CERN Open Day

April 6, 2008

CERN had an open day for the Large Hadron Collider before they turned it on. A group of us, biologists all except for me, headed out bright and early on Sunday morning from Lausanne to see the great machine, along with 15,000 other people.

The primary attractions were of course the detectors and a tour of the tunnels, but by the time we arrived at the Meyrin site at about 11:00, there was at least a four hour wait to get a ticket, assuming there were any tickets left at that point. This didn't stop us: CERN had arranged many other attractions. There were demos of superconductivity and superfluidity, the requisite freezing of things in liquid nitrogen and shattering them for the children, and a display of artwork inspired by the LHC. I didn't see any of this.

I dragged the unfortunate biologists who accompanied me to the magnet factory, to the magnet testing center, and to the prototype of the linear collider that will succeed LHC. They were good sports, even as I quizzed them on the discoveries of the famous physicists whose names the streets bear.

One thing that astonished them was the amount of prefabricated construction. The buildings aren't pretty. I explained that this is a working lab: the buildings have to go up fast, and if you need a hole in the wall, you can't wait for approval. You just grab a drill. Under these conditions, prefab is the best option.

CERN put an enormous amount of effort into this open day. The magnet factory had magnets in various stages of construction set up throughout the room, and the engineers of the facility giving tours in English, French, and German. We were lucky enough to get a tour by one of the head engineers of the division, who gave us a wonderfully detailed description of the construction process.

In any circular accelerator, you have to bend the beam, accomplished with magnetic dipoles, and you have to focus it, using quadrupoles. The LHC ring consists of a series of 50m long dipoles with smaller quadrupoles of 6m interspersed. Protons traverse narrow tubes through the length of these magnets. The magnets look almost straight, but the accelerator is a circle. They must be curved. But if you do the calculation for an accelerator 27km in diameter, a proton only has to shift 7mm to the side in a 50m tube.

Actually, the quadrupoles are straight. The dipoles are ever so slightly curved: the physicists insisted that the beam could deviate no more than 1mm, not 7mm, from the center of its containing tube all the way along the accelerator. Our guide recounted a scene anyone who has dealt with physicists will find familiar:

"We need 1mm precision the whole way."

"Impossible."

"1mm."

"Alright, it's possible, but it will be enormously expensive."

"1mm."

To curve them, they string the plates that form the magnets on the beam tube, put the whole thing in an enormous press, forcefully bend it, then weld it in that shape.

How do they know they have met their required precision? They transfer the magnet to a sealed room where they use a laser/reflector system to measure the geometry to a fraction of a millimeter precision.

Once it has passed that test, the magnet is transferred to the testing facility, which we also visited. Here they seal the magnet into its insulating jacket, insert the tubes that carry liquid helium to cool the coils of the magnet, and check that it's air tight.

The coils are superconducting. This is one of the most important facts about LHC: it's what makes the machine possible. A superconducting wire can carry seven hundred times the current of a copper wire of the same cross section. A comparable magnet made with superconducting wire is 25 times smaller than its copper counterpart. LHC's coils are a few cm across. In copper they would be almost a meter. In the tight spaces of LHC's underground tunnels, this is a vital concern.

The superconductors carry a price though: NbTi, the only one commercially viable when LHC's development began, isn't superconducting above a couple degrees above absolute zero. The only practical coolant at these temperatures is liquid helium. Making and distributing that much liquid helium demands cryogen facilities as expansive as the magnets themselves.

The test facility has a direct link to the tunnels. When a magnet is declared complete, it is lowered 100m to the tunnels and slowly, carefully dragged to its final position. The pit was closed to prevent anyone falling in it, but they had a movie of the magnets being hauled at 2km/h through the tunnels, with a selection of charmingly incongruous background music along the lines of 'Carmina Burana' or the closing march from 'Star Wars.'

All of this occupied our afternoon, after we had eaten lunch at a nearby Indian restaurant, and half our party (including a nine and ten year old boy) had departed. Before lunch was the hilight of the day: CLIC, the Compact Linear Collider, or rather its prototype. LHC smashes protons together. Protons are heavy, which makes it easy to reach high energies, but they consist of three particles. Making sense what happened when two protons, six particles, smashing into each other is difficult. LHC gets us to high energies to see what's there. Then we need a collider that uses truly elementary particles --- in this case electrons and positrons.

The day of circular electron collider is over. Electron radiate their energy as X-rays when dragged in a circle, and it swiftly becomes impractical to push energy in faster than it radiates. Modern facilities using electrons are straight, but unlike in circular accelerators where you can increase the energy just a bit with every circuit, all the energy must be given in one pass. As the energy grows, the distance you need to do this gets longer and longer.

CERN's cost constraints dictate an accelerator no longer than 50km, but you can't get close to the target energy of 3TeV in this distance. CLIC's designers have found an incredibly clever solution.

Instead of accelerating one electron to 3 TeV, accelerate a thousand in a bunch to 3 GeV, which is perfectly possible in a reasonably sized linear collider. How does this get us closer to 3 TeV? It's only the energy of individual particles that count, not the combined energy of all of them.

Someone person figured out how to build a device, two specially shaped metal chambers connected by a mass of fiber optic cable, that saps 96% of the energy from those thousand electrons as they fly into one chamber, and transfers it all to one electron just entering the other chamber. That single electron goes flying out at the required 3 TeV. The technical difficulties are enormous, but suddenly a sub-50km, 3 TeV collider seems possible.

It was a lovely day. My biological colleagues learned something about smashing very small things, and I relived my childhood dreams of building particle accelerators. And I bought a t-shirt with the Lagrangian of the standard model on the front.


Fred Ross
April 2008
Lausanne, Switzerland


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