• N.S. Ginzburg, A.M. Malkin, N.Yu. Peskov, R.M. Rozental, A. Sergeev, V.Yu. Zaslavsky
  IAP/RAS, Nizhny Novgorod

A novel Bragg FEL scheme 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 efficiency 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 novel Bragg resonator is provision of higher selectivity over transverse index than traditional scheme of Bragg FEL. The cold microwave testing of the Bragg structure based on coupling of propagating and trapped waves in the Ka band demonstrated a good agreement with theoretical considerations. The novel feedback scheme should be used on a JINR- IAP FEL as a method of increasing the operating frequency for fixed transverse size of interaction space.