The magnets that steer particles through large accelerators are greedy for electricity. For the LHC, the total current can reach 1.5 million amps. This current is brought in via copper cables of up to 10 centimetres in diameter. In the tunnel, these cables connect the current leads - which provide the transition between the ambient-temperature cables and the magnets in their bath of superfluid helium - to the power supply. In the accelerator, the current leads are connected to the niobium-titanium (Nb-Ti) superconducting cables that bring the current to the magnets.
Though this supply system has not posed any major problems so far, it could become a serious handicap in the future. This is because the electrical power supplies, when the LHC reaches its design energy, will be exposed to streams of very high-energy particles, which could interfere with their operation. “In an ideal world, we would take the power supplies right out of the tunnel,” says Lucio Rossi of the High Luminosity LHC project (HL-LHC). “That would have the added benefit of making them accessible rapidly, without having to worry about precautions for radiation. Unfortunately, the voltage drops that are incurred with copper cables rule out using them over long distances. So what we need to do is find a way to do so with superconducting cables.”
The niobium-titanium superconducting cables in the LHC depend on a sophisticated cryogenic system that uses liquid helium at temperatures between 4.2 K and 1.9 K (-268.8 °C and -271.1 °C).
“Currently we are working with an Italian company called Columbus to develop new SC wires based on magnesium diboride (MgB2),” says Amalia Ballarino, who heads the Superconductor and Devices (SCD) Section in the TE Department. “MgB2 is considerably less expensive than High Temperature Superconductors, and offers the major advantage that it remains functional at up to 25 K (-248 °C). The material has been around since the 1950s, but its SC properties were only discovered in 2001.” With this wire, CERN will be able to build cables that can transport the high currents needed to operate the magnets—above 100 kA - from the surface to underground.
“This superconductor can be cooled using helium gas (as opposed to liquid helium), greatly simplifying the demands made on the cryogenic system,” says Ballarino. “In addition, MgB2 can function with a temperature margin of several degrees, which is a great advantage from the machine operation point of view. However, we have been faced with a difficulty: until now, the MgB2 ex situ has only been available in flat ribbons, which are unsuitable for high-amperage cables.” To overcome this problem, the team with Columbus have developed high-performance round wires.