Scientific Achievement

The Berkeley Center for Magnet Technology, a collaboration between researchers from Berkeley Lab’s Accelerator Technology & Applied Physics and Engineering Divisions, has demonstrated training-free behavior for a 5.4-tesla (T) niobium-tin (Nb₃Sn) canted-cosine-theta (CCT) accelerator dipole magnet impregnated with paraffin wax. The magnet reached the short sample limit of the superconductor without any training quenches in the inner and outer layer coils. The work shows the potential for future training-free low-temperature superconductor (LTS) accelerator magnets that can operate near the limits of the superconducting material.

Significance and Impact

Training campaigns for LTS magnets, which gradually increase the magnet current to the desired amount through repetitive quenches, can be lengthy and costly. Furthermore, a substantial margin—leading to higher conductor quantity—is generally required to ensure that the magnet meets operational requirements. A combination of stress management approaches, like CCT designs, with paraffin wax impregnation can significantly reduce the cost of accelerator magnets by allowing for near conductor short-sample-limit operation while eliminating magnet training.

Research Details

CCT Magnet Program

As part of the U.S. Magnet Development Program (USMDP), Berkeley Lab researchers are developing CCT magnet technology. CCT magnets are composed of layers of mandrels with machined grooves containing the superconducting cable. These grooves take the shape of a tilted solenoid with opposing tilt directions between the layers. The mechanical strength provided by the mandrel leads to reduced stress on the superconductor compared to more traditional magnet designs. Since Nb₃Sn is a brittle superconductor with limited strength, this stress management approach can potentially overcome the stress limitations for traditional magnet designs at high magnetic fields.

Multiple epoxy-impregnated two-layer Nb3Sn magnets with a short sample bore field of 10 T were constructed in the past as part of the CCT technology development program [1]. While the magnets demonstrated the technology’s viability, they still required some training, which is common in LTS magnets. This training behavior is believed to be due to the release of energy from de-bonding, cracking, and frictional events at the interfaces. A sub-scale CCT magnet program was launched as a faster turn-around platform to understand and address training systematically.

Demonstration of a training-free CCT subscale magnet

Five sub-scale CCT magnets have been constructed to improve the understanding of training in CCT magnets. Two of these magnets are highlighted below: a baseline epoxy-impregnated magnet (Sub 2) and a paraffin wax-impregnated magnet (Sub 6). While the epoxy-impregnated magnet exhibited similar training behavior to the previously fabricated larger magnets, the paraffin wax-impregnated magnet exhibited no training inside the coils. It reached the short sample limit of the conductor on the first coil quench (as shown in the figure below).

Test results for the epoxy-impregnated magnet (Sub 2) and paraffin wax-impregnated magnet (Sub 6). (Credit: Berkeley Lab)

By design, the subscale magnets are relatively low-field magnets. To characterize the behavior at higher fields, a larger CCT magnet (with approximately 10 T short-sample field and 90 mm aperture) is currently under construction. It will be impregnated with paraffin wax and tested soon. If similar performance as for the subscale magnet can be demonstrated, this could lead to a new class of training-free accelerator magnets operating close to the short-sample limit of the conductor.

Contact: Diego Arbelaez

Researchers: Diego Arbelaez, Reed Teyber, Jose Luis Rudeiros Fernandez, Lucas Brouwer, Maxim Marchevsky, Giorgio Vallone, Paolo Ferracin, Soren Prestemon (Accelerator Technology & Applied Physics Division)

Funding: The research described in this article is funded by the US Magnet Development Program through the U.S. Department of Energy Office of High Energy Physics.

Publications:

1. Arbelaez et al. “Status of the Nb 3 Sn Canted-Cosine-Theta Dipole Magnet Program at Lawrence Berkeley National Laboratory,” in IEEE Transactions on Applied Superconductivity, vol. 32, no. 6, pp. 1-7, Sept. 2022, Art no. 4003207, https://ieeexplore.ieee.org/document/9730082
2. Diego Arbelaez et al. “Training-free demonstration of a 5.4 T Nb3Sn Canted–Cosine–Theta accelerator dipole impregnated with paraffin wax,” Sci. Technol., 37, 2024, https://iopscience.iop.org/article/10.1088/1361-6668/ad44e6

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