FREE ELECTRON LASER AND STORAGE RING MICROWAVE INSTABILITY

Recent observations and numerical simulations suggest that the Free Electron Lasers (FEL), operating on a storage ring, may provide a negative feedback for some instabilities.The explanation of this effect is related to the interplay between the FEL induced longitudinal dynamics and the conditions supporting the on set and the growth of the instability. The microwave instability (M.I.) is counteracted by the FEL because of the induced energy spread and because of the intensity dependent corrections to the longitudinal damping time.The first mechanism causes a shift of the M.I. threshold.The second provides a faster damping of the higher order modes. In this contribution we show that simple considerations, based on the Boussard criterion,allow to derive the threshold laser power necessary to counteract the instability.