Researchers at ATAP are producing niobium-tin cables to create more powerful superconducting magnets that could extend the capabilities of particle accelerators and realize the promise of fusion.

Particle accelerators and colliders have played a crucial role in advancing scientific research. They have led to significant discoveries in particle and high-energy physics, material science, medicine, and many other fields. They are also an enabling technology for new research areas like fusion.

Superconducting magnets are essential components of many modern accelerators; they shape and direct the particle beams and determine the energy reach of the accelerator. These magnets are fabricated using coils made from superconducting wires that convey electrical currents with virtually no resistance.

During operation, however, these coils must be cooled to extremely low temperatures (hundreds of degrees below freezing) and below their critical temperature (T_C)—the temperature at which they become superconducting. Superconducting materials with a T_C of about 20 K (-424 °F) are called low-temperature superconductors (LTS), while those with a T_C of approximately 77 K (-321 °F) are referred to as high-temperature superconductors.

“While the Tc of a superconductor is important,” explains Ian Pong, a Staff Scientist from the Superconducting Magnet Program at Berkeley Lab’s Accelerator Technology & Applied Physics (ATAP) Division, “it is the current carrying capability at high magnetic fields and manufacturability that are the critical factors when choosing a material for making superconducting coils.”

He notes that because of their superior critical current density and ability to be manufactured cost-effectively, niobium-based superconductors—despite being LTS—dominate the superconductivity industry. “In particular, niobium-titanium (Nb-Ti) and niobium-three-tin (Nb₃Sn) have become the materials of choice for making the cables used in superconducting magnets.”

Pong was recently invited to author a chapter entitled “Processing of Low T_C Conductors: The Compound Nb₃Sn” for the second edition of the “Handbook of Superconductivity” (CRC Press, July 2022). The chapter describes the fabrication methods and design factors relating to Nb₃Sn superconductors. He has extensive experience and expertise in working with Nb₃Sn superconductors. For example, his doctoral studies—for which he won the Institute of Physics best Ph.D. thesis prize in Condensed Matter and Materials Physics in 2009—focused on developing Nb₃Sn superconductors. After initially joining ATAP as a project scientist in 2013, Pong became the cable task leader for the Large Hadron Collider (LHC) Accelerator R&D Program, a U.S. Department of Energy-directed R&D program, and the High-Luminosity LHC Accelerator Upgrade Project (HL-LHC AUP). Before joining ATAP, he was a Monaco fellow at the International Thermonuclear Experimental Reactor (ITER), overseeing worldwide benchmarking efforts and monitoring the production of Nb-Ti and Nb₃Sn, and before that, a postdoctoral fellow at CERN working on the advanced characterization of prototype Nb₃Sn conductors for future accelerators.

Niobium-tin’s time to shine

Although Nb₃Sn was discovered in 1954, seven years before Nb-Ti, the latter’s relative ease of production, greater availability, better ductility, and resilience led researchers to choose it over Nb₃Sn. These properties have made Nb-Ti superconductors the “workhorse” of the superconducting magnet industry. They are used in particle accelerators and colliders, such as the Tevatron at Fermilab and the Large Hadron Collider (LHC) at CERN, the world’s largest and most powerful accelerator, the magnetic resonance imaging and nuclear magnetic resonance machines used extensively in medical diagnosis, and experimental fusion reactors.

However, Nb-Ti superconductors can only generate magnetic fields up to 9-10 tesla. So, upgrades to accelerators, like the (HL-LHC AUP), which aims to extend the capabilities of the LHC and allow researchers to study phenomena in much greater detail, will require Nb₃Sn superconductors because of their ability to generate much higher magnetic fields.

“Not only do Nb₃Sn wires have a critical magnetic field and critical temperature about twice that of Nb-Ti,” explains Pong, “they are also the closest material to Nb-Ti in production maturity and can be fabricated in mile-length strands in tonnage quantities.”

To date, the largest consumer of Nb₃Sn superconductors is ITER. A global scientific partnership between 35 nations, ITER aims to produce a fusion reactor capable of replicating the processes that power the Sun. The technology promises a source of unlimited, carbon-free energy.

“The worldwide production of Nb₃Sn before ITER was about 15 tonnes per year, and during the peak of ITER production, it was about 150 tonnes per year,” notes Pong. “ITER’s toroidal field and central solenoid superconducting magnets, which confine the plasma for fusion reactions, require more than 600 tonnes of Nb₃Sn strands.”

He adds that the HL-LHC, which will be used for high-energy and particle physics research, is another primary consumer driving demand for Nb₃Sn. The project, he says, will require about 30 tonnes of high-performance Nb₃Sn strands for the accelerator’s upgrade magnets.

“This will be the first use of Nb₃Sn in an accelerator and will enable much stronger magnetic fields to create beams with a higher luminosity that could break new ground, potentially leading to new discoveries in high energy and particle physics.”

Developing new generations of superconductors has been critical to progressively advancing the available magnetic fields, a focus of the U.S. Magnet Development Program led by Berkeley Lab.

Fabricating niobium-tin superconductors

According to Pong, three commonly used methods exist for fabricating Nb₃Sn superconducting wires. These include the bronze method, where the tin source is copper-tin-bronze; the internal tin method, which uses pure tin; and the powder-in-tube process, in which the tin is in powder form.

“We typically use the internal tin method for applications requiring wires to carry a very high electrical current, like particle accelerators,” he says. “The bronze route conductor is typically used for lower-critical-current-density and low-loss applications.”

He notes that the application and the fabrication cost are “usually the main factors” in determining which method is employed.

Pong says that while these fabrication methods are effective in making Nb₃Sn wire, designing and making superconducting cables from Nb₃Sn wires “is still very challenging.”

“For example, whereas wires made from Nb-Ti are ready to use once drawn, Nb₃Sn is as brittle as glass—so conventional wire fabrication techniques, such as extrusion, swaging, and drawing, cannot be used after it is formed.”

Because of this brittleness, he adds, Nb₃Sn cannot withstand the cabling process. Consequently, the niobium and tin components are separated inside the wire, and the superconducting Nb₃Sn compound is formed only after the final deformation occurs via reactive diffusion processes, typically after the coil winding step.

To make Nb₃Sn wires superconducting, the unreacted Nb₃Sn wires or cables are heat-treated at about 1,200 °F to 1,300 °F over several days to form the superconducting compound before the final magnet assembly steps—which include coil impregnating with epoxy, coil assembly into a support structure, and pre-stressing of the complete magnet assembly to maintain compression throughout its operational regimes—are performed and the magnet is cooled down to liquid helium temperatures of 1.9 K to 4.2 K (-456 °F to -452 °F).

While Nb₃Sn performs better than Nb-Ti, Pong says it is much more strain-sensitive. “Because of a thermal mismatch among the different components and the massive magnetic forces they experience during energization, the Nb₃Sn superconductors are subjected to complex and intense stresses.”

The challenge, he says, is how to prevent a reduction in its performance from strain or other reasons. “For instance, the mechanical deformation from the cabling process should induce a negligible impact on the post-heat treatment thermal and electromagnetic performance of the Nb₃Sn wires that comprise the coil.”

To achieve this, Pong and his team use a thread-winding machine at the cabling facility at Berkeley Lab. Here, they not only cable all the unreacted strands of Nb₃Sn with controlled tension into a precise geometry but also monitor the product’s quality using state-of-the-art equipment to ensure there is no single defect across the entire cable length.

Commenting on the work, ATAP Division Director Cameron Geddes said: “Ian and all those involved have done an outstanding job in bringing a new higher performance material to applications like the HL-LHC-AUP upgrade, part of an inspiring progression in superconducting materials that are progressively extending the magnetic fields available and the science they enable.”

Learn More

1. Pong, I., Chapter E3.8 “Processing of Low Tc Conductors: The Compound Nb₃Sn” in “Handbook of Superconductivity: Processing and Cryogenics,” Volume 2, 2^nd Edition, CRC Press, July 2022, DOI: https://doi.org/10.1201/9780429183027
2. A Procedural Solution for Determining the Temperature Dependence of Nb₃Sn Superconductor Performance
3. Three National Labs Team Up for Accelerator Focusing-Magnet Record
4. Cabling for High-Luminosity LHC Project Reaches Halfway Mark
5. HL-LHC AUP Receives CD-3 Approval