Smith-Purcell radiation occurs when a charged particle passes close to a periodically varying metallic surface. In the most simple terms one can imagine that an electric dipole is formed from the beam particle and its image in the metal. Since the surface height is periodic, the dipole oscillates and thus produces radiation at a wavelength determined by the period, particle velocity, and angle of emission of the radiation.
The experiments were conducted with a 2.8 MeV/c electron beam from the ATF RF gun. The beam shape was measured using a phosphor coated beam flag at the grating location. The beam charge could be measured using a Faraday cup located downstream of the grating. A series of blazed gratings were used with periods around 1 cm. The distance of the beam to the grating surface could be remotely adjusted. The optical collection system consisted of a remotely rotatable plane mirror, off axis parabodial mirror, and a light pipe. The wavelength of the radiation was determined with a Czerny-Turner monochromator. The radiation power was measured with a liquid helium cooled InSb bolometer.
This experiment detected Smith-Purcell radiation over the wavelength range from 0.6 to 6.0 mm. The wavelengths of the measured Smith-Purcell radiation agree with theoretical predictions. The emitted power from the electron beam is expected to peak in the forward direction (small angles with respect to the beam direction). The experiment observed this forward peaking for the first time in Smith-Purcell radiation. The radiation was detected with grating period to wavelength ratios as high as 16. The measured intensities were much higher than those predicted from incoherent emission.
A new experiment is under construction to measure Smith-Purcell radiation in the wavelength region 10 - 100 microns using the high energy, 50 MeV beam from the ATF linac.
J. Scott Berg <jsberg@bnl.gov>
3 April 2000.
