Coherent transition radiation in the far-infrared region

Coherent transition radiation (TR) has been observed in the wavelength range from 0.2 to 5 mm, which is emitted from bunches of 150-MeV electrons passing through a radiator system composed of an upstream aluminum foil and a downstream aluminum mirror. At the beam current of 1 μA, the intensity at λ=1 mm is enhanced by a factor of 0.9×${10}^6$ compared with incoherent TR. The value of this factor is close to the number of electrons in the bunch. The intensity of the TR has been observed to depend on the emission length or the length of the trajectory of the electrons between the foil and the mirror. The angular distribution of the TR intensity and its dependence on the wavelength and on the emission length have been observed. All results are in good agreement with theory. Dependences of the TR intensity on the cross section of the beam and on the radiation material have also been examined. From the observed spectrum of TR, the longitudinal distribution function of electrons in the bunch is derived to be approximately a Gaussian with the bunch length full width at half maximum of 0.28 mm.