Diffraction selection of modes in planar backward-wave oscillators

We study the possibility of obtaining spatially coherent radiation in a planar backward-wave oscillator fed with a wide sheet electron beam by using natural diffraction divergence of the radiation. This method is efficient for Fresnel parameters being approximately equal to unity. To solve the two-dimensional boundary problem, the spatio-temporal approach is used, which allows one to determine both the self-excitation conditions and the parameters of the stationary generation regime and the boundaries of its stability. The developed theory is used to evaluate the possibility of achieving diffraction synchronization of the radiation in a high-power planar backward-wave oscillator operated in the 8-mm wavelength band and based on a sheet relativistic electron beam produced by the U-3 accelerator of the Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia.