Angular radiation pattern of Smith–Purcell radiation

The power radiation pattern of Smith–Purcell radiation is measured at various latitudes and azimuth angles relative to the electron beam. The experimental data are used to evaluate the various models and the physical mechanisms previously suggested to describe Smith–Purcell radiation. Good agreement is observed between the experimental data and the theoretical curves derived from Van den Berg’s analysis [ J. Opt. Soc. Am.63, 1588 ( 1973)]. The radiation mechanism proposed by Salisbury [ J. Opt. Soc. Am.60, 1279 ( 1970)] was analyzed and shown to be too small to account for the measured radiation. The experiment and Van den Berg’s theory predict stronger emission at azimuthal angles off the plane perpendicular to the gratings. This observation leads to conclusions regarding the design of optical cavities for Smith–Purcell free-electron lasers and orotron millimeter-wavelength-radiation tube devices.