A cathode mounted plasma prefill source for high power microwave experiments

The introduction of a 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. Two methods of generating the plasmas are electron beam impact ionization of a background gas and prefilling through use of an external plasma source. It has been shown that optimally the plasma should be confined near the axis of the SWS, while the electron beam driving the device should be as close to the walls as possible. Previous plasma-filled BWO experiments used external plasma sources that were mounted in the microwave extraction region of the BWO, making use of the electron beam's guiding magnetic field to compress the plasma onto the axis as it enters the SWS. At the University of New Mexico a plasma source has been developed for the UNM Long-Pulse BWO Experiment which can be mounted upstream of the SWS, directly into the cathode stalk of the electron beam diode. This source utilizes a flashboard, similar to the kind used in plasma opening switches, to create the plasma. Plasma is injected directly onto the axis of the SWS, and since the source hardware has been removed from the extraction region, problems with coupling out the microwave power are eliminated, and the amount of plasma in this region is reduced. Data such as plasma density at different axial positions in the BWO without the electron beam is presented, followed by a discussion on the changes in BWO performance when the experiment is fired with the addition of this plasma. Details of the novel plasma injection system are also presented.