The PS complex: the injector for the big accelerators

The Proton Synchrotron (PS) is the oldest and most versatile of CERN's accelerators. The PS was commissioned in 1959 and has been running continuously ever since. With a diameter of 200 metres and reaching a final energy of 28 GeV, it was for a while the most powerful accelerator in the world.

The PS has been modified strongly since then, adapting it to the ever increasing complexity of CERN's accelerator system. Today the PS complex can accelerate all stable and electrically charged particles (electrons, protons), their antiparticles (positrons, antiprotons), and different kinds of heavy ions (oxygen, sulfur, or even lead).

These particles are produced in ion sources: protons are obtained from hydrogen gas using a "duoplasmatron", electrons are extracted from metal surfaces in "electron guns", and the electron shells of lead atoms are stripped off in "electron cyclotron resonance (ECR)" sources.

The first stage of acceleration happens in a linear accelerator (Linac), and each type of particle has its own. This is because of their very different masses - a lead ion is about 200 times heavier than a proton, and almost 400,000 times heavier than an electron. The final energies are 500 MeV for electrons, 50 MeV for protons, and 4.2 MeV/nucleon for lead (Pb) ions.

For proton and heavy ion beams then follows a 1.0 GeV "Booster" synchrotron to increase the energy prior to injection into the PS. Electrons are first stored and further accelerated in the "Electron-Positron-Accumulator (EPA)" ring. Anti-electrons (positrons) are produced in collisions of an electron beam with a heavy metal target, and then also stored and accelerated in the EPA.

The PS complex

Then all particle beams pass through the PS machine itself. Each acceleration cycle takes 2.4 seconds, and the PS control systems are so versatile that different particle beams can be dealt with on each successive cycle. The beams are then injected into the bigger rings for further acceleration (SPS, LEP or - in future - LHC). Proton beams from the PS complex are also used for physics experiments (ISOLDE, East Hall) or for the production of antiprotons (Antiproton Decelerator).