Need to Test the World's Largest Particle Accelerator? Try a Ping-Pong Ball

The Large Hadron Collider, the largest and highest-energy particle accelerator in the world -- the thing some people once feared would destroy the Earth, and the thing which so far has not -- is a complicated piece of machinery. It is composed of eight straight sections joined together by eight arcs -- 17 miles in circumference, in all -- and it features more than 1,600 magnets that bend and focus beams of particles as they travel close to the speed of light. Imagine all the maintenance. And complicating things even more is the fact that the LHC, when it's operating, does so at a temperature well below freezing, which causes its many metal elements to contract ever-so-slightly. Occasionally, the contraction leads to equipment buckling and then bulging and, in turn, blocking particle beams.

Which is not good, when the whole purpose of your existence is to facilitate the movement of particle beams.

So scientists at CERN, as they overhaul the collider to prepare it for future research, need to test the LHC to make sure it offers a pothole-free path for the particles that travel it. And they have developed a delightfully low-tech tool for doing that: a ping-pong ball. Technically a "radiofrequency ball," because Science ... but the lightweight sphere is a pretty faithful rendition of the one you might know from college or camp or world-class sporting events.

CERN's modified version is slightly smaller than a regulation ping-pong ball: 34 mm in diameter, which is also just slightly smaller than the LHC beam pipes' diameter of 36 mm. The thing is carefully sterilized. And it carries a transmitter within its hollow insides. Researchers feed it into a section of the LHC, and the ball travels along the vacuum pipe -- for about half a mile -- sucked forward by a pumping system at the other end. The ball's transmitter, yep, pings its position every 164 feet, and the transmissions are received by Beam Position Monitors (BPMs) studded along the pipe. If all goes well, the test takes about 15 minutes per section. 

And if the ball gets stuck -- if it stops transmitting a signal -- technicians can target their search to the section of the pipe between the last transmission and the silence. They use an endoscope to peer into the pipe and then, if all goes well, diagnose the problem from there.

The ping-pong solution has been in use at CERN since 2007, when researchers developed it as a way to monitor the interconnections of LHC sectors without having to open the device's pipes. ("It takes three weeks to open a sector and five weeks to close it up again," a collider installation leader explained at the time. And it also requires cooling down the collider and warming it up again -- a "months-long, labor-intensive process.") CERN's low-tech-meets-high-tech maintenance method was put to use yet again yesterday. And the collider "passed the ping-pong ball test flawlessly."

H/t Samuel Arbesman