Observation of Longitudinal Quadrupole-mode Oscillations of a Bunch which were Induced by RF Phase Modulation in the Electron Storage Ring

Abstract

It has been reported that the beam lifetime in electron storage rings can be improved by applying a small phase modulation on the rf accelerating voltage at a frequency of two-times the synchrotron oscillation frequency. This effect can be explained as being due to the improved Touschek lifetime that is caused by an excited bunch-shape oscillation in the stored bunches. In order to understand this phenomenon further, we investigated the induced bunch-shape oscillation in the 2.5-GeV Photon Factory storage ring at High Energy Accelerator Research Organization (KEK). Using both a streak camera and a spectrum analyzer, we observed the quadrupole-mode oscillations of a stored single bunch very clearly. It showed a splitting of a single bunch into three or two bunchlets, which depended on the modulation frequency. We report on these experimental results, and discuss them using computer simulations and an analytical formalism.