An intense beam of relativistic electrons (800 A, 6.7 MeV) has been bunched
at 35 GHz by a free-electron laser, in which output power levels exceeding
100 MW were obtained. The beam was then extracted and transported through
a resonant cavity, which was excited by its passage. Microwave power levels
of 10 MW were extracted from the cavity, in reasonable agreement with the
simple formula which relates power to known properties of both the beam and
the cavity.