Initial plasma-filled backward-wave oscillator experiments using a cathode-mounted plasma prefill source

The introduction of plasma into the slow-wave structure (SWS) of a backward-wave oscillator (BWO) has been shown to increase microwave power output and generation efficiency, as well as provide several other benefits. Researchers at Niigata University in Japan have performed a linear analysis that has shown that the optimal plasma-filled BWO configuration is one in which the plasma is confined close to the axis of the SWS, while the electron beam driving the device is kept close to the walls. Previous schemes for preinjecting plasma have utilized sources mounted downstream of the SWS and have relied upon the converging field lines of the electron beam´s guiding magnetic field to compress the plasma close to the axis as it entered the SWS. This paper presents data from experiments using a plasma source mounted internal to the cathode structure of a high-power BWO, thus providing injection of the plasma directly on axis in the strong, uniform field region of the BWO. Data describing plasma prefill density at different axial positions within the SWS prior to operation is presented. The plasma prefill has been found to both enhance and reduce the microwave generation efficiency, depending on the plasma density. Details of the novel plasma injection system are also provided