Why the LHC is like an experiment in space
The Large Hadron Collider (LHC) is inside a 27-kilometre-long tunnel that is 100 metres underground, near Geneva, Switzerland. Why is that like an experiment in space?
Well, for one, the magnets of the LHC are colder than outer space. At 1.9 K, the LHC easily puts outer space (which is at 2.73 K – the temperature of the cosmic microwave background) to shame.
But more than that, it’s the fact that the magnets have to be cooled down to 1.9 K that makes the LHC more like an experiment in outer space – in the sense that if something goes wrong, you can’t just go in there and repair it. You have to wait for the machine to be warmed back up to room (or rather tunnel) temperature, before engineers can go in and do their thing.
But why is the LHC at 1.9 K? For a given tunnel, the more energetic the particles, the more powerful the magnets need to be. The LHC was designed to fit into the tunnel that had previously housed the much less powerful Large Electron Positron (LEP) collider, which at its peak operated at an energy of 209 giga-electronvolts (GeV). The LHC is designed for 14 tera-electronvolts (TeV). It needed the next generation of superconducting coils, both for so-called radio frequency cavities used to accelerate protons around the tunnel and for the supremely powerful magnets needed to bend its high energy proton beams around the tunnel’s tight curve. The RF cavities and the magnets had to be compact to fit into the small-bore tunnel, and hence had to carry extremely high currents for their size.
The designers turned to coils made of niobium-titanium, the only ones that could be made in the industrial quantities required by the LHC. But generating the extra-strong magnetic fields for the machine meant cooling the coils down to 1.9 K rather than 4.5 K (the rather more easily-reached temperature of liquid helium), so that they could carry much more current. This, however, came at a price. At that temperature, liquid helium acts as a superfluid, with weird quantum properties. It has zero viscosity and can slip through microscopic cracks. So, the thousands and thousands of welds in the plumbing had to be at least as good as those in a nuclear plant.
It’s crucial that everything works perfectly, for repairing the LHC and its detectors once they are up and running is far from trivial. The LHC takes about 5 weeks to warm up. After repairs, the LHC’s 40,000 tonnes of magnets need to be cooled back down to 1.9 °K, a process that also takes 5 weeks, requiring nearly 10,000 tonnes of liquid nitrogen and 130 tonnes of superfluid helium.
So, it’s not really stretch to say that the LHC is like an experiment in outer space…almost.