Far-Infrared Smith-Purcell Experiment - Measurement Of Radiated Power

Infrared radiation produced by a fully relativistic, 3.6 MeV, electron beam passing close to a metal grating has been observed and the radiated power estimated by direct comparison with a high pressure mercury vapour lamp. The dependence of wavelength on angle of emission, over angles from 56-degrees to 150-degrees and wavelengths from 350 mum to 1860 mum, agrees remarkably well with the Smith Purcell dispersion relation. The observed radiated power of about 10 muW/cm2 sr A for a 100 mum band width at a wavelength of 1 mm is comparable with or greater than that typical for infrared beam lines at synchrotrons, and also suggests that the Smith Purcell mechanism deserves greater attention as a possible basis for FEL operation in the mm and sub-mm wavelength bands.