Fermilab's giant magnet passes test after cross-country move
A huge collective sigh of relief emanated from scientists, engineers and construction workers three weeks ago at Fermi National Accelerator Laboratory in Batavia.
For the first time in about 10 years, a new-to-the-laboratory particle storage ring and electromagnet was plugged in. And it did what it was supposed to do: turned on, chilled down to 450 degrees below zero, and made a super-powerful magnetic field.
The superconducting wires in the 50-foot-wide ring had survived a 3,200-mile journey two years ago without breaking.
The journey started after Fermilab bought the used ring from Brookhaven National Laboratory in Long Island, then moved it by barge and truck over an ocean, rivers, a canal and land. Building it a home in Illinois has, so far, proved worthwhile.
"You can't put it in to words out significant it was," scientist Hogan Nguyen said of the test. "So now everything can happen."
"Everything" being finishing preparations and then running the Muon g -2 experiment to see if what the Standard Model of particle physics has said about the appearance and disappearance of particles in a vacuum is correct.
<h3 class="breakHead">Punch list</h3>
There's a bunch of work still to be done. Fermilab workers and scientists have to install a vacuum chamber in which muons will travel around the ring, and connect a beam line that will inject the muons to it. Muons are one of six types of subatomic particles known as leptons. Leptons and quarks are considered elementary particles, or the building blocks of the universe. Muons spin and wobble, and scientists want to know why.
A small trolley-like device is also being designed to run inside the ring to monitor the magnetic force being generated, so scientists can make sure the force is uniform throughout the circle.
Insulation will be put over the ring, to aid in temperature control.
And by the spring of 2017, the electromagnet should be ready for a two-year run.
<h3 class="breakHead">Logistics</h3>
Getting the ring in its resting place "was a bit of a challenge," said scientist Adam Lyon. They slid it in, then lowered it by crane about 10 feet onto footings; the ring sits slightly above the floor. That took about a week, he said.
When the storage ring was moved from New York in 2013, the magnet part was welded to the barge, to minimize movement. When being towed by truck, it traveled no faster than 15 mph.
"We were worried about twisting the magnet," Nguyen said.
Twisting it as little as a quarter-inch horizontally or a tenth of an inch vertically could have broken the superconducting wires inside.
The magnet was parked on Fermilab grounds until July 2014 while the building and a liquid helium cryogenic plant were constructed. They also moved the 12-piece iron yoke that encompasses the ring by truck.
The iron amplifies the magnetic field to about 30,000 times greater than the Earth's field, Lyon said. That's why people won't be carrying credit cards into the experiment hall when the ring is powered. Or enter it if they have certain pacemakers, shrapnel or aneurysm clips in them. Items with ferromagnetic components would be sucked right to the ring, and the magnetic strips on credit cards erased.
The helium cools the magnet to the necessary temperature for superconductivity. It took about two weeks to cool.
The team upgraded all the electronic components and has connected it to faster-processing computers than those originally used when the magnet was built in the 1990s at Brookhaven. But in a thrifty move, the experiment is repurposing beam-line magnets from another Fermilab experiment and a solenoid from a third.
"It's been pretty amazing how well it has come together," Lyon said.
<h3 class="breakHead">Why Fermilab?</h3>
One of Fermilab's existing accelerators will rev up protons that will be smashed to create the muons. It can produce more of the muons, and in a cleaner stream, than Brookhaven could, which will make Fermilab's measurements four times more precise. The muons will race around the ring while spinning like gyroscopes. The scientists will measure the precession (think "wobbliness") of the spins and compare it to what has been predicted. All that happens in the 2-millionths of a second that muons typically survive before decay.
"To make that wobble happen, we need a magnet," Nguyen said. The magnet also keeps them moving in a circle.
Quantum mechanics postulates that things appear and disappear in empty space. Do muons wobble because there are heavy, undisclosed particles? Or is there a hidden subatomic force acting on muons?
"The wobbling property," Nguyen said, "is really profound."