Difraction theory of quasi-optical diplexers

We propose a description of the diplexer, a device for control of high-power beams of millimeterwave radiation in electron-cyclotron heating systems for controlled fusion facilities. The diplexer is a four-mirror quasi-optical traveling-wave resonator, in which the transmission lines are connected by a pair of corrugated mirrors forming the resonator. In this paper, we develop a theory of such systems. An integral equation describing the beams of electromagnetic waves in the resonator in paraxial approximation and defining the diplexer characteristics with allowance for losses is deduced. The losses are due to the ohmic dissipation, diffraction, and limited size of the mirrors, as well as the difference between the widths and wave fronts of the diplexer resonator modes and the transmission lines. The integral equation is numerically solved by the iteration method using software with imaging. The eigenfrequencies of such a system with allowance for the vector character of the wave beams are found. Conditions under which the diplexer can be operated irrespective of the polarization of the incident radiation are determined.