Knocking on Heaven's Door - Lisa Randall [71]
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