Abstract: Recent results of this novel amplifier-oscillator system are reported. Parameterization of the TWT's gain and amplification bandwidth as well as the phase coherence between the two TWT's rf outputs as a function of BWO operating frequency are presented. The experimental apparatus is designed to simultaneously generate three intense annular electron beams and propagate them through twin Traveling Wave Tube (TWT) amplifiers and a Backward Wave Oscillator (BWO). The high power BWO and TWTs are composed of cylindrical waveguide with a sinusoidally varying wall and are designed to operate in the X-band frequency range at a total system power level of a few 100 MWs. A broad-band microwave coupler built into the system is used to extract the spontaneously generated microwave radiation from the BWO and redirect it into two TWTs for amplification. The system is frequency tuned by independently varying the parameters of the electron beam driving the BWO while maintaining constant beam conditions for the TWTs. The performance of each amplifier is individually investigated for gain and amplification bandwidth as a function of BWO operating frequency and for gain as a function of beam current. Since the TWTs operate below device saturation levels, the gain and amplification bandwidth results are compared to linear theory. The two TWT output signals are further compared for phase coherence. Techniques used to eliminate mode competition between the TWT's primary TM$-01$/ amplifier mode and higher modes which behave as oscillators are also reviewed.!
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