Dipole model slice made in 1994 by Ansaldo. The high magnetic fields needed for guiding particles around the Large Hadron Collider (LHC) ring are created by passing 12’500 amps of current through coils of superconducting wiring. At very low temperatures, superconductors have no electrical resistance and therefore no power loss. The LHC is the largest superconducting installation ever built. The magnetic field must also be extremely uniform. This means the current flowing in the coils has to be very precisely controlled. Indeed, nowhere before has such precision been achieved at such high currents. 50’000 tonnes of steel sheets are used to make the magnet yokes that keep the wiring firmly in place. The yokes constitute approximately 80% of the accelerator's weight and, placed side by side, stretch over 20 km!

With its 27-kilometre circumference, the Large Electron-Positron (LEP) collider was the largest electron-positron accelerator ever built. The excavation of the LEP tunnel was Europe’s largest civil-engineering project prior to the Channel Tunnel. Three tunnel-boring machines started excavating the tunnel in February 1985 and the ring was completed three years later. In its first phase of operation, LEP consisted of 5176 magnets and 128 accelerating cavities. CERN’s accelerator complex provided the particles and four enormous detectors, ALEPH, DELPHI, L3 and OPAL, observed the collisions. LEP was commissioned in July 1989 and the first beam circulated in the collider on 14 July. The collider's initial energy was chosen to be around 91 GeV, so that Z bosons could be produced. The Z boson and its charged partner the W boson, both discovered at CERN in 1983, are responsible for the weak force, which drives the Sun, for example. Observing the creation and decay of the short-lived Z boson was a critical test of the Standard Model. In the seven years that LEP operated at around 100 GeV it produced around 17 million Z particles. In 1995 LEP was upgraded for a second operation phase, with as many as 288 superconducting accelerating cavities added to double the energy so that the collisions could produce pairs of W bosons. The collider's energy eventually topped 209 GeV in 2000. This object is one of the superconducting cavities from this epoch.

Particle beams circulate for around 10 hours in the Large Hadron Collider (LHC). During this time, the particles make four hundred million revolutions of the machine, travelling a distance equivalent to the diameter of the solar system. The beams must travel in a pipe which is emptied of air, to avoid collisions between the particles and air molecules (which are considerably bigger than protons). The beam pipes are pumped down to an air pressure similar to that on the surface of the moon. Much of the LHC runs at 1.9 degrees above absolute zero. When material is cooled, it contracts. The interconnections must absorb this contraction whilst maintaining electrical connectivity.

Connection between two superconducting magnets Cryostat - Keeping the magnets cold The Large Hadron Collider superconducting magnets are cooled by liquid helium to 1.9 degrees above absolute zero, or around 300 degrees below the ambient temperature in the tunnel. To keep them cold, each 30 000 kg magnet sits inside a cryostat that isolates it from the tunnel. Inside the cryostat, air is pumped out to reduce heat in-flow. Bellows - Allowing expansion and contraction When the magnets are cooled, they contract: normally 15 metres long, each magnet shrinks by 4.5 cm on its way down to 1.9 degrees above absolute zero. One side of each 30 000 kg magnet is held stationary, while the other is left free to move. Stainless steel bellows such as these absorb the contraction. Notice that every single join needs to allow for such movement, even the electrical connections. Helium Pipe - The cooling supply This pipe carries superfluid helium at 1.9 degrees above absolute zero, around 300 degrees below room temperature. As helium is cooled and put under pressure, it becomes a superfluid, with excellent thermal conductivity, ensuring the temperature is the same everywhere in the circuit. However this gives engineers an extra challenge as superfluids have unusual quantum properties. They can even creep upwards – if there are leaks in the circuit a superfluid will find them! The Large Hadron Collider is cooled by sector, of which there are eight in total. Cool down of one sector takes around 6 weeks. When the accelerator is brought back to room temperature for maintenance works, CERN recuperates the helium and stores it, so it can be reused. Niobium Titanium Cable - Bringing current to the magnets This cable carries the 13 000 amps to the Large Hadron Collider magnets. It is made from a Niobium-Titanium superconductor which is embedded in copper, to ensure an electrical connection is maintained even if the superconductor warms up and stops conducting. This happens at around 10 degrees above absolute zero. The LHC is cooled to 1.9 degrees above absolute zero, to keep the current perfectly stable. Look at the joins in the cable, called splices. They allow the wires to move over each other and retain an electrical connection, when the magnet contracts during cooling. Beam-Pipe Fingers - Keeping the electrical connection Fingers of copper slide over the beam-pipe in every connection between magnets in the Large Hadron Collider. These fingers retain an electrical contact whilst the magnets contract during cooling. The beam-pipe has double layers. The outer layer is slightly colder than the inner one so that any residual gas molecules, left behind in the tube after pumping, are drawn outwards through small holes so they cannot be disturbed by the passing proton beam. Diode - Removing the current There are many mechanisms in place to prevent friction between cable windings that might generate heat and stop the superconductor from conducting. In the eventuality the magnets do stop working, around 13 000 amps of current needs to be taken out of the system. This happens via diodes situated at the extremity of every magnet. The diode conducts a current pulse ramping in less than a second up to 13 000 amps and then slowly decaying down to zero. This process raises their temperature by several hundred degrees, so the diodes are cooled by the LHC Helium circuit.

The magnetic field must be extremely uniform. This means the current flowing in the coils has to be very precisely controlled. 50’000 tonnes of steel sheets are used to make the magnet yokes that keep the wiring firmly in place. The yokes constitute approximately 80% of the accelerator's weight and, placed side by side, stretch over 20 km.

The pulse of a particle accelerator. 128 of these radio frequency cavities were positioned around CERN's 27-kilometre LEP ring to accelerate electrons and positrons. The acceleration was produced by microwave electric oscillations at 352 MHz. The electrons and positrons were grouped into bunches, like beads on a string, and the copper sphere at the top stored the microwave energy between the passage of individual bunches. This made for valuable energy savings as it reduced the heat generated in the cavity.

The Large Electron-Positron Collider (LEP) was the largest lepton collider ever built, and gave high precision measurements of the W and Z particles. LEP was commissioned in 1989 and shut down in 2000, to leave room for the LHC. In conjunction with an energy upgrade, new, superconducting, final- focus (low-$\beta$) quadrupole magnets were built. The new magnets resembled much those built for the ISR luminosity upgrade, i.e., the coils were wound with a single rectangular wire. They operated at 4.5 K in a cryostat especially developed to fit into the limited space available in the shield placed in front of the experiment.