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January 9, 2004
Another Magnet, and Another World Record
Only two years after building the world's most powerful dipole electromagnet, a Berkeley Lab team has upstaged itself with an even stronger magnet, a 16-tesla design that paves the way for substantial upgrades to existing and under-construction accelerators like the Tevatron and the Large Hadron Collider (LHC).

The record breaker could also lead to a new breed of powerful yet cost effective magnets that drive the next generation of particle accelerators and help scientists unlock the enduring secrets of physics.

Shlomo Caspi (left) and Steve Gourlay stand next to a dewar that contains their record-holding 16-tesla magnet.

The magnet, which in October generated a magnetic field more than 300,000 times stronger than the Earth's, cements Berkeley Lab's place as the leader in high-field superconducting magnets. Dubbed HD-1, it owes its performance to a niobium tin alloy that offers unprecedented field strengths but requires innovative fabrication and analysis techniques.

"Our goal is to push magnetic field strengths as high as possible," says Steve Gourlay, head of the Accelerator and Fusion Research Division's Superconducting Magnet Group. "To get there, we need to develop ways to use niobium tin at its full potential. That's the trick."

Dipole magnets like the HD-1 are used to steer particle beams at near relativistic speeds as they zip around an accelerator ring. In accelerators with a fixed radius, the stronger the magnets, the greater the particles' energies, and, in the case of colliders, the more fierce the particles' collisions and the more spectacular the shower of debris. The LHC, for example, is slated to use 8.3-tesla magnets to accelerate protons to 7 trillion electron volts before smashing them into each other. The collisions will hopefully yield theorized but never-before-observed particles like the Higgs boson.

When completed in Geneva, Switzerland in 2007, the LHC will be the most powerful accelerator in the world. But it could be even more powerful. If higher-field magnets are installed, then the LHC's luminosity increases, meaning it produces more proton interactions per second. And more interactions means more events to study, a crucial advantage in experiments in which scientifically important debris like the Higgs boson are expected to be extremely rare.

"As the luminosity increases, there are more collisions, which means more junk as well as more interesting phenomena," says Gourlay. "Our work could facilitate upgrades to the LHC when they'll be needed, about eight years after it's turned on."

In addition, stronger magnets could increase the LHC's physics reach, or the probability that it will create particles that have never been observed.

In pursuit of more powerful magnets and their promise of exotic particles, physicists have coaxed ever-stronger magnetic fields from superconducting alloys. The most commonly used alloy is niobium titanium. It's easy to work with, but its field strength tops out at approximately 10 tesla at extremely low temperatures. To push above 10 tesla, the Berkeley Lab team switched to niobium tin, another low temperature superconducting alloy. They used this compound to string together several record-breaking designs, including a 13.5-tesla magnet in 1997, a 14.5-tesla magnet in 2001, and this fall's 16-tesla magnet.

Although they make it seem easy, their success hinged on overcoming a daunting engineering obstacle. Niobium tin has excellent magnetic properties, but it's as brittle as glass—an unfortunate characteristic if it's to be incorporated into a magnet subject to 3 million pounds of pressure, or the equivalent of balancing a dozen trucks on a couple of toy Lego pieces.

"Niobium tin hasn't been embraced for accelerator magnet applications because it has horrible mechanical properties," says Gourlay. "But we knew that it could become an important high-energy physics tool if we overcame its limitations."

This computer simulation reveals the temperature distribution in a superconducting coil 150 milliseconds after a portion of it loses superconductivity (non-blue colors of magnet), a procedure called quenching. The rise in temperature, voltage and stress must be minimized to protect the coil during this event.

First, that meant zeroing in on the optimum magnet design, and this meant using software to digitally render and test the magnet before its first coil is fabricated. To do this, the Berkeley Lab team developed a unique protocol that models how the entire magnet—from its niobium tin core to its hundreds of other components—respond to tremendous swings in thermal, mechanical, and electrical forces in three dimensions and through time. This comprehensive approach is an improvement over conventional computer-aided analyses that only test a magnet's most fundamental parts, an inexact method that postpones much of the trial-and-error phase until after the magnet is built.

"Our software captures the whole magnet," says Shlomo Caspi, a senior mechanical engineer in the Engineering Division who heads the design and analysis team. "It enables us to predict how the magnet's components react to changing voltages, temperatures, and stresses before it's constructed. We can solve problems before they occur."

Armed with this virtual troubleshooting, Gourlay's team worked the superconductor into the world's most powerful dipole magnet. Strands of niobium and tin made by Oxford Superconducting Technologies are formed into a cable and fashioned into double-layer, flat racetrack coils designed to withstand extreme forces. The coils are then heated to 680 degrees Celsius for more than 100 hours. This yields a potentially superconducting but brittle alloy. To strengthen the coils, they're impregnated with an epoxy that protects them from extreme pressure. It becomes bulletproof, as Gourlay says. Finally, the coils are encased in an iron yoke and wrapped in an aluminum shell.

To reach the magnet's peak field strength, it's cooled to 4.2 Kelvin—a temperature at which niobium tin is superconducting—then a current is increased until the magnet reaches it's theoretical limit, in this case 16 tesla.

And they're not stopping there. Now that Gourlay's team knows their design can endure extreme forces, they plan to reduce the operating temperature from 4.2 Kelvin to 1.8 Kelvin and shoot for 17 tesla, niobium tin's intrinsic conductor limit.

So what does this mean for high-energy physics? HD-1 is experimental and not destined for an accelerator. But the lessons Gourlay's team learned in pushing to 16 tesla could translate to relatively inexpensive production magnets for accelerators that eclipse even the LHC in power, such as the aptly named Very Large Hadron Collider. Although still on the drawing board, this collider would probably require hundreds of magnets operating at 10 to 12 teslas.

"And that would be a piece of cake after building and training a 16-tesla magnet," says Gourlay.

In addition to Gourlay and Caspi, members of the record-holding team include Scott Bartlett, Mike Barry, Paul Bish, Sharon Buckley, Dan Dietderich, Paolo Ferracin, Modeste Goli, Ray Hafalia, Roy Hannaford, Hugh Higley, Ruth Hinkins, Bill Lau, Alan Lietzke, Nate Liggins, Sara Mattafirri, Al McInturff, Mark Nyman, GianLuca Sabbi, Ron Scanlan, Jim Smithwick, Jim Swanson, and Kathleen Weber.

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