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What does 7 TeV mean?

Inspired by Regina’s excellent post on the CERN accelerator complex, I thought I’d give you some fun facts about the LHC (in “human units”).

1) What does 7 TeV beam energy mean?

Please look at Wikipedia for a discussion of units. Briefly, 1 Joule is the energy of a 1 Kilogram mass moving with a speed of 1 meter/second (1 J = 1 Kg * (1 m/s)2). In particle physics units, it is about 6*1018 electron volts, i.e., 6*106 TeV.

When operating at design parameters, the LHC will have two beams of protons, where each beam consists of ~2800 individual bunches, and each bunch contains ~1011 protons. Each proton will have energy of 7 TeV, so the energy of each bunch of protons is ~ 7*1011 TeV, i.e., 110,000 Joules (or 110 kilo Joules).

A bullet fired from a rifle typically weighs 4 grams, and can have speeds of up to 1000 m/s when it leaves the barrel. This corresponds to an energy of about 2000 Joules, i.e., roughly 1/55 the energy of one bunch of protons. Anti-tank shells (used in WW II) had energies anywhere from 150-800 kilo Joules.

So, it is crucial that the beam does not hit something that it is not intended to hit! (BTW, I have not included the energy stored in the magnets, which is a whole different story, and is many times larger).

2) How cold is the LHC?

The magnets in the LHC are superconducting. For this, the magnet mass and the wires carrying the electrical current (which generates the magnetic field) have to be cooled to about 2° K, i.e., -271° Celsius, or -455° Fahrenheit; the refrigeration plant uses 50,000 tons of liquid Helium.

By studying the Cosmic Microwave Background, which is a form of electromagnetic radiation filling the universe, astronomers have deduced that the current average temperature of the known universe is about 2.7° K.

This makes the LHC the coolest place in the Universe! (Well, not quite. Some atomic physics experiments attain much lower temperatures – thanks to Tim for pointing this out).

3) How about those magnets?

To keep the proton beam circulating in the accelerator ring at 7 TeV, we need very strong magnetic fields. For this purpose, the LHC has 1232 dipole magnets, each of which is 14 m long, weighs about 35 tons, and the required magnetic field is generated by passing about 11700 Amps of current through 5 Km of superconducting wire.

Then there are about 7066 magnets that focus the beam, and otherwise correct the path of the proton beam. For instance, if nothing was done, a proton will “fall” down due to gravity and hit the beampipe after travelling a mere 850 times around the ring (in one second, it goes around the ring about 11000 times).

To learn more, please take a look at this web page and links therein.

— Vivek Jain, Indiana University

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