Knocking on Heaven's Door (23 page)

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 unrealistically large magnetic fields. However, the proposed European plan had the practical advantage that the tunnel and the CERN infrastructure of science, engineering, and logistics already existed.

One of the most impressive objects I saw when I visited CERN was a prototype of LHC’s gigantic cylindrical dipole magnets. (See Figure 26 for a cross section.) Even with 1,232 such magnets, each of them is an impressive 15 meters long and weighs 30 tons. The length was determined not by physics considerations but by the relatively narrow LHC tunnel—as well as the imperative of trucking the magnets around on European roads. Each of these magnets cost €700,000, making the net cost of the LHC magnets alone more than a billion dollars.

The narrow pipes that hold the proton beams extend inside the dipoles, which are strung together end to end so that they wind through the extent of the LHC tunnel’s interior. They produce a magnetic field that can be as strong as 8.3 tesla, about a thousand times the field of the average refrigerator magnet. As the energy of the proton beams increases from 450 GeV to 7 TeV, the magnetic field increases from 0.54 to 8.3 teslas, in order to keep guiding the increasingly energetic protons around.

The field these magnets produce is so enormous that it would displace the magnets themselves if no restraints were in place. This force is alleviated through the geometry of the coils, but the magnets are ultimately kept in place through specially constructed collars made of four-centimeter-thick steel.

[
FIGURE 26
]
Schematic of a cryodipole magnet. Protons are kept circulating around the LHC ring by 1232 such superconducting magnets.

Superconducting technology is responsible for the LHC’s powerful magnets. LHC engineers benefited from the superconducting technology that had been developed for the SSC, as well as for the American Tevatron collider at the Fermilab accelerator center near Chicago, Illinois, and for the German electron-positron collider at the DESY accelerator center in Hamburg.

Ordinary wires such as the copper wires in your home have resistance. This means energy is lost as the current passes through. Superconducting wires, on the other hand, don’t dissipate energy. Electrical current passes through unimpeded. Coils of superconducting wire can carry enormous magnetic fields, and, once in place, the field will be maintained.

Each LHC dipole contains coils of niobium-titanium superconducting cables, each of which contains stranded filaments a mere six microns thick—much smaller than a human hair. The LHC contains 1,200 tons of these remarkable filaments. If you unwrapped them, they would be long enough to encircle the orbit of Mars.

When operating, the dipoles need to be extremely cold, since they work only when the temperature is sufficiently low. The superconducting wires are maintained at 1.9 degrees above absolute zero, which is 271 degrees Celsius below the freezing temperature of water. This temperature is even lower than the 2.7-degree cosmic microwave background radiation in outer space. The LHC tunnel houses the coldest extended region in the universe—at least that we know of. The magnets are known as
cryodipoles
to take into account their special refrigerated nature.

In addition to the impressive filament technology used for the magnets, the refrigeration
(cryogenic)
system is also an imposing accomplishment meriting its own superlatives. The system is in fact the world’s largest. Flowing helium maintains the extremely low temperature. A casing of approximately 97 metric tons of liquid helium surrounds the magnets to cool the cables. It is not ordinary helium gas, but helium with the necessary pressure to keep it in a
superfluid phase.
Superfluid helium is not subject to the viscosity of ordinary materials, so it can dissipate any heat produced in the dipole system with great efficiency: 10,000 metric tons of liquid nitrogen are first cooled, and this in turn cools the 130 metric tons of helium that circulate in the dipoles.

Not everything at the LHC is beneath the ground. Surface buildings hold equipment, electronics, and refrigeration plants. A conventional refrigerator cools down the helium to 4.5 kelvin and then the final cooling takes place with the pressure reduced. This process (as well as warming up) takes about a month, which means that each time the machine is turned on and off, or any repair is attempted, a good deal of additional time is required to cool.

If something went wrong—for example a tiny amount of heat capable of raising the temperature—the system would
quench,
meaning that superconductivity would be destroyed. Such a quenching would be disastrous if the energy were not properly dissipated, since all the energy stored in the magnets would suddenly be released. Therefore, a special system for detecting quenches and spreading the energy release are in place. The system looks for differences in voltage inconsistent with superconductivity. If detected, the energy is released everywhere, within less than a second, so that the dipole will no longer be superconducting.

Even with superconducting technology, huge currents are needed to achieve the 8.3 tesla magnetic field. The current goes up to almost 12,000 amperes, which is about 40,000 times the current flowing through the lightbulb on your desk.

With the current and the refrigeration, the LHC when running uses an enormous amount of electricity—about the amount required for a small city such as nearby Geneva. To avoid excessive energy expenditures, the accelerator runs only until the cold Swiss winter months when electricity prices go up (with an exception made for the turn-on in 2009). This policy has the extra advantage that it gives the LHC engineers and scientists a nice long Christmas vacation.

THROUGH VACUUM TO COLLISIONS

The final LHC superlative applies to the vacuum inside the pipes where the protons circulate. The system needs to be kept as free as possible of excess matter in order to maintain the cold helium because any stray molecules could transport away heat and energy. Most critically, the proton beam regions have to be as free of gas as possible. If gas were present, protons could collide with it and destroy the nice circulation of the proton beam. The pressure inside the beams is therefore extremely tiny, 10 trillion times smaller than atmospheric pressure—the pressure one million meters above the Earth’s surface where the air is extremely rarified. At the LHC, 9,000 cubic meters of air was evacuated to achieve the welcoming space for the proton beam.

Even at this ridiculously low pressure, about three million molecules of gas still reside in every cubic centimeter region in the pipe, so protons do occasionally hit the gas and get deflected. Were enough of these protons to hit a superconducting magnet, they would quench it and destroy the superconductivity. Carbon collimators line the LHC beam in order to remove any stray beam particles that lie outside a three-millimeter aperture, which is plenty large enough to permit the approximately millimeter-wide beam to pass through.

Still, organizing the protons in a millimeter-wide bunch is a tricky task. It is accomplished by other magnets, known as
quadrupole
magnets, that effectively focus and squeeze the beam. The LHC contains 392 such magnets. Quadrupole magnets also divert the proton beams from their independent paths so that they can actually collide.

The beams don’t collide precisely or completely head-on, but rather at the infinitesimal angle of about a thousandth of a radian. This is to ensure that only one bunch from each beam collides at a time so that the data are less confusing and the beam stays intact.

When the two bunches from the two circulating beams collide, one hundred billion protons are up against another bunch of 100 billion protons. Quadrupole magnets are also responsible for the especially daunting task of focusing the beams at the regions along the beam where collisions occur and experiments that record the events are situated. At these locations, the magnets squeeze the beams to the tiny size of 16 microns. The beams have to be extremely small and dense so that the hundred billion protons in a bunch are more likely to find one of the hundred billion protons in the other bunch when they pass through.

Most of the protons in a bunch won’t find the protons in the other bunch, even when they are directed toward each other so as to collide. Individual protons are only about a millionth of a nanometer in diameter. This means that even though all these protons are kept in bunches of 16 microns, only about 20 protons collide head-on each time the bunches cross.

This is in fact a very good thing. If too many collisions occurred simultaneously, the data would simply be confusing. It would be impossible to tell which particles emerged from which collision. And of course if no collisions occurred, that would be a bad thing as well. By focusing just this number of protons into just this size, the LHC ensures the optimal number of events each time bunches cross.

The individual proton collisions, when they occur, do so almost instantaneously—in a time about 25 orders of magnitude less than a second. This means the time between the sets of proton collisions is set entirely by how frequently the bunches cross, which at full capacity is about every 25 nanoseconds. The beams are crossing more than 10 million times a second. With such frequent collisions, the LHC produces a huge amount of data—about a billion collisions per second. Fortunately, the time between bunch crossing is long enough to let the computers keep track of the interesting individual collisions without confusing collisions that originated in different bunches.

So in the end, the extremes at the LHC are necessary to guarantee both the highest possible energy collisions and the largest number of events that the experiments can handle. Most of the energy just stays in circulation with only the rare proton collision worthy of attention. Despite the massive energy in the beams, the energy of individual bunch collisions involves little more than the kinetic energy of a few mosquitoes in flight. These are protons colliding—not football players or cars. The LHC’s extremes concentrate energy in an extremely tiny region, and in elementary particle collisions that experimenters can follow. We’ll soon consider some of the hidden ingredients that they might find and the insights into the nature of matter and space that physicists hope those discoveries will provide.

THE RETURN OF THE RING

I entered graduate school for physics in 1983. The LHC was first officially proposed in 1984. So in some sense I’ve been waiting for the LHC for the quarter century of my academic career. Now, at long last, my colleagues and I are finally seeing LHC data and realistically anticipating the insights into mass, energy, and matter that the experiments could soon reveal.

The LHC is currently the most important experimental machine for particle physicists. Understandably, as it commenced operation, my physicist colleagues became increasingly anxious and excited. You couldn’t enter a seminar room without someone inquiring about what was happening. How much energy would collisions achieve? How many protons will beams contain? Theorists wanted to understand minutiae that had previously been almost an abstraction to those of us engaged in calculations and concepts and not machine or experimental design. The flip side was true as well. Experimenters were as eager as I’d ever seen them to hear about our latest conjectures and learn more about what they might look for and possibly discover.