Observation of coherent diffraction radiation from bunched electrons passing through a circular aperture in the millimeter- and submillimeter-wavelength regions

Using a short-bunched beam of electrons of 150 MeV, we have generated diffraction radiation from a circular aperture in an aluminum plate in the region of millimeter and submillimeter wavelengths. We have observed superposition of the diffraction radiation and transition radiation from an aluminum mirror. The angular distribution of the observed radiation shows interference of the two radiations. The intensity of the radiation has been observed to be proportional to the square of the beam current. Compared with the theoretical intensity of incoherent radiation, the observed intensity of the radiation from an aperture of 10 mm at the wavelength of 1 mm has been enhanced by a factor of 1.5×10⁸, which is roughly equal to the number of electrons in a bunch. From the observed spectrum, the longitudinal distribution of electrons in a bunch has been derived with a spatial resolution of 0.1 mm, i.e., a temporal resolution of 0.3 ps. (c) 1995 The American Physical Society