| High-power microwaves is an area that has been of interest due to its applicability in an ever increasing range of fields. Virtual cathode oscillators (vircators) are devices capable of producing microwaves at levels above 1 giga-watt (GW) for short duration, less than a micro-second, and have been used in many applications. But conventional vircators are broadband, containing many modes, and very inefficient, with efficiencies around one to two percent. Hence, most research on vircators is based on increasing the efficiency.; The vircator at Texas Tech University is a unique coaxial geometry believed to be able to increase the efficiency. Previous work on the coaxial vircator has shown promising possibilities and the current research is aimed at understanding the physics of the coaxial vircator to be able to increase the efficiency. The work at Texas Tech involved making changes that kept the device simple in its operation while increasing the effectiveness of its operation. The various changes on the device were aimed at the diode region. The changes made include various screen materials, placement of collection rods on-axis in the diode, placement of a hole on axis in the center of the anode base, various voltages applied to the diode, and variations of the size and position of the emitting material.; Numerical simulations were first performed to test a wide variety of geometries and see how the vircator functioned with the changes made without having to physically perform the experiments. MAGIC, a 2{dollar}{lcub}1over 2{rcub}{dollar} dimensional particle-in-cell code was the tool used in the numerical simulations. From these simulations, a set of test geometries were implemented on the vircator at Texas Tech, for a total of eleven geometry changes at two different operating voltages.; Through these experiments, the operation of the coaxial vircator became better understood. The results showed that the highest power microwaves could be obtained with narrow emitting surfaces. Microwave levels above 1 GW were obtained at a power efficiency of 6.5%. With the understanding gained, future improvements may be made that could increase the output power and efficiency even further. |
