Source of horizontal instability at the CERN Proton Synchrotron Booster

The CERN Proton Synchrotron Booster (PSB) has been known to suffer from horizontal instabilities since its early operation. These instabilities appear at specific beam energies and range of working points. The source of the instability and the reason why the instabilities appear at specific energies remained unidentified. In routine operation, the instabilities have not been limiting the performance reach thanks to the horizontal feedback system. Recently, the interest in these instabilities has been sparked by the on-going LHC Injectors Upgrade (LIU) program, as well as, the Physics Beyond Colliders (PBC) study group. Their systematic characterization has been carried out through measurements. Macroparticle simulations and analytical modeling have been applied to explain the measurements and the dependence on the kinetic energy. Finally, the extraction kicker has been unambiguously identified as the source of the instability.