Energy Stability in Recirculating, Energy–recovering Linacs in the Presence of an Fel

Recirculating, energy–recovering linacs can be used as driver accelerators for high power FELs. Instabilitieswhich arise from fluctuations of the cavity fields are investigated. Energy changes can cause beam loss on apertures, or, when coupled toM56, phase oscillations. Both effects change the beam induced voltage in the cavities and can lead to unstable variations of the accelerating field. An analytical model which includes amplitude and phase feedback, has been developed to study the stability of the system for small perturbations from equilibrium. The interaction of the electron beam with the FEL is a major perturbation which affects both the stability of the system and the development of start–up and recovery scenarios. To simulate the system’s response to such large parameter variations, a numerical model of the beam–cavity interaction has been developed which includes low level rf feedback, phase oscillations and beam loss instabilities and the FEL interaction. Agreement between the numerical model and the linear theory has been demonstrated in the limit of small perturbations. In addition, the model has been benchmarked against experimental data obtained during CEBAF’s high current operation. Numerical simulations have been performed for the high power IR DEMO approved for construction at CEBAF.