3-d modeling of broadband multi-cavity circuits

Summary form only given. We investigate a 3-D generalized modal-expansion technique to model the fields and resonance characteristics of low-Q cavities and multi-cavity circuits found in broadband klystron and multiple-beam klystron amplifiers. Such cavities are often complex, multi-gap structures and full 3-D analysis is essential to their design. For broadband input and output circuits, that have low Q_external, a traditional cavity eigenmode representation is more difficult to define due to the strong coupling of the cavity to an external waveguide. Usually, numerical solution requires addition of an absorbing boundary condition inside the waveguide port to determine a leaky eigenmode of the open system, or one must resort to computing many driven-frequency solutions across the frequency band and perform subsequent field analysis to determine the cavity stored energy dependence on frequency and hence extract the resonance frequency and Q. The analysis becomes more problematic when multiple overlapping resonance peaks occur inside the operating bandwidth, as is often the case. In our approach, we avoid the numerical solution of the open cavity-waveguide system, and introduce a new technique using only modes of a finite closed cavity, terminated at the waveguide aperture by short-circuiting either the tangential electric or magnetic field. By representing fields inside the cavity using a dual set of eigenmodes we determine the coupling to an external waveguide to determine the actual frequency and Q of the open cavity-waveguide system. We present the theory behind the analysis, and compare to existing methodology