2.5-Dimensional Multi-Signal Large-Signal Analysis of Helix TWTs

Abstract

A computer model (SUNRAY-3D) for the 2.5-dimensional multi-signal large-signal analysis of helix TWTs has been developed based on the approach developed earlier by the author for the one-dimensional large-signal modelling of helix and coupled-cavity TWTs. Some of the features of O'Malley's work for three-dimensional modelling of coupled-cavity TWTs were incorporated successfully for the modelling of helix TWTs. The present model is capable for the analysis of multiple input signals together with their higher order harmonics and inter-modulation products. The expressions for the circuit field and the induced current at a plane were derived from the electric field in a helical slow-wave structure instead of using the conventional equivalent circuit approach. The program analyses TWTs with arbitrary variations in the helix pitch and rf loss (including sever and tip loss) along the slow-wave structure. Induced backward voltage components and reflected voltage components due to mismatches at the terminations of the helix were included. The program has been shown to be accurate and self-consistent for drive powers from 60dB below saturation to well above saturation. The energy balance factor is satisfied to better than ±0.1% even at and above saturation. The CPU time to compute the performance of a typical TWT for a single drive power with 96 rings and 16 steps per wavelength is less than one minute on a DEC-ALPHA computer and is much less than 20 seconds on an IBM-P-IV.