The longitudinal system controls beam energy and time oscillations about some fixed reference. Most longitudinal focusing and control is provided by the high level RF system with the cavities and power amplifiers. The damper system provides perturbations on the main high level RF control signals which keep the beam stable. The timing and bunch length are measured with a stripline pickup located at F0, and the processing occurs in racks F0110 and F0111. The signals are then applied to the low level and high level RF controls for the Tevatron where they are required.
Two longitudinal oscillation modes are controlled by this system, the dipole and quadrapole modes. A longitudinal dipole oscillation is when the centroid of a bunch vares in energy and time around the synchronous phase and energy of the accelerator. A longitudinal quadrupole oscillation is when the bunch shape ioscillates between a large energy spread and a large time spread.
Coupled bunch mode instabilities are caused by coupling between the oscillations of different bunches in a multibunch machine. The phase of the oscillation of each bunch forms a very well defined pattern around the ring. Coupled bunch mode 0 means that every bunch is oscillating in unison with the same phase of oscillation. Coupled bunch mode 1 means that the phase of oscillation of each bunch, at a particular instance of time, processes 360° around the ring. Coupled bunch mode 2 processes 720° around the ring, coupled bunch mode 3 processes 1080° around the ring, and so on. The first two modes (coupled bunch modes 0 and 1) occur at frequencies within the bandwidth of the accelerating cavities and are generally much more unstable than the other modes. That is why these two modes are the only modes controlled with this system.
Dipole mode dampers measure the phase difference between the bunch crossing and the low level RF reference and determine the strength of the kick required to bring the bunch back in to phase. The Tevatron low level RF system does not provide a reference signal that is always in phase with the beam. Therefore, it must be produced by the damper system. The phase detector used to measure dipole oscillations is also used in a low frequency phase locked loop system to track the synchronous phase of the beam.