Observation and cure of longitudinal coupled-bunch instabilities in the Fermilab Main Ring

One goal of the Fermilab Main Ring accelerator during the fixed target run is to keep the longitudinal beam oscillations as small as possible. It is found that when the proton beam intensity reaches a certain value, longitudinal beam oscillations rapidly grow. This motion can be explained by the longitudinal beam instability model. Installed active feedback system has successfully eliminated the instabilities and record intensities are achieved. We discuss the observations that indicate the damper systems are necessary. The results of the instability growth measurements are compared with a simple model