Accurate representation of attenuation in large - signal helix TWT simulation codes

We report on the results of a comparison study of the CHRISTINE 1D helix TWT simulation code with the small signal theory of a beam interacting with a slow wave supported by a sheath helix in the presence of loss, in order to ascertain whether attenuation is treated with sufficient accuracy by the ad hoc model used in the CHRISTINE code. This study was motivated in part by the fact that losses in both dielectrics and in metals generally increase with increasing operating frequency, making the accuracy of the CHRISTINE code a potential concern for mm-wave helix TWT design. The basic conclusion of this study is that the existing model of attenuation in CHRISTINE is sufficiently accurate for attenuation rates up to a few dB/pitch. For larger attenuation rates, accuracy can be further improved by taking into account the (quadratic) dependence of phase velocity on the attenuation rate.