Attempts to measure and describe beam instabilities have been made ever since they were first observed in particle accelerators thirty years ago. Such collective, coherent effects arise due to the electromagnetic interaction of the beam with its environment, namely, the elements in the beamline. With sufficient intensity, the motion can become unstable, possibly leading to phase space dilution and beam loss. A coupled-bunch instability has long been observed in the Booster, an 8-GeV proton synchrotron at Fermi National Accelerator Laboratory. The accompanying longitudinal emittance growth is a major limit to beam brightness, limiting also the performance of subsequent accelerator stages. Previous studies have indicated that the coupled-bunch mode fluctuations are likely due to the influence of higher-order modes (HOM) in the radio-frequency (RF) accelerating cavities. However, the physics, especially that of the emittance growth, was only partially characterized.; It is my goal in this thesis to expand what we understand about coherent longitudinal phenomena and integrate it with a real machine which does not readily give up her secrets. Building upon prior observations and coupled with the advent of more sophisticated diagnostic and computational tools, this research seeks to characterize the unstable beam behavior in a rapidly cycling synchrotron. Experimental studies are designed to systematically vary parameters in order to establish functional dependencies. Bench measurements are made of the impedance due to RF cavity HOMs. These data are compared with analytic results derived from the standard linear perturbation treatment as well as with simulation.; The major finding of this research is that the theoretical predictions of linear growth rates of the longitudinal coupled-bunch instability based on the measured impedance show quantitative agreement with the data, but only when the beam momentum spread and nonlinearity of the RF potential are incorporated self-consistently. Development and installation in the cavities of passive HOM dampers proved to reduce the emittance by a factor of three and allowed, for the first time, an experimental test of instability thresholds. The linear theory is inadequate in describing the observed emittance growth, for which simulation results are invoked instead to provide a scaling rule.
展开▼
