Warmer amps for the LHC
CERN is working together with an Italian company to develop superconducting cables that can function at temperatures of up to 25 K (-248°C). This will make it possible to move LHC magnet power supplies out of the tunnel, protecting them from exposure to the showers of very high-energy particles produced by the accelerator.
Figure 1: devices of this type, which measure approximately 10 metres in length, are inserted between the accelerating magnets at different points along the LHC.
When it comes to consuming electricity, the magnets that steer particles through large accelerators can be characterised with just one word: greedy. For the LHC, the total current can reach 1.5 million amps. At the present time, this current is brought in via copper cables of up to 10 cm 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 (see figure 1).
Until now, this supply system did not pose any major problems. However, in the future it could become a serious handicap. 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, in charge 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.” This is the heart of the matter. 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). The system is already as big as it can be. So this is where the new candidate comes in. “Currently we are working with an Italian company called Columbus to develop new SC wires based on magnesium diboride (MgB2),” explains Amalia Ballarino, head of 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.
Figure 2: the black tube is the semi-flexible cryostat that will cool the MgB2 cables from the tunnel to the surface. This particular tube, which is currently in SM18 undergoing testing, is 20 metres long with a diameter of approximately 16 cm.
“This superconductor can be cooled using helium gas (as opposed to liquid helium), greatly simplifying the demands made on the cryogenic system (see figure 2),” stresses Amalia 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, Amalia’s team and Columbus developed high-performance round wires. An important step forward!
* This is the name given to a variant of the technology used for manufacturing superconducting wires.
by Anaïs Schaeffer