BEAM PIPE (blue) being installed into ATLAS, one of two main particle detectors built to study collisions in the Large Hadron Collider outside Geneva, which is slated for a key test this weekend. Image: © CERN
After 14 years of construction and $8 billion, the world's mightiest particle accelerator is about to get a taste of what it was built for.
The Large Hadron Collider (LHC), nearing readiness outside Geneva, Switzerland, was designed to smash protons together at the highest energies ever achieved in hopes of unlocking new secrets of the universe. But to date, all that's traveled through its circular beam pipe are ping-pong balls to test for obstructions.
That's all about to change. This weekend, CERN, the European Organization for Nuclear Research, plans to test a key component of the accelerator by injecting a low-intensity beam of protons clockwise into the LHC and letting it travel three kilometers (two miles) through the machine.
Assuming all goes as planned, the lab announced today that it will send the first beam around all 27 kilometers (17 miles) of pipe on September 10, the machine's official start-up date.
This weekend's test will mark CERN's first attempt to feed protons (or, simply, "beam") into the LHC from a chain of smaller accelerators. These feeder accelerators cannot inject straight into the LHC because their pipes are enclosed in bulky magnets that steer the protons.
Instead, protons enter the LHC ring at an angle. That means a magnet has to nudge the protons to enter the circular beam pipe on the tangent. This "kicker" magnet, which CERN has never had the chance to test until now, must switch on at precisely the right moment to nudge the near light-speed beam, and then switch off just as fast.
If the test works, "I think we'll have some very happy people around this weekend," CERN spokesperson James Gillies said. It might take a few attempts, he added.
Then, in coming weeks, CERN plans to inject a second, counterclockwise beam from a different point on the ring. The LHC is designed to collide two opposing beams—one clockwise and one counterclockwise—each with an energy of seven tera-electron volts (TeV), equivalent to the energy of a speeding train. (A TeV is equal to one trillion electron-volts.)
Researchers believe that when the two beams smash together for a combined 14 TeV, never-before-seen particles may pop out, such as the long-sought Higgs boson, thought to be the source of matter's mass.
Powerful superconducting magnets, cooled by liquid helium to within 1.9 kelvins (3.4 degrees Fahrenheit) of absolute zero (–459.67 degrees Fahrenheit, or –273.15 degrees Celsius), keep the protons on course and accelerate them to higher speeds. The circulating beam that CERN plans to inject on September 10 will carry the minimum energy possible for the LHC, only 0.45 TeV. The lab plans to push the beams to a combined 10 TeV by year's end.
Workers are still finishing cooling down the eight sectors of the LHC, which are defined by eight points along the ring where protons may enter or collide. In this weekend's test, researchers will send beam from the second point to a removable obstruction placed at the third—the only sector that has reached operating temperature.
Switching on an accelerator for the first time is liable to uncover a few glitches, says Peter Limon, a physicist at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., who spent the last two years at CERN.
He says that when Fermilab sent its first full beam through the Tevatron machine in 1983, it unexpectedly stopped after a few turns around the six-kilometer (four-mile) ring. The problem: a Kimwipe, a tissue paper used in labs, was clogging the pipe.
A similar obstruction would set back the LHC schedule by weeks. To remove it, researchers would have to go through the slow process of warming the magnets around the obstruction (access to which would otherwise be blocked by the liquid helium cooling jacket) and then re-cooling them.
But engineers are hoping that their pretesting—using compressed air to fire a ping-pong ball down the pipe, Gillies says—will prevent such a mishap. "It's a very simple solution to the problem, but it's very effective, too," he says. "If the ping-pong ball appears, then you know that the beam pipe's clear."
Limon agrees it would be an unlikely outcome. More likely, he says, would be problems in a few of the thousands of sensors strung along the pipe that monitor the position and intensity of beam and look for slight deviations in it such as wobbling to the right or left.
Electrical cables plugged in the wrong way would switch the "polarity" of a sensor, causing it to confuse left and right. "Much of this stuff has been tested, but none of it has been tested with beam," Limon says.
If any of the LHC's 1,200 bending magnets are even slightly tilted, he says, the beam would veer up or down out of the ring's plane. In that case, CERN would adjust the strength of compensating magnets to coax the beam back into shape.
Without a test, it's impossible to know, Limon says: "In the end, there's nothing like beam."