December 12, 2011 | 2
They call it “the machine.”
Thousands of physicists working at the LHC are looking for the Higgs boson and other new particles, and many of them have contributed to building the gigantic detectors that are taking most of the media limelight these days.
But humming 100 meters under the Franco-Swiss border is the apparatus that makes it all possible. The “machine” is the collider itself: the particle accelerator that delivers swarms of protons to the detectors—funneling them through intense magnetic fields, pumping them with energy, and eventually smashing them into each other at an interaction point that is the width of a hair. Building particle accelerators is an entirely different job than building particle detectors or looking for new particles. The specialists who do it are called accelerator physicists.
Particle physicists live in a quantum world—that of the processes that destroy particles and create new ones and that underlie the fundamental forces—and dream of discovering the new laws of nature for the 21st century.
Accelerator physicists toil in relative obscurity, with tools such as radio-frequency waves and giant tesla coils, and mostly rely on physics that is more than a century old—classical electromagnetism, with a good dose of special theory of relativity.
While we all wait here in Geneva for tomorrow’s update on the Higgs boson, I met with Lyn Evans, who recently retired after four decades as an accelerator physicist at CERN. During those years he took part in the inception of the LHC and, starting in 1994, he oversaw its design and construction.
Evans picked me up at CERN’s visitors center this morning. We walked through a maze of connected hallways until we got out to his car. A quick drive took us to another building toward the outer edge of this citadel of science.
There, we sat and chatted in his office. Like every other expert I talked to, Evans says that tomorrow’s announcement will only be a step toward the Higgs, not the final answer. “It’s obvious to everybody that we don’t have enough data yet,” he says.
But to get more data faster, the particle physicists rely on the machine—and so far, the machine has delivered. This year CERN’s accelerator physicists have been able to ramp up the intensity of the beams faster than expected, and to produce five times as many collisions, than the particle physicists were hoping to get. “I think everybody is astonished—even I, a little bit” at how the machine has performed so far.
It was not always this way. Only three years ago, the machine lay crippled after a severe accident. It happened at Sector 34 of the LHC ring. On September 19, 2008, just over a week after the LHC first got started up, a cable connecting two of the 15-meter-long, 35-ton magnets that form the LHC melted down, producing an electrical arc. Suddenly, the liquid helium that keeps magnets at their superconducting temperature of 1.9 kelvin vaporized. Valves designed to release the resulting gas were not able to do so fast enough, and a shock wave ensued–so violent that it gravely damaged 53 magnets.
“It was really hard to pick ourselves up from that one,” Evans says. At the time, he recalls, he was in the personnel department, and he received a call from the accelerator’s control room. He quickly went down to inspect the damage, wearing a respirator as the tunnel had filled with helium gas. Evans says it was not surprising that an electrical joint could fail. “It was the collateral damage that was unexpected.”
The LHC cools helium to low temperatures to make the magnets superconducting, so that they can carry more current and create more powerful fields. But at 1.9 kelvin, Evans explains–the helium is colder than that at the Tevatron, the LHC’s precursor at Fermilab, near Chicago. In particular, it is below a critical temperature at which it becomes a superfluid.
Supefluidity is an exotic state of matter that drastically lowers viscosity, and thus it enables the liquid to soak the porous material the magnet is made of, carrying any stray heat away more efficiently. (Superfluid helium also conducts heat 10,000 times better than any other materials, Evans says.)
(As it happens, both the magnets’ superconductivity and the helium’s superfluidity are quantum effects, so it’s no longer quite true that particle accelerators are based entirely on classical physics.)
While particle physicists gear up for big discoveries, the machine experts at CERN are already looking ahead to the upcoming upgrade. In part as a result of the Sector 34 accident, CERN has decided to do a first run at half the energy. But in 2013, the lab will completely shut down the accelerator for an entire year.
First, the CERN team will pump the liquid helium out. Part of it will be liquefied and stored, but CERN does not have enough storage space for all of its 150 tons of it, so it will sell about half of it on the market. Then, they will circulate helium gas inside the machine to slowly bring all of its 50,000 tons up to room temperature, a process that will take weeks. “There are constraints on the rate you can do it,” Evans says: less-than-gentle temperature gradients could easily break things up.
During the shutdown, CERN will bring the LHC up to its design specs, and then the laborious cool-down process will begin, so the accelerator can restart. Once again, it will be the machine people’s job to make all of that happen.
My previous articles on the Higgs:
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