Linear Analysis of Helix Traveling-Wave Tubes With Nonnegligible Space Charge Using the Improved Helical-Harmonics Approximation

Linear dispersion relation of helix traveling-wave tubes (TWTs) is obtained and numerically solved using the improved helical-harmonics approximation (HHA), which has been recently presented for modeling helix thickness. The presented technique allows simultaneous examination of effects of various parameters, such as helix tape thickness, nonuniform beam current density profile, potential depression across the beam, finite axial magnetic field, and rod shape, on the tube´s small-signal characteristics. It is shown that while the results obtained from the 1-D Pierce theory agree well with those obtained from the field theory in the low-space-charge regime, they show major deviation in the nonresonant and high-space-charge regime. It is also shown that the small-signal gain and phase results obtained by means of the improved HHA agree well with those obtained from the tape-helix model and also the conventional HHA whenever the relevant cold dispersion characteristics are in a reasonable agreement.