  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, but with short rise time, 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. For the THz pulse we obtain and compare
with calculations: the central frequency, the bandwidth, and the spectral
power---compared to a diffraction radiation background signal.
