For the design of microwave tubes, precise electromagnetic simulations of the cold structures are of particular importance. In this paper, we present the importance of fullwave vector simulations for the analysis of components in microwave tubes, particularly in high-power gyro-devices. Fullwave vector simulations based on surface integral equations allow an accurate analysis of various structures. A fast solution of surface integral equations is provided by the recently developed advanced

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... eration techniques. Simulations of quasi-optical mode converters in high-power gyrotrons, calculations of dispersion relations of gyrotron traveling-wave tubes with helical traveling wave structure and simulations of mirror systems are shown as examples for the wide range of applications of full wave vector simulations in the development of microwave tubes.