Broadband Terahertz Undulator Radiation from Short Electron Bunches into the Dominant Mode of an Oversized Waveguide

A possibility to generate wideband terahertz pulses of Coherent Spontaneous Radiation with energies of hundreds of microjoules and a relatively high efficiency in regimes close to group synchronism of electrons with the mode of a circular waveguide has been demonstrated in simulations. As a magnetic system of the THz source providing a large undulator parameter K> 1, it is proposed to use the simplest helical undulator formed by several permanently magnetized ferromagnetic helices. The use of a preliminary energetic chirp of particles and stabilization of the bunch in the field of the emitted electromagnetic pulse makes it possible to mitigate the effects of Coulomb repulsion of particles and to increase the length of the effective interaction of a dense bunch with the field of the operating mode.