Observation of coherent transition radiation at millimeter and submillimeter wavelengths

Coherent transition radiation has been observed in the wavelength range from 0.5 to 5.5 mm emitted from bunched electrons of 42 MeV passing through an Al foil in vacuum. The radiation intensity at λ=4 mm is enhanced by a factor of 3×${10}^6$ in comparison with incoherent transition radiation. The intensity is proportional to the square of the beam current, and it increases with the emission length, or the length of the trajectory of the electrons between the foil and a mirror to observe the radiation. The divergence of radiation increases with the wavelength and it is qualitatively in agreement with theory. A sheet of Eccosorb is also used as a radiator, and a similar spectrum to that from the Al foil has been observed. The bunch shape is discussed on the basis of a bunch form factor derived from the spectrum.