| The submillimeter-wave or terahertz (THz) frequency regime of the electromagnetic spectrum (~ 300-3000 GHz) has enormous technology potential for communications, astronomy, security, medical imaging, biochemistry, and manufacturing. A lack of compact, powerful, reliable, efficient and relatively inexpensive THz coherent radiation sources has motivated research of microfabricated vacuum electronic devices. Traveling wave tubes (TWTs) are high power vacuum devices, which are used in a wide variety of applications, including radar, communications, electronic countermeasures. Regenerative feedback TWT oscillators have been proposed and investigated as attractive candidate sources of high frequency radiation. Recently, a compact TWT oscillator designed and fabricated following this approach and demonstrated a peak power of 50 mW corresponding to a circuit efficiency of 0.45% at 656 GHz. By continuous variation of beam voltage, the operating frequency was varied between 607– 675 GHz. This type of device uses a TWT as an amplifier, with a fraction of the output power recirculated to the input terminal. Broadband, incoherent, small-amplitude noise fluctuations in the TWT's electron beam current drive the system from quiescence into oscillation. By varying the amount of attenuation and the electrical length (phase or time delay) in the feedback circuit, they can operate with steady state single frequency, multi-frequency, or chaotic dynamics. This thesis is based on the study of this type of device, several important and valuable results are achieved and listed as the following:The study of the linear and non-linear model of traveling wave tubes. Based on Pierce theory, the small signal beam and wave interaction mathematical model is given, which contains space charge field, attenuation (loss) and backward wave. The non-linear model is based on the famous and popular large signal model CHRISTINE, where the space charge, loss and working equations in the sever regions are given as well. We have implemented computational codes of these equations, and the comparison with other similar existing models is also shown. These models are used to study the traveling wave tube (TWT) regenerative feedback oscillator. The detailed introduction, analysis and physical model of the TWT regenerative feedback oscillators is given. The oscillation theory is proposed and used to analyze the detailed oscillation of TWT regenerative feedback oscillators.A 560 GHz folded waveguide traveling wave tube (FWG-TWT) regenerative oscillator is modeled and simulated. The non-linear beam and wave interaction working equations are used to perform the process of the oscillation. The results show single oscillation frequencies exist between 550– 580 GHz, where the maximum output power is near 560 GHz.Based on the 650 GHz FWG-TWT oscillator observation, the oscillation frequency exhibited a step-tuning behavior, rather than a smooth variation with beam voltage. To confirm the hypothesis we theoretically model a 40 GHz FWG-TWT oscillator and experimentally study a 5 GHz helix-TWT oscillator. The FWG-TWT results exhibit an inclined step-tuning characteristic, similar to previously unreported observations of the THz-regime oscillator. Similar characteristics are also observed, along with multi-frequency oscillations, in the helix-TWT experiments.Microfabricated integration requires intensive attention before the feedback loop is fully built into a TWT, which makes it very important to investigate the effect of feedback attenuation in a high frequency TWT regenerative oscillator. Not only a helix TWT regenerative oscillator is used to predict this effect, but also the oscillation condition is given to demonstrate the agreement of experiment and theory.A time-transient TWT regenerative feedback oscillator measuring system has been built, several experimental results have been recorded and shown, including single frequency oscillation mode and a few multifrequency oscillation examples, chaos observation is also recorded and shown. The corresponding frequency domain experimental results are also presented.
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