3-D Nonlinear Theory for Sheet-Beam Folded-Waveguide Traveling-Wave Tubes

A 3-D frequency-domain nonlinear model for sheet beam (SB) traveling-wave tubes (TWTs) based on folded waveguides (FWs) is presented in this paper. The SB with finite thickness is a 3-D asymmetric system. To characterize such a system, an accurate description of the radio frequency (RF) fields and the space charge (SC) fields in the interaction gap in all three dimensions is required. An analytical expression for the interaction electric fields is derived. Beside of the interaction electric fields, a modified analytical model for the dispersion and interaction impedance is also imbedded in our code predicting the cold characteristics of the waveguide, which agrees with the results of the 3-D simulation code CST. As a result, the model provides a through predictive capability from the geometrical parameters to the device performance, such as saturation power and gain. Due to the short computing time, our code is very useful for the large-scale optimizations of TWTs. For the calculation of the SC field, a particle-in-cell method is used by solving the 3-D Helmholtz equation and the motion equation. Such analytical technology is much more preferable to interpret the complicated beam-wave interaction than the conventional electron beam models, which attributes to the ever-increasing computing capacity of the computers. These features have been incorporated in the development of our large-signal code SB-FWTWT-3-D. The validation of the model is performed and comprehensive results are presented.