One-Dimensional Nonlinear Model for Helix TWTs

Abstract

This paper describes the development of a one-dimensional large signal (nonlinear) model for helix travelling wave tubes (TWTs). Lagrangian formulation (large-particle method) has been used to model the electron beam in terms of the well defined cylindrical charged discs. The motion of each charged disc is tracked, in small distance steps down the tube, in presence of the rf circuit field and the space charge field forces. The helix slow wave structure has been represented by generalised transmission line for defining the circuit field and for calculating the energy transfer from the modulated beam to the circuit. The large signal model is self-consistent and fast enough for interactive use. The internal energy balance check is typically satisfied within 1 per cent of the input beam power even beyond saturation, showing high numerical accuracy of the model. The program takes less than 3 minutes in one pass through a tube of length nearly 30 beam wavelengths for number of discs per rf cycle 24 and number of integration steps per rf beam wavelength 16, on VAX 11/730 computer. There is further scope to increase the speed of the program. The computed results were compared with some experimental results of a C-band, 20-W helix TWT, developed earlier at this institute, and they show reasonable accuracy.