Numerical Determination of Interaction Impedance on Birdsall Slow-Wave Structures D. T. Lopes1and C. C. Motta2 1 Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN, Sao Paulo-SP, Brazil, 05422-970 2 Centro Tecnologico da Marinha em Sao Paulo, Sao Paulo-SP, Brazil, 05508-900 Abstract: This paper presents the results of the initial stage in the development of a computer code based on the tangential vector finite element method for computing the dispersion and the interaction impedance characteristics of a Birdsall type slow-wave structure (SWS). This code has been developed in order to consider the coupling wave- guides effect on the interaction impedance. Keywords: Interaction impedance; microwave devices; slow-wave structures; traveling-wave tubes. Introduction In the last years the numerical simulation of microwave vacuum devices had a notable growth and progress have been made on computing the effects of the structure thickness and length, dielectric supports, input and output couplings, etc, in its electromagnetic behavior. To compute these effects is vital for increase the device efficiency. Following this trend we present a numerical tool for solving the Maxwell equations for a TWT slow-wave structure (SWS) based on the Birdsall structure [1] in order to obtain its dispersion and interaction impedance characteristics. The code is based on the tangential vector finite element method and it has an automated mesh generator especially developed to create 3D meshes for SWS of the Birdsall type including the coupling guides since all dimension parameters are given. The inherent generalized eigenvalue problem is solved according to interaction subspace techniques. Mesh generation For the application of the tangential vector finite element method we developed an automated parametric and adaptive mesh generator especially developed to construct meshes for Birdsall type SWS. The finite element used is the Nedelec prism. This code has (for while) three considerable improvements over a previous code [2] developed for the same role. The first is the removal of the Floquet periodic boundary conditions in order to consider the fmite length of the SWS. The second is the consideration of the effects of the coupling guides in the SWS extremities. The third is the consideration of the helix support rods. The SWS is divided in five regions where one can refine the discretization, making the tool more versatile. An example of the structure discretization is shown in figure 1. Excitation guide Birdsall helix region Circular waveguide interior region (vacuum) helix support region /s _ I Load %mMsND~~ ~ ~ ~ ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~ _Io I Figure 1. An illustrative example of the discretization produced. This work was supported in part by the National Council for Scientific and Technological Developement (CNPq) under grant proc. 134966/2005-8 1-4244-0108-9/06/$20.00 © 2006 IEEE 119