Why does the LHC need to be so cold?

17 January 2010


Yeah, G’day. I'm Greg from Australia. I got a question about the Large Hadron Collider, I was hoping you could answer. I'm curious about the temperatures that they run the experiments at. Just wanted to know if you could explain why the temperatures are so low. I think it’s around minus 270 degrees and also, how they get the temperature that low, and how they maintain it?


Dave - There are temperatures in the LHC of about 1.9 Kelvin. That's 1.9 degrees above absolute zero, so about minus 271 (and a bit) degrees centigrade. There's a couple of main reasons for doing this. One of the big ones is that inside the LHC there are huge magnets which are used to bend the particles around corners. They have incredibly strong magnets and you can't do that with a normal permanent magnet, you just can't get them strong enough. So you have to use an electromagnet. If you made the electromagnet out of normal copper wire, it would just melt in no time because of the huge current you need to get that strong a field. So, what they have to use is superconducting magnets.

A superconductor is a material which, when you cool it down enough, its resistivity goes to zero, so it will pass a current with no resistance at all. This is great for a magnet because you just start a current flowing in it and keeps on flowing forever.

The problem with superconductors is the bigger magnetic field you apply to them, the lower the temperature they start working. So, if you want to make incredibly strong magnets, you have to use very, very low temperatures and they're using about 1.9 degrees above absolute zero. This is actually below the temperature of liquid helium at normal atmospheric pressure so what they have to do is they take liquid helium which they have made essentially with a very, very big fridge by compressing gases and letting them expand. And as they expand, they get colder. You then take liquid helium and pump on it, so you reduce the pressure above it. So then it boils and boils, until it gets colder and colder, down to about 1.8 Kelvin. Then they pump that around the system and cool down the magnets. Chris - I would come off there by saying, "Cool!" but that would seem like an awful pun. But the bill for doing that must be absolutely huge or once you've got it to that liquid state, is it so well-insulated that it just stays that way? How does it work? Dave - It is going to need a huge amount of energy because although they're not moving that much heat out of the system, they're probably moving a couple of hundred kilowatts of energy, which by the standards of huge industrial processes isn't very much. The problem is the colder the object you are taking energy from, the harder work it gets. So, if you move energy from somewhere at room temperature to somewhere a couple of degrees hotter, then that uses very little electrical energy if you move it from something at 1.2 Kelvin to a room temperature that - with quite an immense amount of energy. Chris - And will you get a magnetic field once you've fired up the magnet that would just sustain itself because there's no resistance and will you only be losing the magnetic field because the particles will sap energy from it? So, it will therefore need to be topped up for that reason.the magnetic field that will be generated at those low temperatures,

Dave - The superconducting magnets, once they start going, they essentially work like a permanent magnet. People have done experiments with superconducting magnets and as far as they know, some of them will keep on going for billions of years. The real problem is if part of the superconductor warms up, at which point, you get a huge amount of electrical energy running around it. All of which is dumped into a very small area of the superconductor, so it gets very, very hot. All this liquid helium which you've got inside the magnet suddenly boils. And that produces a huge amount of gas which expands very, very rapidly. This was the problem they had last year when it went horribly wrong. This huge amount of gas, then pushed the magnets around. Billions of pounds worth of stuff just shuffled around this system and it caused an immense amount of damage.

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