The Proton Synchrotron Booster (PSB) is the first and smallest circular proton accelerator (a synchrotron) in the accelerator chain at the CERN injection complex, which also provides beams to the Large Hadron Collider.

[2] However, with the development of new experiments (mainly at the Intersecting Storage Rings ISR), the demanded beam intensities in the order of 1013 protons per pulse exceeded the capabilities of this setup.

[3] Eventually, it was decided to go for a setup of four vertically stacked synchrotron rings with a radius of 25 meters, which was proposed in 1964.

[4] The first proton beams in the PSB were accelerated on May 1 in 1972, and the nominal energy of 800 MeV was reached on May 26.

This new machine would provide protons with the same energy as before (50 MeV), but with higher beam currents of up to 150 mA and a longer pulse duration of 200 μs.

[6] From the beginning of the 1980s until 2003, the PSB was also used to accelerate light ions like oxygen or alpha-particles, which were delivered by Linac 1.

This changed in 1992, when the On-Line Isotope Mass Separator (ISOLDE) became the second recipient of PSB's protons.

In 2010, the cornerstone for another upgrade of the LHC was laid: the High Luminosity Large Hadron Collider.

[9] The much higher required beam intensity made it necessary to increase the PSB's output energy to 2.0 GeV.

These proton bunches are then recombined at the exit of the PSB and further transferred down the CERN injector chain.

With a series of different magnetic structures, the beams from the four rings are brought to one vertical level and are then directed towards the PS.

[11] The only direct experiment that is fed by PSB's protons is the On-Line Isotope Mass Separator (ISOLDE).