Field Theory of Traveling-Wave Tubes with Loss: Application to the Study of Attenuator Saturation Effects

Abstract

Propagation constants have been calculated for a lossy traveling- wave tube by means of a field theory. These results have been applied to the prediction of an attenuator power loss of the order of 2 or 3 db. compared to attenuatorless operation. It is shown that the gain of the higher order modes is negligible. Admittance matching by means of radial admittance transformation is the underlying method used throughout. The Pierce-Fletcher theory in common use at this time is examined in some detail to determine its range of validity. It is found to break down when the ratio of beam to helix radius is almost unity and when p_(o)a is small. Furthermore, it is shown that complex parameters must replace the real ones, Q, K, and C, if one wants to extend the Pierce notation correctly to lossy tubes. Not doing so leads to discrepancies of approximately 10% in the propagation constants calculated here. There is reason to believe that parameters other than those used here will not result in greater discrepancy. The effects of space charge bunching on saturation has been treated. Criteria have been set for determining whether bunching is important or not in determining saturation.