  A method for producing narrow-band THz radiation proposes passing an
ultra-relativistic beam through a metallic pipe with small periodic
corrugations. We present results of a measurement of such an arrangement at
Brookhaven's Accelerator Test Facility (ATF). Our pipe was copper and was 5 cm
long; the aperture was cylindrically symmetric, with a 1 mm (radius) bore and a
corrugation depth (peak-to-peak) of 60 um. In the experiment we measured both
the effect on the beam of the structure wakefield and the spectral properties
of the radiation excited by the beam. We began by injecting a relatively long
beam compared to the wavelength of the radiation to excite the structure, and
then used a downstream spectrometer to infer the radiation wavelength. This was
followed by injecting a shorter bunch, and then using an interferometer (also
downstream of the corrugated pipe) to measure the spectrum of the induced THz
radiation.
