the model.
“For instance, the cables must maintain high critical current densities while being subjected to large electromagnetic forces, which generate mechanical stresses and strains that can deform the cables, negatively impacting their performance, while delivering uniform, high-quality magnetic fields. In addition to these forces, the winding process degrades the cable’s performance.”
Since the strength of the magnetic field is determined by how much current can be transported by the superconducting cables, Viarengo says it is essential to understand how much current these superconducting materials can carry under different conditions to determine their suitability for use in high-field strength superconducting magnets.
“Our model, for the first time, considers convective cooling in the cables by coupling the thermal and electromagnetic characteristics of the ReBCO tape,” she explains. “This allowed us to accurately simulate the thermal, magnetic, and electrical behavior of the tape and the layers formed when the tape is wound around the magnet’s core, and how this behavior impacts the current distribution among the tapes.”
To characterize the behavior of the ReBCO tapes, the researchers built a three-dimensional numerical model that simulates a Conductor-on-Round-Core (CORC) cable arrangement wound with ReBCO tapes for use on a canted cosine-theta (CCT) magnet design.
The CORC cable is an HTS cable arrangement that combines scalability, flexibility, mechanical strength, ease of fabrication, and high current density, making it a possible candidate as a conductor for large, high-field magnets. The CCT design provides the uniform, high-strength magnetic fields required in accelerators and can also be fabricated using relatively simple manufacturing processes.
According to Ferracin, the CORC cable geometry is particularly complex and challenging to model. “The cables comprise several ReBCO tapes wrapped around a copper core, making them electrically and thermally coupled. So it’s computationally very burdensome to simulate how the current is shared between the different layers of tape.”
He adds that despite these challenges, the model showed excellent agreement with other simulations and experimental data (provided by Berkeley Lab) and could accurately simulate the current distribution among the tapes even if electrical or thermal disturbances occur, which, he notes, has never been done before.
The work significantly improves our understanding of how current densities are distributed inside CORC cables under various conditions. It could also help to elucidate the mechanisms that lead to sudden and unpredictable losses in superconductivity—a phenomenon referred to as quenching—in HTS magnets during operation. Quenching can generate temperatures high enough to destroy the magnets, costing millions of dollars in damage, and the conditions that lead to it in a CORC conductor are extremely difficult to detect and predict.
“We plan to improve the computational time and stabilization of the model, evaluate the thermal stability of the CORC cable, and scale up the model by simulating what happens when many of these cables are combined,” says Viarengo.
Commenting on the research, ATAP Division Director Cameron Geddes said: “This robust new model demonstrates the importance of simulation in the development of advanced superconducting magnets, which are an essential enabling technology for advancing scientific research and developing new applications across many fields.”
This work presented here was partly supported by Politecnico di Torino through the U.S. Magnet Development Program and partly by the U.S.—Japan High Energy Physics Collaboration from the U.S. Department of Energy Office of High Energy Physics.
Learn More
S. Viarengo, L. Brouwer, P. Ferracin, F. Freschi, N. Riva, L. Savoldi, and X. Wang. “A New Coupled Electrodynamic T – A and Thermal Model for the Critical Current Characterization of High-Temperature Superconducting Tapes and Cables,” IEEE Access Journal 11, 2023, https://doi.org/10.1109/ACCESS.2023.3321194
