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 in the 1970s. 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. However, in routine operation, the instabilities have not been limiting the PSB performance reach thanks to the use of a horizontal feedback system, which can suppress their onset for all beam intensities needed in the PSB. Recently, the interest in these instabilities has been revived by the ongoing LHC Injectors Upgrade (LIU) program, as well as, the Physics Beyond Colliders (PBC) study group. In fact, the PSB is being upgraded to a new energy range and higher beam intensities will be requested in future operation. To ensure that these instabilities will not limit the PSB performance in the future parameter range, a systematic characterization has been carried out through several measurements and models. At fixed energy, the dependence of the instability on the working point has been fully studied experimentally. Macroparticle simulations and analytical modeling have been applied to explain the measurements, suggesting that the main driving term behind these instabilities is the unmatched termination of the PSB extraction kickers. Analytical studies also explained for the first time why the instability appears at specific energies in the PSB cycle and showed that no other instability is expected above 1.4 GeV. Finally, the hypothesis on the main instability driving term has been verified experimentally by performing measurements with the kicker terminations temporarily matched. In this configuration, no sign of instability was observed and the extraction kicker could be unambiguously identified as the source of the instability. Some options to permanently suppress the source of the instability are proposed.