Mode conversion losses in highly overmoded waveguide cavities

A technique to quantify the effects of mode conversion losses in highly overmoded waveguide cavity resonators using the WKB method is presented. The WKB theory is accurate for weakly irregular waveguide cavities with small amounts of mode conversion. This approach to the problem has the advantage that the mode conversion losses in waveguide cavities can be quantified quickly and cheaply. The objective is to compute the amplitude and phase of a resonant mode in the cavity to zeroth order (i.e. without regard to mode conversion effects), and then use amplitude and phase of the computed zeroth-order fields to compute the amplitude of the converted mode. This approach is valid to the extent that the converted modes are not a major perturbation to the resonant mode. The results of the theory are presented in the context of the trade-offs associated with the design of an 85 GHz TE_1.3 phase-locked gyroklystron oscillator experiment. Mode conversion losses in the input cavity are a particular problem, and place limits on the amount of usable beam current (and therefore the output power capabilities of the gyroklystron). The isolation of the input and output cavities due to mode conversion must be carefully considered in any design