Accurate Representation of Attenuation in Helix TWT Simulation Codes

We report on the results of a study that compares an ad hoc model of circuit attenuation used in helix traveling-wave tube (TWT) simulation codes with the exact 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 model used in those codes. This study was motivated in part by the fact that losses in both dielectrics and metals generally increase with increasing operating frequency, making the accuracy of the model a potential concern for millimeter-wave helix TWT design. Our basic conclusion is that the ad hoc model is sufficiently accurate for attenuation rates up to a few decibels per pitch in cases that we have studied. For much larger attenuation rates, as may occur in a sever region, accuracy can be improved by taking into account the (quadratic) dependence of the cold circuit phase velocity on the attenuation rate.