The effect of the Bragg scattering on the performance of a relativistic backward-wave tube

A relativistic backward-wave tube with an overmoded corrugated waveguide as an electrodynamic system is considered. It is shown that, as the beam part icles' velocity in such an oscillator is close to the velocity of light and transverse dimensions of the system are large in terms of the operating wa velength, the Bragg scattering of the operating wave into one of the low-or der waveguide modes plays a significant role. This scattering noticeably ch anges the performance parameters of the microwave oscillator: the starting current increases, the spatial structure of the output radiation experience s distortions, and the scenario of developing oscillations becomes differen t. These effects are analyzed theoretically.