Shutting Off the Large Hadron Collider
How particle beams are brought to a safe halt

Engineers at particle accelerators must be able to halt intense beams of particles during routine shut-downs or emergencies. At the Large Hadron Collider researchers have devised an elaborate off-ramp procedure able to bring beams of protons traveling at nearly the speed of light 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 proton beam 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 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 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.

The protons in the beams race around the LHC machine many thousands of times per second. When the machine is to be turned off 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.

Source: InsideScience.org