CERN Accelerating science

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How does the AD work ?

Antiparticles have to be created from energy (remember: E = mc2). This energy is obtained with protons that have been previously accelerated in the PS. These protons are smashed into a block of metal, called a target. We use Copper or Iridium targets mainly because they are easy to cool (but a piece of English beef would serve the same purpose - it would just roast very quickly and is rather messy).

Then, the abrupt stopping of such energetic particles releases a huge amount of energy into a small volume, heating it up to such temperatures that matter-antimatter particles are spontaneously created (this is explained in our briefing room).

In about one collision out of a million, an antiproton-proton pair is formed. But given the fact that about 10 trillion protons hit the target (about once per minute), this still makes a good 10 million antiprotons heading towards the AD.

The newly created antiprotons behave like a bunch of wild kids; they are produced almost at the speed of light, but not all of them have exactly the same energy (this is called "energy spread"). Moreover, they run randomly in all directions, also trying to break out 'sideways' ("transverse oscillations"). Bending and focussing magnets make sure they stay on the right track, in the middle of the vacuum pipe, while they begin to race around in the ring.

At each turn, the strong electric fields inside the radio-frequency cavities begin to decelerate the antiprotons. Unfortunately, this deceleration increases the size of their transverse oscillations: if nothing is done to cure that, all antiprotons are lost when they eventually collide with the vacuum pipe.

To avoid that, two methods have been invented: 'stochastic' and 'electron cooling'. Stochastic (or 'random') cooling works best at high speeds (around the speed of light, c), and electron cooling works better at low speed (still fast, but only 10-30 % of c). Their goal is to decrease energy spread and transverse oscillations of the antiproton beam.

Finally, when the antiparticles speed is down to about 10% of the speed of light, the antiprotons squeezed group (called a "bunch") is ready to be ejected. One "deceleration cycle" is over: it has lasted about one minute.

A strong 'kicker' magnet is fired in less than a millionth of a second, and at the next turn, all antiprotons are following a new path, which leads them into the beam pipes of the extraction line. There, additional dipole and quadrupole magnets steer the beam into one of the three experiments.