RF transfer in the coupled-cavity free-electron laser two-beam accelerator

A significant technical problem associated with the coupled-cavity free-electron laser two-beam accelerator is the transfer of RF energy from the drive accelerator to the high-gradient accelerator. The author examines one possible solution in which the drive and high-gradient cavities are directly coupled to one another by means of holes in the cavity walls or coupled indirectly through a third intermediate transfer cavity. Energy cascades through the cavities on a beat frequency time scale which must be made small compared to the cavity skin time but large compared to the free-electron laser pulse length. The transfer is complicated by the fact that each of the cavities in the system can support many resonant modes near the chosen frequency of operation. A generalized set of coupled-cavity equations has been developed to model the energy transfer between the various modes in each of the cavities. For a two-cavity case transfer efficiencies in excess of 95% can be achieved.<>