Fermilab breaks ground on coil fabrication for Jefferson Lab collaboration

The Magnet Systems Department recently successfully completed a prototype torus magnet coil for the Jefferson Lab CLAS12 upgrade. They devised a relatively inexpensive system, seen here, for winding the 2,500-pound coil. While the price of a standard coil-winding table that can hold a 4,000-pound fixture is $190,000, the Fermilab team built an adequate system for less than $10,000. One layer of coil, sitting at the winding fixture with a 12-foot-diameter cable spool installed above the fixture, and the second spool on the tensioner, is almost completely wound. Photo: Douglas Howard, TD

There is perhaps no greater challenge, mentally, than taking on a project that has been attempted previously but not successfully completed.

This is the position a team of Fermilab engineers and physicists found themselves in more than a year ago, when Jefferson Lab, based in Virginia, came to Fermilab for help on a project: fabricating magnet coils for an upgrade to its CEBAF Large Acceptance Spectrometer (CLAS) experiment.

It turned out to be a good move. In late November, a Magnet Systems Department fabrication team in the Technical Division successfully wound a full-size coil, called a practice coil, of the type to be installed in the new torus magnet for the upgrade of Jefferson Lab’s CLAS detector. Jefferson Lab’s upgraded facilities will provide scientists with unprecedented precision and reach for studies of atomic nuclei.

Jefferson Lab had initially hired a contractor to design and fabricate the new magnets for the CLAS12 upgrade, but it was not able to effectively build the large, superconductive coils to the required specifications. When the project then came to Fermilab, the fabrication team was unsure whether it would be able to do what others had not, especially as it would have to work under a now very tight schedule and restricted budget.

“Now we can say we can definitely do this job,” said Fermilab engineer Sasha Makarov. “It seems like Jefferson Lab is very satisfied with our achievement.”

Getting to this point was not easy. Because the components of the coil, which is 2,500 pounds, 14 feet long and 7 feet wide, are themselves so big and heavy, the team could not use standard equipment for the project. So they designed and built several brand-new pieces of equipment that had to be tested and verified for use with the coils.

“We pretty much started from scratch,” Makarov said. “You have to design not just tooling, which is normal, but also create new, unique machines that allow you to build these coils. We wanted solutions that would be quick and cheap but still work well.”

One of the steps involves removing gas from inside the coil and replacing it with an epoxy. The epoxied coil sits in a potting mold. Typically gas is removed using a standard vacuum oven. Just one problem: The giant coil wouldn’t fit in that oven. So the team instead made a vacuum-tight potting mold with its own vacuum pump and heating system. No oven was needed: The team installed the system in a custom-built, heat-insulating box. Using this technique, they successfully outgassed the coil and impregnated it with the epoxy.

The practice coil made it through the entire fabrication process, setting the stage for Fermilab to complete its contribution later this year. When all is said and done, the team will have wound eight identical coils: six for the detector’s torus superconducting magnet and two to spare. They hope to have all eight coils built, wound and transported to Jefferson Lab by September.

“Knowing others had fallen short, we were not absolutely confident we could do it, especially without the possibility of using our standard equipment,” Makarov said. “Now we can say we did it, and this is a very good feeling.”

Sarah Witman