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 26GeV with the 10MHz RF system.
On the flat-top, all bunches are then transformed to
flat bunches by adiabatically adding RF voltage at h = 42
from a 20MHz 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. The results of beamdynamics simulations
and their comparison with the measured data are presented.
