Study of planar-helix slow-wave structure for application to travelling-wave tubes

A planar helix, constituted of a pair of unidirectionally conducting screens conducting in different directions, is suggested as a slow-wave structure for application in a traveling-wave tube (TWT). Circuit parameters, such as interaction impedance and space-charge parameter, are derived for the suggested planar-helix TWT. Computed results for the planar helix indicate a performance comparable with that of its circular helix counterpart. Also, the change in interaction impedance and the dispersion characteristics of the planar helix, are considered in the presence of dielectric substrates and a metal shield. Results are obtained for a few different possible configurations of the planar helix on dielectric substrates with or without a metal shield. The phase velocity and interaction impedance reduce, both as the substrate thickness increases and as the metal shields are brought closer to the planar helix. However, the resulting degradation in the characteristics of the structure with commonly used substrate materials, for example alumina and beryllia, is less severe than in the case of the circular helix.