Linear accelerator 4 (Linac 4) is designed to boost negative hydrogen ions to high energies. It is scheduled to become the source of proton beams for the Large Hadron Collider (LHC) after the long shutdown in 2019-2020.
Linac 4 will accelerate ions to 160 MeV to prepare them to enter the Proton Synchrotron Booster, which is part of the LHC injection chain. Negative hydrogen ions are pulsed through the accelerator for 400 microseconds at a time.
Linear accelerators use radiofrequency cavities to charge cylindrical conductors. The ions pass through the conductors, which are alternately charged positive or negative. The conductors behind them push particles and the conductors ahead of them pull, causing the particles to accelerate. Small quadrupole magnets ensure the hydrogen ions remain in a tight beam. As particles approach the speed of light, the energy imparted by the conductors is converted into mass.
Linac 4 accelerates negative hydrogen ions, which consist of a hydrogen atom with an additional electron. The ions are stripped of their two electrons during injection from Linac 4 into the Proton Synchrotron Booster to leave only protons. This allows more particles to accumulate in the synchrotron, simplifies injection, reduces beam loss at injection and gives a more brilliant beam.
Linac 4 is 80 metres long and located 12 metres below ground. Beams have begun to be produced in 2013 and the milestone energy of 50 MeV was reached in 2015. During the long shutdown planned for 2019-20, it will replace Linac 2, which currently accelerates protons to 50 MeV. It is an important milestone in the project to increase the luminosity of the LHC during the next decade.