Longitudinal Instabilities in the Fermilab 400-GeV Main Accelerator

At beam intensities greater than 8 × 1012 protons per pulse longitudinal bunch instabilities are observed in the Fermilab main accelerator. The oscillations are primarily dipole coherent bunch motion with small mixtures of higher order modes. The oscillations grow during acceleration to amplitudes of about ± twenty degrees and remain constant at that amplitude, apparently in equilibrium with the Landau damping process. If sufficient bucket area exists there is no significant beam loss associated with the longitudinal motion but selective extraction and irregular spill structure occurs. "Mountain range" data on bunch motion and spectrum analysis of Fourier components of the circulating beam have indicated that the primary sources of the observed instabilities are resonances with high shunt impedances within the accelerating cavities. Although the RF cavities had already been provided with spurious mode damping devices additional damping systems consisting of ferrite loaded coaxial waveguide filters have been designed and are being installed on all accelerating cavities.