Accurate 3-D simulation of coupled-cavity TWT structures

Summary form only given. We describe a new 3D electromagnetic simulation code capable of fast, accurate modeling of both resonant cavities and slow-wave structures. We also present results of a new method to obtain complete dispersion properties for a 3D coupled-cavity TWT structure based on analysis of only the computed /spl pi/- and 2/spl pi/-modes. The underlying electromagnetic algorithm uses the finite element method on an unstructured conformal mesh, with both quadratic elements and quadratic basis functions to rapidly achieve a high degree of accuracy. For example, the /spl pi/-mode frequency of a coupled-cavity: TWT can be determined to 0.1% accuracy in 2-3 minutes on a 2 GHz PC. To determine dispersion curves for a coupled-cavity TWT, we model a chain of cavities using an open-mode expansion method. Fields in each cavity are represented as a linear combination of a small set of precomputed /spl pi/- and 2/spl pi/-eigenmode fields of the cavity, so that just a few mode amplitudes represent the complete 3D field. Coupling between adjacent cavities is calculated using a set of integrals over the cavity interfaces, derived directly from Maxwell´s equations. This formulation allows us to obtain a small matrix representation for the complete system that may be solved directly, for either the discrete modes of a chain of cavities or by applying a Floquet boundary condition to reduced representation of a single cavity. Results of this method are compared to experimental data. The core code has been written in object-oriented C++ for high performance, but most of the functionality is made directly available from within the Python language. This provides a simple yet highly flexible interface for specifying problem data, such as boundary conditions or material properties, or for automating complex calculations. At present, the mesh is imported from a commercial mesh generation tool, supporting a CAD interface, though a variety of mesh file formats are supported.