Effect of Attenuation in Synchronous and Non-Synchronous Beam-Wave Interactions

Summary form only given. The gain of a traveling wave tube, in which the beam velocity is closely matched to the phase velocity of a slow circuit wave, decreases as the circuit loss increases. However, sufficiently far from synchronism an increase in circuit loss will cause an increase in gain - a phenomenon well known as the resistive wall instability. Between these extremes there are parameter regimes in which the gain is relatively insensitive to loss. A careful analytical study of a beam interacting with the slow wave fields propagating on a sheath helix, itself supported by a lossy dielectric sleeve, illustrates these effects. The central goal of this study is to determine whether the approximate treatment of circuit attenuation in the CHRISTINE ID large signal helix TWT simulation code is sufficiently accurate for the largest values of attenuation encountered in TWTs (in and near severs), for all operating voltages. Our approach has been to develop an exact small signal theory and associated code for the analysis of a beam interacting with a slow wave supported by a sheath helix. In this paper we will describe the approximate model for attenuation used in CHRISTINE as well as the exact ID small signal model. Numerical results from both models and comparisons using a C-band helix TWT will be presented.