Photowalk 2015 - Andrew Hara
Behind-the-scenes at Linac4, a linear accelerator under construction. It is the future starting point of the proton beams circulating in the Large Hadron Collider. (Image: CERN)

Linear accelerator 4 (Linac4) is designed to boost negative hydrogen ions to high energies. It became the source of proton beams for the Large Hadron Collider (LHC) in 2020.

Linac4 accelerates negative hydrogen ions (H-, consisting of a hydrogen atom with an additional electron) 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. Quadrupole magnets ensure the hydrogen ions remain in a tight beam.

The machine comprises a H- ion source and four types of accelerating structure: the particles are accelerated in several stages, first to 3 MeV by a radio-frequency quadrupole (RFQ), then to 50 MeV by drift tube linacs (DTLs), then to 100 MeV by coupled-cavity drift tube linacs (CCDTLs), and finally to 160 MeV by Pi-mode structures (PIMS). Linac4’s hardware also includes an equipment called a chopper line. Its role is to cut up the beam at the same frequency as that of the PS Booster.

The ions are stripped of their two electrons during injection from Linac4 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.

Linac4 is 86 metres long and located 12 metres below ground. Beams began to be produced in 2013 and the milestone energy of 160 MeV was reached in 2016, after the commissioning of all the accelerating structures. During the long shutdown 2019–20, it replaced Linac2, which had previously accelerated protons to 50 MeV. The Linac4 is a key element in the project to increase the luminosity of the LHC during the next decade.