Can anyone explain in simple terms what is meant by a beam of protons at 3Tev? I think it means just the speed or equivalent energy, but I am not sure.
As others have already said, pretty much. When dealing with the energies involved, the speed is pretty much irrelevant since it's so close to the speed of light that you can't really tell the difference. This also means that when you add more energy, the speed hardly changes at all. So for all intents and purposes, the particles going at 3.5TeV are travelling at the same speed as they were at 1TeV, and the same speed as they will at 7TeV (and maybe beyond, but it'll take some upgrades to get past there).
The electronvolt thing is really just jargon. It's a measure of energy derived, as already noted, as the amount of energy an electron gains when it travels over a potential of one volt. It came about because the energies involved are actually very small relative to the units we usually use (1TeV is still only around 100 nano-Joules), and because particles are usually accelerated using a voltage, so the unit kind of falls out naturally. The reason it's electrons rather than protons is really a matter of tradition more than anything else.
How does one measure such a thing?
An interesting question. The answer is - with great difficulty. One of the big problems with particle accelerators is that there is no direct way to actually measure the energy. Particle detectors can do so, but only destructively which isn't much use if you're trying to keep beams circulating. Instead, there are a variety of indirect methods that can be used.
The easiest is to simply look at your magnets. A particle with a particular energy and charge will bend with a particular radius in a given magnetic field. If you know how strong your dipole magnets are, and you know how big the ring you're using is, you can work out the energy. The problem with this approach is that any error in measuring the strength of the magnets or the length of the ring will be carried into the error on the energy. A particular problem is that you can't actually measure the magnetic field in the first place. That would require sticking probes inside the magnet, which is something of a problem if you want to keep it under vacuum and fire particles around inside it. Instead, you have to rely on calibration of the power supplies done previously, which adds another layer of errors to things. There are other similar methods, but most of them suffer from similar problems.
Another method, that I've worked on, is resonant spin depolarisation. The principle is that the spin of particles tends to polarise all in the same direction when they spend a long time spinning round in accelerators. By measuring the parameters involved with the polarisation, and then other parameters involving finding a resonance to depolarise the beam again, you can get a very accurate measure of the beam energy. However, this is something that takes time to do, and so isn't something that can work as realtime measurement.
The LHC energy is most likely taken simply from the dipole magnets. Just remember that when they say the energy is 3.5TeV, there could easily be as much as a 1% error on that.
Does it mean you need 3TeV insulation in the beam tube?
No. The idea is that you don't want the beam to actually hit the beam pipe. Remember, the 3.5TeV refers to individual particles, but there are an awful lot of particles involved - eventually there will be 156 bunches each containing around 10
10 protons. That's somewhere around 10MJ stored in each beam. With the amount of energy involved at the LHC, if the beam were dumped into the beam pipe, there would be no way of stopping it burning straight through, and drilling a reasonably sized hole in the wall outside.
There needs to be enough cooling that the particles and radiation that does escape won't heat anything up too much. However, in order to actually get rid of the beam, there is a series of fast kicker magnets and diffusers that spread the beam out and direct it to an absorber specifically designed to cope with it.
Incidentally, I just noticed that their schedule had preparation for 3.5TeV down for the 29th March, so looks like things are going pretty much to plan at the moment.
