Impulse amplification in a traveling-wave tube - II: Large signal physics

The helix traveling-wave tube (TWT) is a vacuum electronic device which utilizes an electron beam to amplify electromagnetic waves. Although having a broad frequency gain bandwidth compared to other vacuum electron devices, the helix TWT has been used in relatively narrowband ways in its applications in communications and electronic countermeasures. Recent interest in ultrawideband (UWB) radio and radar, combined with the recent development of UWB helix TWTs, has stimulated interest in the amplification of very broadband multifrequency signals and impulses. A number of studies have been conducted on the harmonic and intermodulation effects of multiple signals in a TWT, but there has not been much investigation into impulse amplification. Utilizing a nonlinear time-domain model developed for this purpose, the response of a wideband (helix) TWT to an input Gaussian pulse is examined. Differences between the large signal response to positive and negative Gaussian inputs are examined, and it is shown that a positive impulse input yields better pulse shape preservation, higher saturation level, and possibly greater efficiency. The growth and saturation mechanisms of a positive input Gaussian are then investigated, showing distinct differences from steady-state growth and saturation mechanisms. This study indicates that the possibility of amplification of impulses in a TWT is very promising. The results and conclusions of this study generally apply to all types of TWTs with appreciable bandwidth (e.g., couple-cavity folded waveguide). However, as the helix TWT has the largest bandwidth, it is the most interesting and is the specific TWT studied here.