Pulse stacking in the SCA/FEL external cavity

There are many potential advantages to be obtained by coherently stacking successive FEL micropulses inside a resonator external to the FEL itself. Although the concept is simple, its paractical implementation requires dealing successfully with several issues. The accumulation of energy from the FEL requires that the external cavity have reasonably low losses, that its length and alignment be stable, and that the FEL beam be properly matched. If the accumulated energy is to be extracted from the cavity, then additional problems arise. At the Stanford FEL Center we have been studying these questions, both experimentally and theoretically, with the goal of implementing a practical system. Accumulation of micropulses with more than 75 times the energy of the incident micropulses has been demonstrated, and a system has been designed utilizing photo-induced reflectivity to provide mid-infrared (4-10 μm) micropulses at a 1 kHz rate and with 35 times the energy of individual FEL micropulses.