MD at the PS: Beam Break-Up Instability Near Transition

Machine study experiments have been performed at the CERN PS during the week of June 18th, 2012, with the objective to continue the investigations of the fast (below ms) transverse high-frequency (0.7GHz) instability near transition, without $γ_t$ jump. Special attention is paid to the instability time- and space structure. The role of Landau damping is confirmed on the instability appearance time in the cycle, and on the intensity thresholds for different rf voltage. Other damping mechanisms, as the synchrotron motion, and the ratio between the bunch length and the perturbation wave length, are studied. The bunch parameter measurements demonstrate that the bunches are in the regime of strong space-charge regarding bunch dynamics, but the transverse emittance variations indicate that the space-charge effect does not affect the instability thresholds. Stability enhancement due to octupole magnets is measured for different beam parameters.