An Improved Treatment of AC Space Charge Fields in Large Signal Simulation Codes

An accurate representation of the AC space charge electric field is required in order to be able to predict the performance of linear beam tubes, including TWTs and klystrons, using a steady state simulation code like CHRISTINE (Antonsen Jr and Lebush, 1997). The model implemented in the current release version of CHRISTINE is based on the assumption that the space charge reduction factor R is insensitive to axial wavenumber k, for a small range of k near k_b equiv omega/v_b, where omega is the circular frequency of the wave and v_b is the beam velocity. This assumption is usually a good one, but we have found cases in which the variation of R with k must be taken into account. We show how this can be done, and illustrate the effect in an example