Coherent Smith-Purcell radiation in the millimeter-wave region from a short-bunch beam of relativistic electrons

Coherent Smith-Purcell radiation, generated by the passage of short-bunched electrons of 150 and 40 MeV above a lamellar-type grating, has been observed in the millimeter-wave region. The intensity of the coherent radiation is proportional to the square of the beam current, and is enhanced by a factor of ∼10⁸ in comparison with theoretical intensity of ordinary Smith-Purcell radiation. The enhancement factor is of the same order of magnitude as the number of electrons in a bunch. The intensity decreases with the increase of the beam height, or the distance of the beam from the grating, and the dependence on the beam height is expressed approximately by the modified Bessel function of the zeroth order. Owing to a relativistic effect the radiation is emitted in a narrow direction along the plane normal to the grating. The intensity of the radiation varies in a periodic way when the groove depth of the grating was changed. The observed properties of radiation are explained well by a three-dimensional theory of Smith-Purcell radiation, in which the coherence effect due to bunched electrons is taken into account.