COMPUTATIONAL INVESTIGATION OF EXPERIMENTAL INTERACTION IMPEDANCE OBTAINED BY PERTURBATION FOR HELICAL TRAVELING-WAVE TUBE STRUCTURES

Conventional methods used to measure the cold-test interaction impedan ce of helical slow-wave structures involve perturbing a helical circui t with a cylindrical dielectric rod placed on the central axis of the circuit. It has been shown that the difference in resonant frequency o r axial phase shift between the perturbed and unperturbed circuits can be related to the interaction impedance. However, because of the comp lex configuration of the helical circuit, deriving this relationship i nvolves several approximations. With the advent of accurate three-dime nsional (3-D) helical circuit models [1]-[3], these standard approxima tions can be fully investigated. This paper addresses the most promine nt approximations made in the analysis for measured interaction impeda nce by Lagerstrom [4] and investigates their accuracy using the 3-D si mulation code MAFIA. It is shown that a more accurate value of interac tion impedance can be obtained by using 3-D computational methods rath er than performing costly and time consuming experimental cold-test me asurements.