Three-dimensional simulations of PPM focused helical traveling-wave tubes

A three-dimensional (3D) helical traveling wave-tube (TWT) interaction model has been developed using the electromagnetic particle-in-cell (PIC) code MAFIA. The model includes a short section of helical slow-wave circuit with excitation fed by RF input/output couplers, electron beam with initial transverse velocity distribution, and periodic permanent magnet (PPM) focusing. All components of the model are simulated in three dimensions allowing several azimuthally asymmetric TWT characteristics to be investigated for the first time. These include (1) C-magnets, shunts and magnet misalignment and their effects on electron beam optics, and (2) 3D helical RF fields and their effects on RF circuit/beam interaction. The development and validation of each component of the interaction model is presented, and 3D TWT electron beam characteristics are compared in the absence of and under the influence of the azimuthally asymmetric characteristics described. Of the most significant results, the 3D beam optics simulations showed that inclusion of an output C-magnet caused a substantial beam shift off the tube's central axis. This implies a commensurate reduction in overall TWT efficiency from excess beam/RF circuit interception and inefficient collection of the asymmetric spent beam. Comparisons between the 3D helical TWT interaction model with a commonly used 2.5D model showed good agreement with small beam diameters, with increasing discrepancies with increasing beam diameter and input drive power where 2.5D approximations become less accurate and more prominent. These results demonstrate the effects 2.5D approximations have on predicted TWT performance, and indicate that the approximations also contribute to disagreement between measured TWT performance and that predicted by conventional 2.5D codes. This 3D time-domain helical TWT interaction model can be used for the first simulated investigations of data transmission through the TWT in order to correlate signal degradation with TWT geometry and operating parameters. Intermodulation products, harmonic generation and backward waves can also be monitored with the model for similar correlations. The advancements in computational capabilities and corresponding potential improvements in TWT linearity and efficiency may prove to be the enabling technologies for realizing unprecedented data rates for near real time transmission of increasingly larger volumes of data demanded by planned satellite communications applications.