Question: How does the large hadron collider (LHC) work? and how do they manage to get the particles within to travel at such high speeds?

Peter Williams answered on 16 Nov 2011:

The LHC is really the last link in a whole chain of accelerators on the CERN site, some of which go back to the 1950’s. All of them accelerate protons. They start off at what’s called an ion source, you take hydrogen and leak a very small amount in to a vacuum chamber – this is under a very large static voltage. Some jiggery-pokery goes on and the electrons are stripped off the hydrogen to make bare protons. These see the big electric field and are accelerated toward the negative voltage part of the source – the cathode. They are extracted through a hole and pass to LINAC2, this is a tubular metal structure that has alternating electric fields in it. The protons “surf” this field as they pass through the structure, getting faster and faster. At the end, they are guided by magnetic fields into the Proton Synchrotron Booster. This is the first of four synchrotrons which are linked together, the protons pass through each in turn getting higher and higher energy as they do. The next is the Proton Synchtrotron, then the Super Proton Synchrotron, then finally the Large Hadron Collider. There’s a good picture of the CERN accelerator chain here http://en.wikipedia.org/wiki/File:Cern-accelerator-complex.svg

A synchrotron works a bit like swinging a ball on a string above your head in a circle. Every time the ball (or protons) go round they get a little kick forwards. In the case of a synchrotron this comes from seeing a voltage difference over a short section called a radio-frequency cavity. As the energy or speed increases you need to keep the protons in the same orbit, with the ball on the string the tension of the string increases. In a synchrotron the bending force is provided by magnetic fields instead. So to get to the highest energies, the magnitic fields are increased. Once this is done the protons will keep going around the ring for days, even weeks. When a synchrotron is operated like this it is called a storage ring. In the case of the LHC there are two counter-rotating beams that are brought into collision at 4 points around the 28km ring. But only a very small number of protons interact each time, so the same bunches of protons are used for many hours.

In summary – protons in the LHC are accelerated by seeing very large electric fields and being attracted to the negative pole of that field. The exact way that this happens varies depending on what energy you are working at.