Knocking on Heaven's Door - Lisa Randall [70]
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.