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Simulation-based-design (SBD) techniques to achieve "first-pass design success" depend on the development of fast, accurate, realistic models that can handle material properties, geometry, and appropriate boundary conditions. This paper describes a new three-dimensional (3-D) electromagnetic and large-signal simulation tool. Cold-Test and Large-Signal Simulator (CTLSS), which has been developed as part of an SBD tool suite for vacuum electron devices. Computational electromagnetic codes are essential for applying the SBD methodology to the design of vacuum electron devices and components. CTLSS offers the unique advantage that its computational electromagnetics model is linked intimately with a large-signal simulation tool for computing the electron-wave interaction in the radiating structure. Currently, this link has been implemented for helix traveling-wave tubes (TWTs) only, using the CHRISTINE code as the large-signal model, but a new, general, large-signal model is under development and is described in this paper. The electromagnetic simulation engine in CTLSS has been designed and implemented as a volumetric frequency-domain model that can handle both resonant eigenvalue problems, using the Jacobi-Davidson algorithm, and nonresonant driven-frequency problems, using the quasi-minimal residual (QMR) technique to invert the non-Hermitian matrices that often occur in real problems. The features and advantages of this code relative to other models and results from the code for several classes of microwave devices are presented.