Constructing the LHC
Chris - Joining us from CERN is Guy Crockford. He's one of the engineers on the project. Hello, Guy.
Guy - Hello.
Chris - Thank you for joining us. You must be very excited.
Guy - It's a very exciting time for all of us here in the control room.
Chris - What's the launch procedure building up towards Wednesday?
|The Large Hadron Collider experiments and the preaccelerators. The path of the protons (and ions) begins at linear accelerators (at p and Pb). They continues their way in the booster, in the Proton Synchrotron (PS), in the Super Proton Synchrotron (SPS) and finally they get into the 27-km-long LHC tunnel. In the LHC. © Arpad Horvath @ wikimedia|
Guy - Ok so at the moment we've reached a point where the machine is ready to take the first beams. In fact we've already made some pre-tests where we've managed to synchronise the LHC with the injector machines and inject a beam into the LHC. We just circulate particles around the 27km circumference.
Chris - How do you actually generate those particles in the first place?
Guy - The beam starts off inside of a bottle of hydrogen. A hydrogen atom contains a proton nucleus with an electron orbiting around it. We simply strip off the electron and we start to accelerate beams of protons from this hydrogen bottle. They have to go through a chain of four separate accelerators before it gets up to the energy we can inject it into the LHC.
Chris - So you spin it up to speed first in a smaller version and once it gets going quickly enough, this is presumably using magnetism to accelerate it -
Guy - Well we use magnets to guide the machine [particles] round a circular path, these circular accelerators and focus the beam. We use radio frequency cavities which actually give the beam its energy. The reason for having circular machines is that the beam can make many passages through these cavities.
Chris - As it gets going how fast will the particles end up travelling?
Guy - When the beam is up to the collision energy of the LHC it will be extremely close to the speed of light, actually 99.99999% the speed of light.
Chris - That's pretty fast!
Guy - That's about as fast as you can go, yes.
Chris - Once you've got that stream of particles going at that speed the whole circumference of the LHC is 27km so how quickly will they do a lap?
Guy - It's about the speed of light so they'll travel the 27km 11,000 times in one second.
Chris - That's pretty fast. You've got one group of protons going in one direction, one group going in the other direction. When they get to the critical speed do you then make them run into each other?
Guy - Yes so the particularity of the LHC is that you have two vacuum pipes in which the two counter-rotating beams can oscillate. They actually cross over each other at the four experimental detectors and when the beam reaches top collision energy we then make some fine adjustments at these crossing angles to bring the beams into collision inside the detectors.
Chris - When they smack into each other what happens?
Guy - Most of the protons will just pass straight through each other like clouds passing through each other. A few protons will actually impact with each other and annihilate and transform into energy. This can hopefully create these strange particles which existed after the beginning of the universe.
Chris - To put it into perspective so people can appreciate the kind of energies you're dealing with, when they run into each other how hard are they going to collide?
Guy - Well the actual collision is not very remarkable. If you clap your hands together you're probably making a collision of greater energy than the particles. The remarkable thing about the LHC is that these collisions are taking place on an atomic scale. It is very concentrated: not very large amounts of energy but extremely concentrated.
|The beam pipe surrounded with a magnet © Julian Herzogeigene|
Chris - When you want to build something on the scale of the LHC what are the sorts of engineering constraints there? How do you go about doing this?
Guy - First of all to be able to make a beam circulate round the LHC you need a vacuum pipe. Inside the vacuum pipe you need an ultra high vacuum. The vacuum of the LHC is about 20 times lower than the pressure on the surface of the moon. Once you have a beam circulating you need to keep it on its circular trajectory using very strong magnets. We have our two main kinds of magnets in the LHC: dipolar or two pole magnets which keep the beam on its trajectory round the ring; and quadrupole magnets which focus the very intense beams of protons to keep them inside the aperture of the vacuum pipe. The fields inside these dipole and quadrupole magnets are so high: about 170,000 times the Earth's magnetic field that we need to resort to superconducting technology to make these magnets. The coils of the magnets are made of a special alloy called niobium titanium. If you reduce the temperature of these coils down to very low temperatures, you're talking -270 degrees Celsius; the coil will conduct electricity with zero resistance. That way we can pass huge currents that are almost 12,000 amperes through the coil and produce these very powerful magnetic fields to keep the beam under control.
Helen - We actually had a question from Joshua on our forum which was about the temperature. He wanted to know why was it they needed to be kept so cold. It's nearly absolute zero, isn't it? How do you keep it so cold as well?
Guy - In fact we have a huge cryogenic refrigeration plant to cool down the whole machine. We need to use liquid helium to keep the magnets cold. The first stage of this process is to pre-cool the gaseous Helium using liquid nitrogen which is very cheap. We don't need huge quantities of liquid nitrogen. That will bring the temperature down to about 80 degrees Kelvin: something like -200 Celsius. Then we use cryogenic refrigeration plants to reduce the temperature down to the boiling point of helium. Now we're getting down to about four 4 degrees Kelvin. This is still not cold enough for the fields required for the currents that we require inside the LHC. We have further cold compressors, refrigeration plants which reduce the pressure in the Helium to 150 millibars. At this point the temperature reduces to 1.9 Kelvin which the operating temperature.
Chris - That's really quite cold, isn't it?
Guy - Colder than outer space, in fact.
Chris - Colder than Pluto, that's for sure! One last question, Guy: at this time of rising energy bills, what is your electricity bill?
Guy - In fact, the energy consumption of the LHC is similar to the whole of the city of Geneva (domestic consumption). Let me see, this is about 120 megawatts of power so -
Chris - Roughly how many households are there in Geneva?
Guy - There's roughly 150,000 households in Geneva.
Chris - That's quite a lot of energy, isn't it? But it's all for a good cause!
Guy - Yes!