The effect of electron beam shape on TWT performance

Most electron beams in TWT's are not perfect. The current density varies with distance and with radius (i.e., beam scalloping occurs). There are nonlaminarities due to initial beam formation and due to beam focusing peculiarities and which are most prevalent in PPM tubes, and which appear to be most severe in dual mode PPM TWT's. These non-laminarities result in beams that can be approximated by laminar beams which vary in beam current density as a function of beam radius and circuit distance. The major effects of beam shape variation from an ideal beam are to reduce the efficiency of the TWT by factors of 2 to 4. The reduction in efficiency is highly dependent upon beam size as well as beam shape. Most computer programs use a beam with uniform current density in the beam as a function of radius and distance. This paper presents results from two different computer models, one relating beam shape (i.e., radius), the other to beam scalloping (i.e., distance).