Electron emission from slow-wave structure walls in a long-pulse, high-power backward wave oscillator

Pulse shortening is a phenomenon common among all long-pulse, electron-beam-driven high-power microwave sources. Although the electron beam driving the source may continue to propagate through the interaction region of the device for several microseconds or more, the duration of the emitted microwave pulse is typically no more than ~100 ns. Most explanations of this phenomenon put forth involve the introduction of plasma into the interaction region and/or the degradation of beam quality. This paper describes experiments conducted on the University of New Mexico´s Long-Pulse Backward Wave Oscillator (BWO) Experiment which investigate the behavior of beam electrons in the slow-wave structure (SWS) during microwave generation. A current probe having a small aperture at a variable radius is placed within the SWS to monitor the beam current profile at different radii as a function of time. Results from these experiments reveal the appearance of electrons between SWS ripples at times corresponding to the peaks of the microwave pulses in separate experiments. A drop in the main beam current is observed shortly thereafter. The source of the electrons within the ripples is thought to be field emission or secondary electron emission from the SWS walls or emission from plasmas generated there