Nonlinear analysis of beam-wave interaction in a planar THz travelling-wave tube amplifier

Large-signal analysis is presented for analysing non-linear beam-wave interaction in a planar THz TWT with a sheet electron beam. The sheet beam within one RF beam wavelength is represented by uniform rectangular charged discs for confined flow of the beam, and the discs are tracked in small distance steps of forward integration. For a planar THz TWT, a staggered double-vane loaded rectangular waveguide slow-wave structure (SDVSWS) is used. Frequency-domain analysis is used to define the RF circuit field in the SDVSWS and the interaction of the RF circuit field with the electron beam is carried out for steady-state TWT operation. The RF circuit field is considered to be continuous as in a helix SWS and it is represented by transmission line theory at each integration plane. A large-signal analysis code of a helix TWT with pencil beam (SUNRAY-1D) was modified for a planar TWT with a sheet beam. The accuracy of the modified SUNRAY code was checked against the 3D e.m. field simulator (CST-PS) by comparing the output performance for a 0.22-THz planar TWT. The results for the gain and the output power over the operating band given by the two codes were found to be comparable. A significant advantage of the SUNRAY code is that it is very fast compared to the CST code. It takes less than a minute to simulate the RF performance of a typical THz TWT at a single frequency and drive power on a standard PC. The SUNRAY code can therefore, be used interactively for design a complete SWS with input/output couplers, sever, loss profiles and phase velocity taper for a high efficiency high gain THz TWT.