Coherent transition radiation at submillimeter and millimeter wavelengths

A spectrum of coherent transition radiation (TR) emitted from 150-MeV electron bunches from a linear accelerator has been observed in the wavelength range from 0.6 to 5 mm. The intensity at a wavelength of 4 mm is enhanced by a factor of about 6×${10}^5$ in comparison with that of ordinary TR. The factor is about a half of the number of electrons in a bunch. The intensity shows nearly a quadratic dependence on the electron-beam current. The interference between TR from adjacent bunches has been observed. The electron distribution in a bunch has been derived from the observed spectrum. It has a full width at half maximum of 0.56 mm and the shape is similar to a Gaussian function.