  High conversion efficiency between electrical and optical power is highly
desirable both for high peak and high average power radiation sources. In this
paper, we discuss a new mechanism based on stimulated superradiant emission in
a strongly tapered undulator whereby an optimal undulator tapering is
calculated by dynamically matching the resonant energy variation to the
ponderomotive decelerating gradient. The method has the potential to allow the
extraction of a large fraction (~50%) of power from a relativistic electron
beam and convert it into coherent narrow-band tunable radiation, and shows a
clear path to very high average power radiation sources.
