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In this paper, an improved model for the beam-wave interaction of sheet beam in traveling wave tubes (TWTs) considering ohmic losses and reflections is presented. The ohmic losses are obtained by field analysis and equivalent method. The space charge magnetic field is derived from the active Helmholtz's equation. An algorithm to obtain the S-matrix by the equivalent circuit method is presented. The relativistic Boris method is applied to accelerate macroparticles. The exchanged power is computed by the work the electromagnetic field applied to the macroparticles. The theoretical model is applied for validation to a G-band staggered double vane TWT and validated by comparison with CST Particle Studio and simulations without losses and reflections. The convergence of this algorithm is also discussed. The simulation time of the model is substantial faster than 3D Particle-in-cell (PIC) simulations.  Index Terms-beam-wave interaction, losses, reflections, sheet beam TWT I. INTRODUCTIONHMIC losses and reflections of slow wave structures (SWSs) are important parameters for a reliable and accurate simulation of TWTs. This is particularly important at millimeter wave (30 -300 GHz) and Terahertz (THz, 300 GHz -1000 GHz) frequencies due to the small dimensions that affects the fabrication accuracy and the decreasing skin depth that requires high quality surface roughness to keep losses low.The 3D fast time-domain nonlinear algorithm (FTDNA-3D) presented in [1] is a PIC-method-based approach for

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