Forward directed Smith-Purcell radiation from relativistic electrons

The requirements for the generation of forward directed Smith-Purcell emission from relativistic electrons are introduced, and the first experimental evidence of this phenomenon is presented. The experiments were conducted with a 2.8 MeV/{ital c} electron beam interacting with a 1 cm period grating. Radiation emitted in two bands with wavelengths ranging from 600 to 650 {mu}m and 1.2 to 1.7 mm were measured. Grating period to wavelength ratios were as high as 16. The measured intensities are higher than that predicted by incoherent emission.