Variational analysis of helical slow wave structures

A variational technique has been used in predicting impedance and dispersion characteristics for the cold test measurements of helical traveling wave tubes. The variational formula of Bevensee is used in this method. An optimal combination of slow wave trial fields is found using Rumsey's reaction integral as a measure of equivalence to the true fields. In contrast to other approximate analyses, the variational formulation has the advantage of including the exact geometry and dielectric of the helical support. In order to verify the technique, two trial field regions have been used in successfully analyzing the sheath helix and the case of a homogeneous dielectric support. A composite metal-ceramic helix has also been analyzed. Simulation for the composite helix shows a reasonable dispersion for some combinations of harmonics over a wide frequency range when results are compared to experiment. However, the impedance computation is not yet satisfactory. In future developments, major modifications will be made to the trial fields in order to satisfy all boundary conditions simultaneously and improve the predicted impedance. In addition, the technique will be applied to other devices including wedge and rod supports.