  Accelerator-based terahertz (THz) radiation has been expected to realize a
high-power broadband source. Employing a low-emittance and short-bunch electron
beam at a high repetition rate, a scheme of coherent diffraction-radiation in
an optical cavity layout is proposed. The scheme's stimulated radiation process
between bunches can greatly enhance the efficiency of the radiation emission.
We performed an experiment with a superconducting linac constructed as an
energy recovery linac (ERL) test facility. The electron beam passes through
small holes in the cavity mirrors without being destroyed. A sharp THz
resonance signal, which indicates broadband stimulated radiation correlated
with beam deceleration, was observed while scanning the round-trip length of
the cavity. This observation proves the efficient beam-to-radiation energy
conversion due to the stimulated radiation process.
