Large Hadron Collider (LHC) Dipole Magnet Cutaway

Dipole Magnet - Guiding the protons around the ring This is a cut-through of the coil of a dipole magnet, that generates the magnetic field used to bend the paths of circulating protons. Looking closely, you can distinguish insulated cables made of individual wires. High and extremely stable magnetic fields are needed for guiding the proton beams, so a superconducting material called Niobium-Titanium was chosen for the wires. At very low temperatures, superconductors have no electrical resistance and therefore no power loss. They carry a very stable current of 13.000 amps, about 20.000 times that used to power this screen. In addition to dipole magnets, the Large Hadron Collider contains quadrupoles and other higher order magnets, used to prepare the proton beams for collision. Dipoles are two pole magnets used for bending the beams of protons around the ring. Quadrupoles have four magnetic poles and are used for focusing the beam, squeezing protons closer together to increase the chance of collision when the beams cross inside the experiments. In total, the LHC uses more than 50 different types of magnet to adjust the particle beams even more finely. The Beam-Pipe - Where the beams of protons circulate Proton beams can circulate for over 10 hours in the Large Hadron Collider. Over this time, protons make four hundred million revolutions of the 27 km machine, traveling a distance equivalent to the diameter of the solar system. They must travel in a pipe that is emptied of air, to avoid collisions with molecules of gas. The beam-pipes are therefore pumped down to an air pressure similar to that on the surface of the moon. There are two pipes, one for each direction of the circulating beams. The two beams only meet inside the four experiments where collisions take place. Liquid Helium - Bringing in the cooling fluid This pipe carries liquid helium through the Large Hadron Collider magnets to keep them at 1.9 degrees above absolute zero - about 300 degrees below room temperature. 800'000 litres of superfluid helium are used to cool down the 36'000 tonnes of equipment. This is the world's biggest cryogenic installation and its reliability and efficiency is essential for the magnets. The pipe connects to the main cryogenic line that you can see running along behind the blue magnets via "jumper connections" like the one to your right. Support Post - Insulating and extremely tough The magnet supports bridge a difference in temperature of nearly 300 degrees! Electrical connections, instrumentation and the posts on which the magnets stand are the only points where heat transfer can happen through conduction. They are all carefully designed to draw off heat progressively. The posts are made of 4 mm thick glass-fibre - epoxy composite material. Each post supports 10'000 kg of magnet and leaks just 0.1 W of heat. There are three per magnet. Magnet Collars - Preventing the wires from moving The LHC accelerates two proton beams moving in opposite directions, so it is really two accelerators in one. To keep the machine as compact and economical as possible, two magnets are built into a single housing that must withstand enormous electromagnetic forces. These forces tend to open-up the coils, and squeeze them. At full field, the force on one metre of coil is comparable to the weight of a jumbo jet. Great care must be taken to prevent movements as the field changes - any friction could create hot spots that would cause the wire to lost its superconducting stage. Magnet collars made from reinforced steel keep the coils firmly in place. Insulation - Preventing heat from leaking In The LHC, beam-tube and magnets are inside a vacuum tank to reduce to a minimum the heat flowing in through convection. To prevent heat inflow through radiation, they are surrounded by a super insulator - multi-layer, reflective, aluminized Mylar. Then to prevent heat flow via conduction, ingenious solutions had to be found for the electrical connections and the support posts. Iron Yoke - Shielding the magnetic field The LHC magnet cables are surrounded by a layered iron yoke that shields the powerful magnetic field - 100'000 times stronger than the Earth's - so that stray fields outside the magnet are negligible. This action also helps enhance the magnetic field within the beam-pipe, where it is needed for control of the proton beams. In addition, the layers of iron yoke, called laminations, play a role together with the magnet collars in keeping cables from moving when the magnet powers up. The technical challenge of manufacturing the laminations centred on ensuring both strength and magnetic homogeneity across a large-scale production. Over 6 million laminations are needed for the 1232 dipole magnets installed around the LHC's 27km ring.