Knocking on Heaven's Door - Lisa Randall [79]
In 2006, after five years of construction, the last of the 1,232 dipole magnets was delivered. In 2007, the big news was the last lowering of a cryodipole and the first successful cooldown of a 3.3-km-long section to the design temperature of—271 degrees Celsius—which allowed the whole thing to be powered up for the first time, with several thousand amps circulating in the superconducting magnets in this section of the tunnel. As often happens at CERN, a champagne celebration marked the occasion.
In 2006, after five years of construction, the last of the 1,232 dipole magnets was delivered. In 2007, the big news was the last lowering of a cryodipole and the first successful cooldown of a 3.3-km-long section to the design temperature of—271 degrees Celsius—which allowed the whole thing to be powered up for the first time, with several thousand amps circulating in the superconducting magnets in this section of the tunnel. As often happens at CERN, a champagne celebration marked the occasion.
A continuous cryostat section was closed in November 2007 and everything was looking pretty good until yet another near disaster struck, this time involving the so-called plug-in modules, known as PIMs. In the United States, we didn’t necessarily follow all the reports about the LHC. But news spread about this one. A CERN colleague told me about the worry that not only had this piece failed, but it could be a ubiquitous problem all around the ring.
The problem is the almost 300-degree differential between a room-temperature LHC and a cool operating one. This difference has an enormous impact on the materials with which it is constructed. Metal parts shrink when cooled and expand when warmed. The dipoles themselves shrink by a few centimeters during the cooldown phase. This might not sound like much for a 15-meter object, but the coils must be accurately positioned to within a tenth of a millimeter to maintain the intense uniform magnetic field required to properly guide the proton beams.
To accommodate the change, dipoles are designed with special fingers that straighten out to ensure electrical continuity when the machine is cooled down and that slide back when warmed. However, due to faulty rivets, the fingers collapsed instead of recessing. Worse yet, every interconnection was subject to this failure, and it wasn’t clear which ones were problematic. The challenge was to identify and fix each faulty rivet—without introducing a huge delay.
In a tribute to the ingenuity of the CERN engineers, they found a simple method of exploiting the existing electrical pickup located every 53 meters along the beam that was initially installed so that the electronics would be triggered by the beam passage. The engineers installed an oscillator into an object about the size of a Ping-Pong ball, which they could send around the tunnel along the path a beam would take. Each sector was three kilometers long and the ball could blow through, triggering the electronics each time it passed a pickup. When the electronics didn’t record a passage, the ball had hit the fingers. The engineers could then go in and fix the problem without having to open every single interconnect along the beam. As one LHC physicist joked, the first LHC collisions were not between protons, but between a Ping-Pong ball and a collapsed finger.
After this last resolution, the LHC seemed to be on track. Once all the hardware was in place, its operation could begin. In 2008, many human fingers crossed when at long last the first test took place.
SEPTEMBER 2008: THE FIRST TESTS
The LHC forms proton beams and after a series of energy boosts injects them into the final circular accelerator. It then sends those beams around the tunnel so that they return to their precise initial position, allowing the protons to circulate many times before being periodically diverted to collide with great efficiency. Each of these steps needs to be tested in turn.