Investigation of the RF phase modulation at DELTA

At DELTA, a third generation 1.5 GeV synchrotron radiation facility, we utilize an RF phase modulation of the master-RF frequency at two times the synchrotron frequency. This provides additional damping capabilities for longitudinal coupled bunch mode instabilities and a significant increase in beam lifetime, due to bunch lengthening and reduced Touschek effect. This leads to an enhanced beam lifetime of approximately 20%. Despite the apparent influence on beam lifetime and longitudinal beam stability, the actual impact of the RF phase modulation on the time dependent acceleration gradient of the normal conducting cavity and its influence on the longitudinal phase space dynamics has not been studied sufficiently yet. We present an overview of the existing modulation system as well as studies of its effect on the longitudinal beam dynamics and the small bandwidth accelerating cavity.