Accurate Space Charge Modeling for 1-D Large Signal Simulation of Sheet Beam TWTs

An improved space charge model for a 1-D large-signal simulation algorithm for sheet beam (SB) traveling wave tube (TWT) is proposed in this article, which is finally integrated into the simulation software KlyC. Rather than assuming a smooth rectangular waveguide for drifting tunnel as a conventional approach, the periodical geometric modification of metallic wall accounted for better modeling accuracy. The effect of the complex space charge field of modulated electrons in the slow wave structure (SWS) is approximated by modifying Green functions from the smooth waveguide system. To evaluate such modification coefficient, electrostatic problems are to be solved to calculate a series of Green functions of the space charge field within one period of SWS, before comparing with the Green function obtained from smooth rectangular boundaries. For most SB TWTs, the quasi-2-D SWS is employed, of which the modification coefficient could be calculated by 2-D numerical algorithms proposed in this article, aiming to speed up the evaluation. The 1-D large-signal simulation results conducted by KlyC with the improved space charge model are in good agreement with the CST particle-in-cell (PIC) simulations, where overall accuracy reaches several percent, with much faster computational speed (at least two orders of magnitude). Moreover, the proposed method of accurate space charge modeling can be further applied to any TWT with 3-D SWS theoretically.