Shutting Off the Large Hadron Collider

Jul 15, 2010 By Phillip F. Schewe
Credit: CERN Geneva

On hilly parts of the Interstate highway system, road engineers provide steep-grade areas with gravel off-ramps for trucks that lose their brakes. The ramps bring the big rigs to a rough but safe halt.

Engineers at particle accelerators must also be able to halt intense beams of particles during routine shut-downs or emergencies. At the largest accelerator of all, the in Geneva, Switzerland, researchers have devised an elaborate off-ramp procedure able to bring beams of protons (particles found inside every atom) traveling at nearly the to a dead halt in a fraction of a second. The beams carry enough energy to melt a ton of copper.

At LHC, the "road" the beams travel is a sixteen-mile ring-shaped tunnel, and the off-ramp looks like an immense pencil -- it's a piece of graphite about three feet wide, 26 feet long, wrapped with steel, water cooled, and encased in concrete.

The difficulty of stopping the isn’t the large number of protons involved, a hundred trillion or more at any moment. That sounds like a lot, but all those protons -- if they were atoms at room temperature -- wouldn’t be enough to inflate a basketball.

Rather, the difficulty lies in the amount of energy in the protons. "When the machine achieves full operation," said Robert Appleby, "the energy of the protons will be about 360 mega-joules, equivalent to the energy of an aircraft carrier moving through the ocean at a speed of 20 knots." And all that energy is concentrated in a beam that’s thinner than a frail bit of thread. Appleby is one of the scientists who worry about seeing that the LHC runs smoothly.

The LHC is in the business of doing exotic physics -- using this energy to create unstable particles that, the researchers hope, will reveal information about the fundamental structure of the universe.

The protons in the beams (parceled into bunches) race around the LHC machine many thousands of times per second. When the machine is to be turned off, said Appleby, the beam can be siphoned off, bunch by bunch, and shot sequentially into the graphite dump. The bunches are aimed so that they don’t all hit the graphite at the same place. This prevents a meltdown of the dump. This is how you can absorb an aircraft-carrier-amount of energy in a fraction of a second.

Appleby's study of what happens in the case of an LHC-shut-off will be published in an upcoming article in the journal Physical Review Special Topics.

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User comments : 5

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not rated yet Jul 15, 2010
Ho ho! Not as easy as emergency quenching an MRI field. Who knows how?

Vent the helium refrigerant, de-superconducts, sinks the energy and all for only $100K or so.
5 / 5 (7) Jul 15, 2010
That aircraft carrier comparison caught my attention.

By my calculation, 360 MJ at 20 knots corresponds to about 6,800 metric tons. According to wikipedia, a Nimitz class carrier is about 101,000 metric tons. So that would be a pretty small aircraft carrier.

It's still an impressive amount of energy for less than a nanogram of protons.
not rated yet Jul 15, 2010
@kcameron: Good one!
not rated yet Jul 16, 2010
Knots are not SI accepted units.
20 knots are roughly 10 m/s
Note that the true destructive power (penetrative capability) is caused by the impulse and not by the energy.
Seems they are in need of impressing us.
5 / 5 (1) Jul 16, 2010

To be fair, the Nimitz class is one of what are commonly called Supercarriers. 20,000 (short) tons is a more common Aircraft carrier size. Though, still your right that 6,800 metric tons is a bit small for the comparison. (Incidently, a 20,000 short ton ship @ 20 knots over 1 meter would be ~3,730 Mega-Joules! 8D )

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