Linear accelerator 2

Linac2 was the starting point for the protons used in physics experiments at CERN from 1978 until 2018

Linear accelerator 2 (Linac2) was the starting point for the protons used in experiments at CERN for 40 years. 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 was a bottle of hydrogen gas at one end of Linac2. The hydrogen was passed through an electric field to strip off its electrons, leaving only protons to enter the accelerator. By the time they reached the other end, the protons had reached the energy of 50 MeV and gained 5% in mass. They then entered the Proton Synchrotron Booster, the next step in CERN's accelerator chain, which took them to a higher energy.

The proton beams were pulsed from the hydrogen bottle for up to 100 microseconds per pulse. The pulses were repeated again and again until enough protons were produced.

Linac2 started up in 1978, when it replaced Linac1. It was originally built to allow higher intensity beams for the accelerators that follow it in CERN's accelerator complex. Linac2 was switched off for the last time on 12 November 2018. It was replaced by Linac4, which was inaugurated in 2017 and connected to the PS Booster in 2020 during CERN’s Long Shutdown 2.

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