A Study of Longitudinal Coherent Effects of Unbunched Beams Near Transition in the Fermilab Accumulator

Abstract

The Fermilab Accumulator was built primarily as an 8 Ge V kinetic energy antiproton storage ring. Recently it was modified for use in a medium energy experiment studying charmonium states. In the experiment, an intense antiproton beam was decelerated from the operating energy to specific energies in the charmonium region between 6.8 Ge V and 3.8 Ge V, and collided with protons from an internal hydrogen gas jet target. In order to decelerate the beam below 5 Ge V, transition must be crossed. The method adopted for Accumulator transition crossing was to raise the value of $\gamma t$ slowly while the antiproton beam circulated in the ring unbunched. During Accumulator transition crossing with this coasting beam, longitudinal coherent instability was observed which limited the machine performance. Measurements for identifying the major source of the longitudinal instability were made with a resistive wall current monitor and a dedicated Schottky detector. Measurement results are presented and compared with the simulations. The Accumulator operation in the experiment is also recorded in this thesis, which includes deceleration of the $\bar{p}$ beam, transition crossing, and measurements of the beam energy and energy spread. In addition, since knowledge of the longitudinal impedance is important for instability studies, beam-based measurements of the Accumulator longitudinal impedance is described. The principle, the experimental setup, and measurement results are discussed.