Large Hadron Collider (LHC) magnet interconnection

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.