RF system modeling for the high average power FEL at CEBAF

High beam loading and energy recovery compounded by the use of superconducting cavities, which requires tight control of microphonic noise, place stringent constraints on the linac RF system design of the proposed high average power FEL at CEBAF. Longitudinal dynamics imposes off-crest operation, which in turn implies a large tuning angle to minimize power requirements. Amplitude and phase stability requirements are consistent with demonstrated performance at CEBAF. A numerical model of the CEBAF RF control system is presented and the response of the system is examined under large parameter variations, microphonic noise and beam current fluctuations. Studies of the transient behavior lead to a plausible start-up and recovery scenario