Browsing by Subject "Microwave devices"
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Item Experimental studies on magnetic field and pressure dependence of high power microwaves at a dielectric surface(Texas Tech University, 2002-05) Hemmert, David J.The number of anticipated military applications of high power microwaves has greatiy increased in the past few years. Several limitations in the operation of high power microwave devices have been identified including breakdown at dielectric interfaces. These limitations prevent modem devices from being used to their full capabilities. Current research has focused on different materials, geometries, and coatings of dielectric windows. To truly understand how to maximize a dielectric window's capabilities, the basic physics of breakdown at a dielectric interface must be understood. In the work reported here, studies are made using a simple interface geometry for testing at power density levels up to 10 MW/cm^. An alumina sample with a planar geometry is placed in a rectangular waveguide such that the sample surface is parallel to the microwave electric field and perpendicular to the microwave propagation. A 4 MW, S-band magnetron is coupled to a traveling wave resonant ring to produce power levels for breakdown up to 100 MW. High speed (nanosecond) and high spatial resolution diagnostics are used to study the breakdown phenomena at the sample surface. The studies identified evidence of secondary electron emission avalanche occurring on the sample surface initiating breakdown as well as evidence of the microwave magnetic field contributing significantly to the avalanche effect for high power breakdown. This was corroborated by a significant difference in breakdovm levels on the upstream interface side of the sample compared to breakdown on the downstream side. Further studies compared gas breakdown to gas/interface breakdown and identified pressure regions in which breakdown is avalanche dominated or collision dominated.Item Microwave emission in a plasma filled nonuniform backward wave oscillator(Texas Tech University, 1998-05) Young, Douglas T.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.Item Microwave window breakdown(Texas Tech University, 1997-08) Elliott, Jason PaulDielectric windows separating vacuum from atmospheric pressure are essential in the operation of high power microwave devices and linear accelerators. Due to the large electric fields associated with high power microwave and linear accelerator devices, breakdown and flashover across the dielectric window is a commonly occurring phenomenon. Since the physical mechanisms leading to dielectric window failure are not fully understood, the advancement of high power devices utilizing dielectric windows is currently limited. A microwave system capable of monitoring and acquiring data geared toward the pre-window breakdown mechanisms has been created through the union of a traveling wave resonator and coaxial magnetron. The system consists of a high voltage DC power supply to charge a pulse forming capacitor network. The capacitor network is discharged to a pulse transformer, which is directly attached to the cathode of the magnetron. The microwaves are axially extracted from the magnetron to the traveling wave resonator, or ring resonator, and are applied to a dielectric window contained in the ring. As the traveling wave circumnavigates the ring, the power level will increase eventually causing a dielectric window breakdown. High speed diagnostics present on this system include forward and reverse power couplers and field probes to monitor the microwave power and field at the dielectric window. Luminosity and soft x-ray emission were recorded through high speed optical sensors. High purity alumina is the focus for this experiment, but rexolite is investigated and compared to the alumina results.Item The design of a high power electron beam generator(Texas Tech University, 1989-08) Augsburger, Blake W.The object of this experiment is to study high power microwave breakdown across a dielectric window. In order to achieve this objective a microwave source is designed and constructed. This report discusses the design of a pulsed power system used to drive this microwave source. This experiment is funded by the AFOSR/AFWL. The experiment was named THOR, after the Norse God of lightning and thunder.