Prebunched high-harmonic FEL for short-pulse millimeter wave emission

A novel wide-band millimeter-wave source is described where a “point-charge” electron beam rotating within a bifilar wiggler will generate a short-pulse TE_mn wavepacket at the mth-harmonic of the fundamental free electron laser (FEL) frequency. In this first-order interaction, all electrons lose energy to the wave if the electron bunch size is much less than the wavelength. For a grazing intersection of the beam mode with the waveguide mode, there is no slippage of the electron bunch in the wave and the narrow width of the broad-band output pulse is limited only by the waveguide´s dispersion. A linear theory of this new interaction is presented along with the design for a prebunched tenth harmonic FEL proof-of-principle experiment that is predicted to generate a 140-ps pulsewidth 30-GHz wavepacket at kilowatt power levels