Beam Break-up Instability in the CERN PS near Transition

Fast beam losses, due to a vertical coherent instability of high frequency, have been observed in the PS near transition energy, with the high-intensity single-bunch beam for the neutron Time-of-Flight facility (n-ToF). By increasing the longitudinal emittance, the beam could be stabilised. These phenomena can be described by the beam breakup theory, since near transition the longitudinal positions of particles are almost frozen, as in the linac case. Comparison between observations and theory, using Brandt and Gareyte's formula for single-bunch beam breakup in circular accelerators, shows good agreement.