Pulling together: Superconducting electromagnets
Particles zipping round the LHC at close to the speed of light must follow precise paths. Powerful magnets keep the beams stable, accurate and safe
The Large Hadron Collider (LHC) is currently operating at the energy of 6.5 TeV per beam. At this energy, the trillions of particles circle the collider's 27-kilometre tunnel 11,245 times per second. Before they reach the LHC, the particles are sped up in a series of interconnected linear and circular accelerators: once they reach the maximum speed that one part of the accelerator chain can achieve, they are shot into the next. Without any other force involved, the particles would drift apart and their momentum would carry them in a straight line. More than 50 types of magnets are needed to send them along complex paths without their losing speed.
All the magnets on the LHC are electromagnets. The main dipoles generate powerful 8.3 tesla magnetic fields – more than 100,000 times more powerful than the Earth’s magnetic field. The electromagnets use a current of 11,080 amperes to produce the field, and a superconducting coil allows the high currents to flow without losing any energy to electrical resistance.
Thousands of "lattice magnets" on the LHC bend and tighten the particles’ trajectory. They are responsible for keeping the beams stable and precisely aligned.
Dipole magnets, one of the most complex parts of the LHC, are used to bend the paths of the particles. There are 1232 main dipoles, each 15 metres long and weighing in at 35 tonnes. If normal magnets were used in the 27 km-long LHC instead of superconducting magnets, the accelerator would have to be 120 kilometres long to reach the same energy. Powerful magnetic fields generated by the dipole magnets allow the beam to handle tighter turns.
When particles are bunched together, they are more likely to collide in greater numbers when they reach the LHC detectors. Quadrupoles help to keep the particles in a tight beam. They have four magnetic poles arranged symmetrically around the beam pipe to squeeze the beam either vertically or horizontally.
Dipoles are also equipped with sextupole, octupole and decapole magnets, which correct for small imperfections in the magnetic field at the extremities of the dipoles.
When the particle beams enter the detectors, insertion magnets take over. Particles must be squeezed closer together before they enter a detector so that they collide with particles coming from the opposite direction. Three quadrupoles are used to create a system called an inner triplet. There are eight inner triplets, two of which are located at each of the four large LHC detectors, ALICE, ATLAS, CMS and LHCb. Inner triplets tighten the beam, making it 12.5 times narrower – from 0.2 millimetres down to 16 micrometres across.
After the beams collide in the detector, enormous magnets aid the measurement of particles. For example, physicists look at how charged particles bend in the magnetic field to determine their identity. Charged particles are deflected by the magnetic field in the detector, and their momentum can be calculated from the amount of deflection.
After colliding, the particle beams are separated again by dipole magnets. Other magnets minimize the spread of the particles from the collisions. When it is time to dispose of the particles, they are deflected from the LHC along a straight line towards the beam dump. A "dilution" magnet reduces the beam intensity by a factor of 100,000 before the beam collides with a block of concrete and graphite composite for its final stop.
Insertion magnets are also responsible for beam cleaning, which ensures that stray particles do not come in contact with the LHC’s most sensitive components.