Step-tuning characteristics of traveling wave tube regenerative-feedback oscillators

Summary form only given: A lack of compact, powerful, and efficient sources in the submillimeter-wave or terahertz-regime of frequencies has motivated research of microfabricated vacuum electronic devices. Regenerative-feedback traveling wave tube (TWT) oscillators have been proposed as a candidate source. Recently, a compact TWT oscillator was fabricated and demonstrated with an intrinsic efficiency of -0.34% at 638 GHz. This device utilized a folded waveguide structure as the slow wave circuit. By continuous variation of the beam voltage, the operating frequency was varied between 607-675 GHz. However, the oscillation frequency exhibited a step-tuning behavior, rather than a smooth variation with beam voltage. In this talk we will present the results of a computational and experimental study of the frequency tuning characteristics of regenerative-feedback TWT oscillators. Simulations utilized the Christine-ID nonlinear TWT code to simulate multi-pass physics of a regenerative TWT oscillator. The initial (and ever-present) stimulus of beam current micro-fluctuations was modeled as shot noise. Experiments were conducted with a low power, microwave-frequency, helix TWT. Both studies revealed the same step-tuning behavior as reported. The physical mechanisms responsible for the step-tuning behavior will be described.