Quasioptical Model of Terahertz Undulator Superradiance from Picosecond Electron Bunches

Undulator superradiance from extended electron bunches can be considered a promising mechanism for generating high-power short pulses in the short-wavelength ranges, including the terahertz band. Due to the Doppler effect, the emission frequency of electron bunches moving at a translational speed close to the speed of light can significantly exceed the frequency of particles' bounce oscillations in a periodic magnetic (undulator) field. In this work, a quasi-optical theory of undulator superradiance is developed, which takes into account the effects of radiation diffraction. It is shown that for spatially coherent radiation with a narrow angular spectrum, the transverse size of the bunch should be on the order of unity on the scale of the Fresnel parameter, in which the inverse gain is used as a characteristic length. Under such conditions, diffraction effects provide synchronization of radiation from the entire volume of the electron bunch. Simulation demonstrate that for kiloampere picosecond electron bunches formed on the basis of photoinjectors, the radiation power in the terahertz range can reach several tens of MW.