Fermilab powers up electromagnet dormant for a decade

Fermilab announced September 23 that scientists there have successfully brought a fragile, expensive and complex 17-ton electromagnet back to life, cooling it down to -450°F and then powering it up. The ring is the centerpiece of an experiment to probe the mysteries of the universe with subatomic particles called muons. The experiment will trap muons in the magnetic field and use them to detect theoretical phantom particles that might be present, impacting the properties of the muons.

Bringing the ring down to its operating temperature required cooling it with a helium refrigeration system and liquid nitrogen for more than two weeks. It was a tricky business according to Chris Polly, project manager for the experiment, since the magnet as a whole shrank by at least an inch while it cooled down. This could have damaged the delicate coils inside if not done slowly. Once cooling was complete, the ring had to be powered with 5,300 amps of current to produce the magnetic field. This was another slow process, with technicians easing the ring up by less than 2 amps per second and stopping every 1,000 amps to check the system.

Slow processes, of course, are something this magnet is accustomed to. Two years ago it was laying dormant at Brookhaven National Laboratory in New York. Moving the magnet to Fermilab for new experiments cost roughly ten times less than building a new magnet, but it involved a tricky 3,200 mile land and sea trek. Scientists used a barge to bring the magnet south around Florida and then up a series of rivers to Illinois where a specially designed truck gently drove it the rest of the way to Fermilab.

Over the past year, workers have reassembled the magnet’s steel base. Two dozen 26-ton pieces of steel, and a dozen 11-ton pieces, had to be maneuvered into place with tremendous precision. “It was like building a 750-ton Swiss watch,” says Polly.

Workers also have replaced the control system for the magnet, redesigning it from scratch. Del Allspach, the project engineer, and Hogan Nguyen, one of the primary managers of the ring, oversaw much of this effort, in addition to the construction of infrastructure systems (helium lines, power conduits) needed before the ring could be cooled and powered. “That work was very challenging,” Nguyen says. “We had to stay within very strict tolerances for the alignment of the equipment.”

The tightest of those tolerances was 10 microns. For comparison, the width of a human hair is 75 microns and a red blood cell is about 5 microns across. While assembling the components around the ring, the team tracked down and sealed a significant helium leak previously documented at Brookhaven. The successful cooldown proved that the leak had been plugged. “That’s where the big relief comes in,” says Hardin. “We had a good team, and we worked together well.”

The next step for the magnet is a long process of “shimming,” or adjusting the magnetic field to within extraordinarily small tolerances. Fermilab is in the process of constructing a beamline that will provide muons to the magnet, and scientists expect to start measuring those muons in 2017.