All the focus on the LHC is generally on the enormous, 26km circumference of the main accelerator ring. But, according to Paul Collier, the head of CERN's Beams Department, the protons that form the main beams of the LHC travel roughly 6 million kilometers before even reaching the LHC, and these early steps are essential to making the LHC the highest energy, highest intensity particle collider ever built. And once inside the LHC, the proton beams still need to be accelerated and shaped even further before collisions can take place. To help you understand how it all works, we'll take a complete trip from the proton source to the detectors within the LHC, and explain how each step contributes to the final output.

The first thing that needs to be understood is that, although the LHC control system refers to beams, that's not an accurate description of the protons that are circulating within it. Instead, a "beam" is actually a collection of proton bunches, a bit like a series of beads without the string to connect them. Each bunch is about 20-30µm in diameter, and a few centimeters long. The timing and control provided by the LHC is so precise that bunches only cross paths—and produce collisions—within the four areas of the LHC that have detectors present.

Bunches and their properties are the keys to determining the efficiency of the LHC. The more bunches circulating at once, the more collisions occur per unit time (a property called "luminosity"). In the same way, the more protons per bunch, the higher the LHC's luminosity. And, although the maximum energy of collisions at the LHC places it in a class by itself, it also will have the highest luminosity of any particle collider, giving it a greater chance of spotting events so rare that it would take decades to spot them with existing hardware.

So to a certain extent, key to the LHC is getting lots of bunches, each with lots of protons, into the ring at once. And to do that you have to start with the proton source.

Starting straight

The LHC will actually run experiments with both protons and heavy ions, and these two start on slightly different paths. But each starts with a linear accelerator, which produces bunches of ions/protons, and gets them up to an energy of 150MeV.

The full complex of accelerators that dumps protons into the LHC and other CERN experiments

From there, the protons are dumped into the first (roughly) circular accelerator, called Booster. This hardware dates from 1972, and manages to get the protons up to 1.4GeV in 1.2 seconds. It also starts squeezing the bunches down so that they have a smaller cross-section. According to Collier, it's harder to perform this compression at low speed—as Chris Lee noted, slower moving particles have less mass, and therefore the charge repulsion has a larger impact on them. To limit this effect in Booster, the hardware actually consists of four separate rings, stacked vertically.

When not being used to fill the LHC, protons from Booster can be diverted to the ISOLDE experiment, where they're used to bombard other atoms to produce exotic isotopes.

From there, the protons take a trip to the PS ring, which was built in 1959. It has a 628m circumference (which means a 100m radius, which Collier said "makes the math easier"), and takes 3.6 seconds to get two injections of bunches up to 26GeV. Here, carefully timed radiofrequency pulses start chopping up the bunches in halves and thirds, converting a handful of bunches into a stream.

PS can also spool out a second's worth of protons for other experiments. Some of these hit a target that produces neutrons, for the nTOF experiment. There's also some talk of using protons at these energies to "burn" nuclear waste, converting it into unstable isotopes that rapidly decay. Elsewhere, tungsten can be targeted in order to produce antiprotons, which are combined with antielectrons to produce antihydrogen atoms (CERN is testing whether they experience the same force as regular hydrogen).

The next stop on the way to the LHC involves a long trip underground to France, where protons enter the SPS. Four cycles of injections increases the bunches even further, and the SPS takes about 20 seconds to get the protons up to 450GeV. When not filling the LHC, these protons can be sent to a large collection of physics experiments in a dedicated area of CERN, or used to create neutrinos that are sent off to an underground facility in Italy.

Alternately, multiple transfers of bunches can be sent in each direction around the LHC. The ultimate goal is to have over 2,800 bunches circulating at once, but so far progress has been cautious. Collier told Ars that the initial goals were to do the hard things—more protons per bunch, beam shaping, compression, and control—first. These are all central to ensuring that the LHC can meet its designed luminosity, but the total number of collisions was kept low by a low number of bunches (initially, 10 in each direction). With the challenging stuff sorted out, the number of bunches is being ramped up rapidly, meaning that a new physics run can produce as much data as the total output of all earlier runs.

Inside the LHC

Those of you following at home may have already noticed that 450GeV is a long way off from the 3.5TeV that's the target energy for current experiments—the LHC clearly has more work to do. A glance at the screenshot below, taken from the main LHC status page, shows that beam energy remains at a stable 450GeV as several sets of bunches are transferred into the main accelerator (red and blue stepped lines). With everything in place, the black line, showing the total energy, then begins a long ramp up to 3.5TeV.

After bunches of protons are injected (red and blue lines), the beams' energy is ramped up to 3.5TeV

The slow pace of this acceleration is a product of the magnets that control the beam as it rockets around the ring. Frédérick Bordry, the head of CERN's technology department, told Ars that there are over 1,200 dipole magnets to steer and contain the beam, 400 quadrupole magnets to shape it, and over 7,000 correcting magnets. At full strength, each of these magnets contains 7MJ of energy on average, but they start off at significantly lower energies when the bunches are first injected. Injecting more energy into the magnets has to be done slowly, which dictates the extended energy ramp.

The process is carefully choreographed. At a single point in the LHC, radiofrequencies are used to accelerate the protons slightly—Collier compared the process to getting protons to surf on the radio waves. This process also helps reinforce the bunching, since any proton that has slipped off the right part of the wave will either be pushed forward into the bunch, or fall back due to a lack pushing.

If the magnets were kept at constant strength as energy is imparted to the beam, it would start to rotate at a greater and greater diameter, eventually plowing into the equipment. So the power to the magnets is increased to compensate, pulling the beam back towards the center of the LHC. This balancing between acceleration and control continues for about a half-hour, until both beams are stabilized at 3.5TeV.

With the beams at the right energy, the only thing left is to shape them for collision. The various control magnets and devices called collimators shave off any protons that have wandered too far from the bunch, and ensure that the beam reaches its narrowest point just as it enters the crossover points, where collisions take place inside one of the four detectors. That ups the density of protons, and increases the probability that some of them will collide. Once past the detector, the beam is allowed to spread a bit before being squeezed again before the next detector.

Dealing with disasters, preparing for the future

Over time, the luminosity of the collisions in the LHC's detectors declines due to the loss of protons to collisions

The number of protons lost to shaping the beams is actually relatively minor, but the beams do decay over time for a very simple reason: each time around the loop, some of their constituent protons get destroyed by collisions. Over time, this decay will reduce the luminosity of the LHC, eventually making it better to just dump the existing beams and reload the accelerator.

Dumping the beams isn't a trivial matter, either. At full power, they carry over 300MJ of energy, which various people compared to the kinetic energy of an aircraft carrier at 30 knots, and noted was sufficient to melt a ton of copper. For safety reasons, the LHC has to be able to dump the beams within 90 microseconds; the protons are simply targeted at a large mass of concrete and other inert material underground.

The longer we can avoid this process, however, the more physics will get done. Bordry suggested one way to stretch out the physics is to start with more protons per bunch, and not compress the bunch as much at first. This would still allow a high rate of initial collisions, and further compression could be started once the number of protons start to decline sufficiently. This sort of adaptive approach probably won't be implemented until the first big LHC shutdown, which will be used to upgrade it to reach its full, 14TeV collision energy.

That shutdown and upgrade were necessitated by problems with some of the electrical junctions in the hardware, which led to a catastrophic failure in 2008. Bordry said that after the accident, his team went back and rechecked every one of the over 7,000 junctions of this sort—the one that failed was the only one that was out of spec.

Nevertheless, the consequences of the failure have made things more challenging. Normally, CERN likes to keep 30 spare LHC segments available, but the repairs required 50, which is why they took a while (they're currently rebuilding their stock of spares). Installing the segments is also a painstaking process. There's only one site in the ring that they can be lowered down, and moving them within the tunnels is limited to two kilometers an hour. The junctions between the segments also have to be precisely aligned, since cooling the hardware causes them to contract by four centimeters.

Long-term, Bordry suggested that the ultimate solution to many of these challenges would be to develop magnets and an electrical system that provide the same performance, but don't require the whole machine to run at liquid helium temperatures. That technology, however, doesn't exist now, and it certainly wasn't a possibility when the LHC was being designed and built. Putting it in place would require a complete overhaul of the LHC, but the machine and its components were designed to run for decades, and plans are already in place for a major upgrade after 10 years, one intended to create the Super LHC.