Development of FEM based on coupling of propagating and trapped waves in advanced Bragg resonators

A novel Bragg FEM scheme [1] is discussed in which an electron beam synchronously interacts with a propagating wave, and the latter is coupled to a quasi cut-off mode. This coupling is realized by either helical or asimuthally symmetric corrugation of the waveguide walls. The quasi cut-off mode provides feedback in the system leading to self-excitation of the whole system while the energy extraction in steady-state regime of generation is almost completely determined by the propagating mode, synchronous to the beam. Analysis based on averaged time domain approach as well as on direct PIC code simulation shows that the efficiency of such a device in the single mode single frequency regime can be rather high. The main advantage of the advanced Bragg resonator is provision of higher selectivity over transverse index than that in the traditional scheme of Bragg FEM. This method can be used for increasing of operating frequency at fixed transverse size of interaction space.The novel feedback scheme should be used on a 8 mm FEM based on high current accelerator (0.7 MeV/2 kA/200 ns) at Kanazawa University, Japan.