Injection of a short electron bunch into THz radiation section with an undulator and strong guiding magnetic fields

To implement an efficient source of coherent radiation with negative mass longitudinal stabilization, the methods of the formation and injection of a dense electron bunch onto a stationary helical trajectory in a combined helical undulator and a strong uniform magnetic field are studied. Using a magnetic or electric lens permits sending particles almost along the converging lines of the magnetic field (magnetic following) and obtaining a nearly rectilinearly moving compressed bunch inside a solenoid. After that, the bunch can be injected into an adiabatically increasing field of the helical undulator. In this way, it is possible to excite operating undulator oscillations of particles, significantly mitigating the effects of destructive bunch expansion, the excitation of parasitic cyclotron oscillations, and velocity spread, thereby providing stabilization and terahertz radiation of a dense bunch. Due to a significant mode selection for an axis-encircling bunch, as well as due to the long-term interaction of the particles with a dominant mode, which is closest to the group synchronism conditions, radiation with a relatively narrow spectrum and high efficiency can be obtained even in a strongly oversized waveguide. An additional efficiency enhancement can be obtained due to the reduction of the velocity spread caused by the mutual Coulomb repulsion of electrons during injection due to the initial energy chirp of the bunch.