Stabilizing Effect of a Double-Harmonic RF System in the CERN PS

Motivated by the discussions on scenarios for LHC upgrades, beam studies on the stability of flat bunches in a double-harmonic RF system have been conducted in the CERN Proton Synchrotron (PS). Injecting nearly nominal LHC beam intensity per cycle, 18 bunches are accelerated on harmonic h=21 to 26 GeV with the 10 MHz RF system. On the flat-top, all bunches are then transformed to flat bunches by adiabatically adding RF voltage at h=42 from a 20 MHz cavity in anti-phase to the h=21 system. The voltage ratio V(h42)/V(h21) of about 0.5 was set according to simulations. For the next 140 ms,longitudinal profiles show stable bunches in the double harmonic RF bucket until extraction. Without the second harmonic component, coupled-bunch oscillations are observed. The flatness of the bunches along the batch is analyzed as a measure of the relative phase error between the RF systems due to beam loading. Measurements of electron cloud effects induced by the beam are also discussed. The results of beam dynamics simulations and their comparison with the measured data are presented.