DESIGN OF TRAVELING-WAVE TUBES BASED ON FIELD-THEORY

A method is described for the design of helix traveling wave tubes (TW T) which is based on the linear field analysis of the coupled beam-wav e system. The dispersion relations are obtained by matching of radial admittances at boundaries instead of the individual field components. This approach provides flexibility in modeling various beam and circui t configurations with relative ease by choosing the appropriate admitt ance functions for each case. The method is illustrated for the case o f a solid beam inside a sheath helix which is loaded externally by los sy dielectric material, a conducting cylinder, and axial vanes. Extens ion of the analysis to include a thin tape helix model is anticipated in the near future. The TWT model may be divided into axial regions to include velocity tapers, lossy materials and severs, with the helix g eometry in each region varied arbitrarily. The relations between the a c velocities, current densities, and axial electric fields are used to derive a general expression for the new amplitudes of the three forwa rd waves at each axial boundary. The sum of the fields for the three f orward waves (two waves in a drift region) is followed to the circuit output. Numerical results of the field analysis are compared with the coupled-mode Pierce theory. A method is suggested for applying the fie ld analysis to accurate design of practical TWT's that have a more com plex circuit geometry, which starts with a simple measurement of the d ispersion of the helix circuit. The field analysis may then be used to generate a circuit having properties very nearly equivalent to those of the actual circuit.