Simulations on longitudinal coupled-bunch instabilities due to higher-order modes of five RF cavities at the Pohang Light Source

During the last several years of operating with a train of bunches, coupled-bunch instabilities have been sometimes observed at the 2.5-GeV Pohang Light Source (PLS), depending on the parameters of rf system and machine parameters of the storage ring. In the next year, one more rf cavity will be inserted into the storage ring to raise the stored beam current. The performance of the 2.5GeV storage ring may be limited by the longitudinal coupled-bunch instability as the beam current is increased. If the performance of the PLS is to be improved, the longitudinal coupled-bunch instabilities that may be generated by higher-order modes in the five rf cavities have to be cured. A longitudinal multibunch simulation code was developed to investigate the longitudinal coupled-bunch instabilities for the cases of various bunch filling patterns. It is shown in this paper that the beam energy spread due to the longitudinal coupled-bunch instabilities at higher beam currents for the five rf cavities can be suppressed by using a proper bunch filling-pattern. We present the results of detailed computer simulations on the longitudinal beam dynamics. The simulations included the effects of fundamental and higher-order modes in the five if cavities.