Concepts and present status for multi-mode quasi-optical converters in gyrotrons

It is a demand and a challenge for high-power microwave electronics to develop a megawatt power level gyrotron capable to operate at several modes and correspondingly at several frequencies. The main approaches for designing effective converters of a set of operating modes into a paraxial wave beam are discussed. Two main principles: "pre-shaping" of the wave beam before its launching from the waveguide end and near-confocal mirror optics are considered and their limitations studied.