Microwave emission in a plasma filled nonuniform backward wave oscillator
Young, Douglas T.
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High power microwave radiation is used in science, industry, and by the military for a variety of purposes including the acceleration of particles in high energy physics, heating of plasma particles, radiation treatment of surfaces in manufacturing, and for electronic warfare in the military. Among the many sources of high power microwaves, the backward wave oscillator (BWO) is one of the oldest devices and has undergone a continuous effort to produce higher output power and better efficiency. Motivated by recent experimental observations of an improved efficiency in a BWO by either applying a nonuniform slow wave structure (SWS) or using a plasma filling in a BWO, this dissertation focuses on the combined effects both a nonuniform SWS and plasma filling. The particle-in-cell computer simulation conducted in this study revealed the mechanism of microwave generation through the interaction of an electron beam and slow wave structure, either uniform or nonuniform, with and without the presence of plasma. The first result the simulations is that the electrons within the plasma are quickly driven out of the device by the interaction with the electron beam, leaving the electron beam in the BWO flowing in an ionic background. This resuh is significant since all analytical treatments of plasma filled BWOs to date have assumed that the electron beam is interacting with the plasma to produce electron plasma waves that, in turn, interact and enhance the output power. From the simulations, this could not be the case, since the plasma electrons are nearly depleted from plasma by the time power production begins. The second major result of this work is that a plasma filled uniform BWO behaves differently than and a plasma filled nonuniform BWO. The uniform BWO does show a substantial power enhancement when filled with plasma, and the radiation frequency is upshifted from 9.75 GHz to 9.91 GHz when plasma is introduced. The power in the nonuniform BWO is also enhanced by the plasma, but not as much as in the uniform BWO. In addition, the nonuniform BWO shows very little frequency shifting with plasma filling.