Control of heavily-beam-loaded sns-ring cavities

In each of four RF stations that make up the RF buncher system in the SNS accumulator ring, cavity voltage and phase are control led through a negative-feedback system employing digital electronics. With peak beam currents as high as 75 amperes near the end of each 1.1 ms machine cycle, the RF cavities in the SNS ring are strongly driven by the beam. To provide adequate regulation of cavity fields in the presence of high SNS beam currents, basic feedback loop parameters are pushed to levels where stability becomes a major concern. This note presents a LabVIEW simulation of the ring RF system that demonstrates how Smith compensation can be used to mitigate the destabilizing effect of dead-time delay in the feedback loop and assure adequate regulation of cavity fields. A digital implementation of Smith compensation is outlined that could be incorporated into the LLRF system being provided by BNL