Efficiency enhancement of THz radiation from an electron bunch in a waveguide due to low-frequency stabilization

Simultaneous generation of pulses of high-frequency Super-Radiance and low-frequency Coherent Spontaneous Radiation from an ultra-relativistic electron bunch moving in a waveguide placed in a spatially periodic and / or uniform longitudinal magnetic field is studied. The low-frequency radiation can weaken expansion of the bunch and decrease a velocity spread. This significantly improves electron micro-bunching and increases energy of high-frequency pulses. Such effect makes possible fairly efficient high-frequency cyclotron radiation of the bunch near an autoresonance regime, when the wave phase velocity is very close to speed of light.