The Rayleigh hypothesis applicability criterion for periodic plasma-filled waveguides

The necessary and sufficient conditions for applicability of spatial harmonic expansions (the Rayleigh hypothesis) for plasma wave branches in periodic plasma-loaded waveguides have been obtained for the first time, based on the exact solution for the relevant boundary-value problem. Conditions of oscillation appearance beyond the scope of such expansions and their expected effect on the operation of plasma-filled backward-wave oscillators are discussed.