Mechanisms for phase distortion in a traveling wave tube

We present a view of the physics of phase distortion in a traveling wave tube (TWT) based on unique insights afforded by the MUSE models of a TWT [ J. Wöhlbier, J. Booske and I. Dobson IEEE Trans. Plasma Sci. 30 1063 (2002)]. The conclusion, supported by analytic theory and simulations, is that prior to gain compression phase distortion is due to harmonic frequencies in the electron beam and the resulting “intermodulation” frequency at the fundamental, and not the often cited “slowing down of electrons in the electron beam.” We draw these conclusions based on MUSE simulations that allow explicit control of electron beam frequency content, an analytic solution to the S-MUSE model [ J. Wöhlbier, J. Booske and I. Dobson IEEE Trans. Plasma Sci. 30 1063 (2002)] that reveals that phase distortion is due to the fact that the fundamental frequency is an intermodulation product of itself, and large signal LATTE [ J. Wöhlbier, J. Booske and I. Dobson IEEE Trans. Plasma Sci. 30 1063 (2002)] simulations that are modified to remove the effect of the slowing down of electrons in the electron beam. As applications of the theory we compare S-MUSE simulations to an amplitude phase model using the analytic phase transfer curve, we study dependence of phase distortion on circuit dispersion and electron beam parameters at the second harmonic with large signal LATTE simulations for narrow and wide band TWT designs, and we consider the phase distortion theory in the context of TWT linearization.