This nobium- tin superconducting dipole magnet, designed and built at Berkeley Lab, reached a world-record field strength of 13.5 Tesla, which is about 250,000 times the magnetic field of earth. The magnet will be a critical component of future high-energy physics experiments.

large team led by materials scientist Ron Scanlan designed, built and tested the new magnet, which is one meter in length and diameter, weighs about seven tons, and features coils wound out of 14 miles of niobium-tin wire. After being "trained," the magnet reached a peak field strength of 13.5 Tesla, which is about a quarter of a million times stronger than the magnetic field of Earth. This far-surpasses the previous high of 11.03 Tesla, and is about triple the strength of the superconducting dipole magnets at the Tevatron, the highest energy particle accelerator in the world.

"We were in unknown territory, and even though we carefully tested all of the components during construction, we could not know for certain what we had until we tested the completed magnet," Scanlan said of the new magnet, which is the first to use niobium-tin for its superconducting coils.

The inherent limitations of conventional electromagnets which cannot attain a dipole field strength much above 2 Tesla has necessitated the continuing development of new and better superconducting alloys. However, the use of high-field strength superconducting electromagnets has always been a considerable technical challenge, because superconductivity tends to weaken and disappear in the presence of a strong magnetic field.

In recent years, the alloy of choice for accelerator magnets has been niobium-titanium. Superconducting magnets made from this alloy operate in all of today's most powerful machines and will be used in the Large Hadron Collider (LHC) now being built at CERN. The LHC magnets are expected to operate at field strengths approaching the 10 Tesla mark that is considered to be the upper limit of niobium-titanium accelerator magnets.

In the search for superconductors capable of reaching even higher field strengths, it was determined that niobium-tin could, in principle, fit the bill. However, unlike niobium-titanium, niobium-tin is a non-ductile material, thought to be too fragile and brittle to withstand the stress of fabrication.

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Scanlan and his team overcame the brittleness obstacle by making their cable from separate strands of niobium and tin in a copper composite strand while the materials were still ductile. Only after they wound their cable into four magnet coils did they meld the separate niobium and tin strands into the compound that is so brittle. Once the four coils were assembled into a dipole magnet they had to be cooled to a temperature of about 4.3 Kelvin (-270 degrees Celsius) to make them superconducting. To withstand this and other stresses, the wound coils were impregnated with an epoxy filler.

After being filled with epoxy, each coil was encased in an iron yoke that contributed to the strength and stability of the magnetic field. The coils were then wrapped in 18 layers of sheet stainless steel, forming a collar that prevents the coils from separating under the force generated when their tremendous magnetic field is energized.

In addition to serving as the model for the dipole magnets that will be used in the next generation of high-energy particle accelerators, this new magnet will also be used at Berkeley Lab as a test facility for evaluating superconductors that could yield even more powerful magnets in the future.

- Lynn Yarris