High-power, coaxial vircator geometries
Woolverton, Kevin Scott
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High-power microwave research 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 military applications, by universities as research tools in high-energy physics experiments and more recently, high-power microwaves are beginning to be used in commercial applications. Conventional vircators are broadband, usually containing many modes, and very inefficient, with efficiencies around one to two percent. The vircator at Texas Tech University is a 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 changes made on the device were changes on the diode and included the testing of 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 ^-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 geometries 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 close to 3 GW were obtained at a power efficiency of 13 %. With the understanding gained, future improvements may be made that could increase the output power and efficiency even further.