Enhancing the efficiency of waveguide terahertz free-electron lasers using a bow-tie cavity.

Free-electron lasers (FELs) fill an important gap in the terahertz (THz) source domain, offering exceptional average and peak power capabilities as well as broad spectral tunability. The integration of optical waveguides into THz FELs significantly enhances the outcoupling efficiency between relativistic electrons and the radiation field. However, the introduction of optical waveguides brings new physical effects, the most significant of which is the spectral gap phenomenon. In this work, taking a 50-600 m FEL design as an example, we demonstrate the advantages of a novel, to the best of our knowledge, compact waveguide THz FEL. This design employs a bow-tie cavity configuration that can effectively prevent the occurrence of the spectral gap phenomenon and significantly improve long-wavelength laser performance. Micro-pulse energies on the order of tens of microjoules and peak powers on the megawatt scale are achieved across the 50-600 m wavelength range.