Andrei V. Soukhov

Wide-band semivane and heavily dielectric loaded helix traveling-wave tubes

Theoretical simulation of the dispersion and interaction impedance characteristics of a semivane loaded negative dispersion helical slow-wave structure, which is a slight variant of a conventional vane-loaded helix of a wide-band traveling-wave tube (TWT), was validated for the nonresonant perturbation measurement. While the experimental and theoretical results agreed well with respect to interaction impedance for both the semivane structure and the structure without vanes, there was disagreement found with respect to the dispersion characteristics of the semivane structure, which was explained quantitatively by the deformation that had taken place in assembling the structure. The output performance of a TWT using the semivane structure fairly agreed with the prediction by an in-house one-dimensional nonlinear Lagrangian code, with respect to saturated output power (/spl sim/50 W), saturated gain (/spl sim/40 dB), and lower-band-edge second harmonic level (/spl sim/-5 dBc), in the operating frequency range of 6-18 GHz, except at the low frequency end of the operating band, where the saturated power was less and the second harmonic level higher than predicted, a finding that may be attributed to the departure of the deformed structure from exhibiting a negative dispersion at the lower band edge.

Positive phase-velocity tapering of broadband helix traveling-wave tubes for efficiency enhancement

A positive phase-velocity tapering of 1.5 octave broadband helix traveling-wave tubes for efficiency enhancement, where the phase velocity is linearly increased in the output section, was studied by using the one-dimensional nonlinear theory. At high frequencies, the electromagnetic wave in the positively tapered section traps the fastest electrons in the decelerating electric field, extracting more energy from the electron beam. At low frequencies, a decreased velocity difference between the electron beam and the electromagnetic wave destroys the phase condition for second-harmonic generation, retaining fundamental wave efficiency as well as reducing its second-harmonic power.

Effect of conductive perturber diameter on nonresonant measurement of interaction impedance for helical slow-wave structures

The measurement accuracy of the interaction impedance for a helical slow-wave structure (SWS) using the nonresonant perturbation method has been studied using conductive wire perturbers with different diameters. Data obtained by the measurement were compared with a rigorous numerical analysis. It is shown that the measured values of the interaction impedance for the helical SWS converge to those obtained by using a three-dimensional finite-element computational method when the diameter of the perturber is reduced to less than 10% of the helix diameter.

Method for measuring interaction impedance in helix TWT

A nonresonant method for measuring the interaction impedance in a helix TWT using a hairline conductive wire is studied. The theory of the interaction impedance measurements using a dielectric and/or a conductive perturbing rod with a finite diameter, becomes complex when the upper space harmonics of a slow wave structure are taken into account. This problem may be overcome if one uses a hairline conductive wire instead of a dielectric rod with a finite diameter. The diameter of this conductive wire placed precisely along the helix axis, is about 1-2% of the helix diameter. It is shown that in this case the change of the propagation constant characterises the interaction impedance of the fundamental space harmonic independently of the upper harmonics.

Design of wideband helix TWT

The wideband helix traveling-wave tube (TWT) using an electron beam of voltage 4 kV and current 140 mA is designed over the frequency range of 6-18 GHz by a nonlinear theory employing one-dimensional particle-in-cell (PIC) method. The nonlinear theory involves a large particle beam model in which the electron beam is treated as the electron disk instead of the hydrodynamic fluid in the linear theory for the numerical calculations. A linear theory is used to verify the numerical algorithm of the nonlinear theory in a small-signal mode.