Comparison of Smith-Purcell radiation characteristics from gratings with different profiles

Gratings with different profiles such as lamellar, triangular, sinusoidal (so-called "volume" gratings) and "flat" gratings consisting of thin conductive strips separated by vacuum or dielectric gaps, have been used for investigation of Smith-Purcell radiation (SPR) in a number of experimental and theoretical reports. Some theoretical models predict that the flat grating is more preferable in respect to the radiation intensity. The azimutal dependence of SPR yield from volume and flat gratings is drastically different. However, till now the direct comparison of SPR characteristics for these profiles for the same experimental conditions is absent. We present some experimental results for such comparison obtained with the 6-MeV microtron in millimeter wavelength region under coherent conditions. Radiation angular distribution from gratings of different profiles has been studied in different spectral regions. The flat thin periodical target was demonstrated to have a superior radiation capacity. An additional result of this experiments is the observation of anomalous large long wave radiation intensity at small observation angles, which is not predicted by existing SPR theory. Calculations of SPR characteristics from volume and flat gratings using traditional models, presented in this article are in agreement with experiment only for a large polar angle.