Linear accelerator 2 (Linac 2) is the starting point for the protons used in experiments at CERN. Linear accelerators use radiofrequency cavities to charge cylindrical conductors. The protons pass through the conductors, which are alternately charged positive or negative. The conductors behind them push the particles and the conductors ahead of them pull, causing the particles to accelerate. Small quadrupole magnets ensure that the protons remain in a tight beam.
The proton source is a bottle of hydrogen gas at one end of Linac 2. The hydrogen is passed through an electric field to strip off its electrons, leaving only protons to enter the accelerator. By the time they reach the other end, the protons have reached the energy of 50 MeV and gained 5% in mass. They then enter the Proton Synchrotron Booster, the next step in CERN's accelerator chain, which takes them to a higher energy.
The proton beams are pulsed from the hydrogen bottle for up to 100 microseconds per pulse. The pulses are repeated again and again until enough protons are produced.
Linac 2 started up in 1978, when it replaced Linac 1. It was originally built to allow higher intensity beams for the accelerators that follow it in CERN's accelerator complex. Linac 2 will be replaced by Linac 4 in 2017 or 2018.