Efficiency of terahertz undulator radiation from short electron bunches moving in the field of permanently magnetized helices

The motion and radiation of short dense bunches of ultrarelativistic electrons produced by laser-driven accelerators and moving in an undulator in the form of magnetized helices have been studied. Simulations demonstrate the possibility of generating wideband THz pulses with energies of hundreds of microjoules and relatively high efficiency in regimes close to the group synchronism of electrons with the waveguide mode.