Imagine a tunnel, circular in shape and 17 miles long. Its interior is 20 feet across (see the picture above). It is deep underground — as deep as 300 feet. In this tunnel lives the largest particle accelerator ever built.
When the machine is running, the accelerated particles — usually these will be protons (the particles which combine with neutrons to make up the nuclei of atoms) — will go round and round in the tunnel, some clockwise, some counterclockwise. They will travel in packs, or “bunches”, with about 100,000,000,000 protons in each bunch. The protons will travel at a speed that is just a tiny bit below the speed of light, circling the tunnel about 10,000 times a second.
At specified locations the two beams of bunches of protons will be aimed straight at each other. There, up to 40,000,000 times a second, a bunch heading clockwise will pass through a bunch heading counterclockwise — 100,000,000,000 protons heading to the right meeting 100,000,000,000 protons heading to the left — and everything will be arranged by the machine’s operators so that there will be a high probability that a few collisions of two protons, one from each bunch, will occur every time…
The energy of each collision will be more than 14000 times the energy that one would need to create a proton from scratch (using E = m c-squared). That’s 7 times more per collision than the collisions currently obtained at the Fermilab accelerator near Chicago. (As of 2011, the machine is operating with 1/2 the maximum energy — still plenty for discoveries!)
The protons are not free to move about the tunnel; they are carefully contained within a tiny pipe (actually two pipes, one for each beam, that cross and join at the “special locations” mentioned above). Each pipe is a few centimeters across and 17 miles around. It has to be maintained at a fantastically good vacuum — we don’t want those protons running into stray gas molecules and falling out of step with their friends, thereby disrupting the carefully organized beam.
A particle will move in a straight line unless acted on by a force; and protons are no exceptions. To keep them moving in a circle instead, inside the beampipe where they belong, requires extremely powerful magnets… a chain of them, thousands of them, as much as 30 feet long, surrounding the beam pipes and extending throughout the tunnel, one after another after another…
These are no ordinary magnets. To get them to be so powerful, they must be superconducting — unlike ordinary wires, which get hot when power flows through them (think about all the heat your computer generates,) superconductors, which only work at low temperatures, stay cool. They are therefore extraordinarily efficient. But to get the superconductivity, and the strong magnetic fields needed to operate the LHC, requires that these magnets be colder than outer space! They must be maintained at 1.9 degrees above absolute zero. (Outer space is at 2.7 degrees.) To do this requires a refrigeration system, which surrounds the magnets with liquid helium. This refrigeration system is by far the largest of its type ever built.
For a cross-section through the LHC, see http://www.computerweekly.com/galleries/235347-2/CERN-LHC-bending-magnets.htm , which shows the two beampipes side by side, surrounded by a magnet, which is housed inside the refrigeration system. (The beampipes are each a couple of inches — several centimeters — across.)
These are the main ingredients for the accelerator. Particles to accelerate. A pipe to hold them. Magnets to steer them. A refrigeration system to keep the magnets extremely cold, and therefore very powerful. There’s more (I haven’t told you how the particles are brought up to speed) but during normal operation, these are the key elements of the machine.
But that’s the accelerator itself. The science is actually done by studying the collisions that occur at those specialized locations I mentioned. Other, separate machines, called “detectors”, the size of a small office building, have been built around each of these locations, to allow the debris from the collisions to be observed, studied and interpreted. These detectors are a story unto themselves… to be told later…
April 2, 2009