Operational experience and techniques for controlled longitudinal phase space dilution in the AGS using a high harmonic cavity

A fixed-frequency, 930-MHz harmonic cavity has been installed in the alternating gradient synchrotron (AGS) ring. The primary purpose of this harmonic cavity is to increase (dilute) the longitudinal phase space area occupied by the beam by as much as a factor of four (to 4.0 eV-s) in a controlled, rapid manner while maintaining a smooth local density. This will then permit essentially lossless passage through transition energy (8.0 GeV) at intensities higher than previously achieved. Although the system controls permit as many as five dilutions per AGS cycle, only one is used at present. Effects of the various phase modulation programs, excitation period, and RF voltage programs on the dilution rate have been modeled. The parameters predicted to yield the best dilution have been implemented with satisfactory results. Further, the parameter space is being explored experimentally in order to determine the programs leading to the desired degree of dilution without loss of beam in the minimum time