Knocking on Heaven's Door - Lisa Randall [69]
The final accelerating electric fields in this tunnel are not arranged in a precisely circular fashion. The LHC has eight large arcs alternating with eight 700-meter-long straight sections. Each of these eight sectors can be independently heated up and cooled down, which is important for repairs and instrumentation. After entering the tunnel, protons are accelerated in each of the short straight sections by radio waves, much as they were in the previous acceleration stages that brought them up to injection energy. The acceleration occurs in radio-frequency (RF) cavities that contain a 400 MHz radio signal, which is the same frequency you use when you remotely unlock your car door. When this field accelerates a proton bunch that enters such a cavity, it increases the energy of the protons by a mere 485 billionths of a TeV. This doesn’t sound like much, but the protons orbit the LHC ring 11,000 times a second. Therefore, it takes only 20 minutes to accelerate the proton beam from its injection energy of 450 GeV to its target energy of 7 TeV, about 15 times higher. Some protons are lost during collisions or stray loose, but most of those protons will continue to circulate for about half a day before the beam is depleted and needs to be dumped into the ground and replaced by fresh newly injected protons.
By design, the protons that circulate in the LHC ring aren’t uniformly distributed. They are sent around the ring in bunches—2,808 of them—each containing 115 billion protons. Each bunch starts off 10 centimeters long and one millimeter wide and is separated from the next bunch by about 10 meters. This helps with the acceleration since each bunch is accelerated separately. As a bonus, bundling the protons in this way guarantees that proton bunches interact at intervals of at least 25–75 nanoseconds, which is long enough apart that each bunch collision gets recorded separately. Since so many fewer protons are in a bunch than in a beam, the number of collisions that happen at the same time is under much better control because it is bunches, rather than the full quota of protons in the beam, that will collide at any one time.
CRYODIPOLE MAGNETS
Accelerating the protons to high energy is indeed an impressive achievement. But the real technological tour de force in building the LHC was designing and creating the high-field dipole magnets necessary to keep the protons properly circulating around the ring. Without the dipoles, the protons would go along a straight line. Keeping energetic protons circulating in a ring requires an enormous magnetic field.
Because of the existing tunnel size, the major technical engineering hurdle LHC engineers had to contend with was building magnets as strong as possible on an industrial scale—that is, they could be mass produced. The strong field is required to keep high-energy protons on track inside the hand-me-down tunnel that LEP had bequeathed. Keeping more energetic protons circulating requires either stronger magnets or a bigger tunnel so that the proton paths curve sufficiently to stay on track. With the LHC, the tunnel size was predetermined, so the target energy was governed by the maximum attainable magnetic field.
The American Superconducting Supercollider, had it been completed, would have resided in a much bigger tunnel (which in fact was partially excavated), 87 kilometer in circumference, and was planned with the goal of achieving 40 TeV—almost three times the LHC’s target energy. This vastly greater energy would have been possible because the machine was being designed from scratch, without the constraint in size of an existing tunnel and the consequent requirement of