Knocking on Heaven's Door - Lisa Randall [117]
CMS’s enormous solenoidal magnet made of refrigerated superconducting niobium-titanium coils is 12.5 meters long and six meters in diameter. This magnet is the defining feature of the detector and is the largest magnet of its type ever made. The solenoid has coils of wire surrounding a metal core, generating a magnetic field when electricity is applied. The energy stored in this magnet is the same as that generated by a half-metric ton of TNT. Needless to say, precautions have been taken in case the magnet quenches and suddenly loses superconductivity. The solenoid’s successful 4-tesla test was completed in September 2006, but it will be run at a slightly lower field—3.8 tesla—to ensure greater longevity.
The solenoid is sufficiently big to enclose the tracking and calorimeter layers. The muon detectors, on the other hand, are on the outer perimeter of the detector, outside the solenoid. However, the four layers of muon detector are interlaced with a huge iron structure surrounding the magnetic coils that contains and guides the field, ensuring uniformity and stability. This magnetic return yoke, 21 meters long and 14 meters in diameter, reaches to the full seven-meter radius of the detector. In effect, it also forms part of the muon system since the muons should be the only known charged particles to penetrate the 10,000 metric tons of iron and cross the muon chambers (though in reality energetic hadrons will sometimes also get in, creating some headaches for the experimenters). The magnetic field from the yoke bends the muons in the outer detector. Since the amount muons bend in the field depends on their momenta, the yoke is vital to measuring muons’ momenta and energy. The structurally stable enormous magnet plays another role as well. It supports the experiment and protects it from the giant forces exerted by its own magnetic field.
The ATLAS magnet configuration is entirely different. In ATLAS, two different systems of magnets are used: a 2-tesla solenoid enclosing the tracking systems and huge toroidal magnets in the outer regions interleaved with the muon chambers. When you look at pictures of ATLAS (or the experiment itself), the most notable elements are these eight huge toroidal structures (seen in Figure 34) and the two additional toroids that cap the ends. The magnetic field they create stretches 26 meters along the beam axis and extends from the start of the muon spectrometer 11 meters in the radial direction.
Among the many interesting stories I heard when visiting the ATLAS experiment was how when the magnets were originally lowered by the construction crews, they started off in a more oval configuration (when viewed from the side). The engineers had factored in gravity before installing them so they correctly anticipated that after some time, due to their own weight, the magnets would become more round.
Another story that impressed me was about how ATLAS engineers factored in a slight rise of the cavern floor of about one millimeter per year caused by the hydrostatic pressure from the cavern excavation. They designed the experiment so that the small motion would put the machine in optimal position in 2010, when the initial plan was to have the first run at full capacity. With the LHC delays, that hasn’t been the case. But by now, the ground under the experiment has settled to the point that the experiment has stopped moving, so it will remain in the correct position throughout operation. Despite Yogi Berra’s admonition that it’s “tough to make predictions, especially about the future,”52 the ATLAS engineers got it right.
COMPUTATION
No description of the LHC is complete without describing its enormous computational power. In addition to the remarkable hardware that