Browsing by Author "Krile, John T."
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Item A Model of high power microwave surface flashover at atmospheric pressures(2012-05) Ford, Patrick; Neuber, Andreas A.; Krompholz, Hermann G.; Krile, John T.High Power Microwave Surface Flashover across a dielectric boundary between a high power microwave source and atmospheric environment can cause significant reflection and attenuation of the incident power, leading to a reduction in source efficacy or damage to sensitive components; therefore, it is important to examine the potential causes and effects of surface flashover in high power microwave systems (such as radar). The development of a finite-difference simulation that models the microwave power response to the surface flashover plasma is presented in this thesis, along with background theory related to microwave breakdown in atmospheric gases. Surface flashover is reproduced in the laboratory using a 2.85 GHz source to produce a 3.18 MW, 3 us pulse at atmospheric pressures that are typical for high altitudes (60 - 400 Torr); the full experimental apparatus is discussed.Item Anode material testing in a vacuum diode(2011-05) Vara, John; Dickens, James C.; Krile, John T.The gas evolved during the operation of a virtual cathode oscillator (vircator), which can be detrimental to the maximum output power, repetition rate, and pulse width was studied. Gases are known to evolve both from processes at the cathode, such as explosive electron emission, and anode processes such as heating by the electron beam. A residual gas analyzer (RGA) and pressure measurements have been used to perform characterization of background gases before and after the operation of the vircator. Multiple anode materials have been tested, with measurements made of both the quantity and types of gases evolved during firing. The test materials include stainless steel, copper tungsten, tantalum, nickel,molybdenum, and oxygen-free high-conductivity copper. For Nickel, two anodes are machined and given different treatments before operation in the sealed vircator. The first being a high temperature bake-out under vacuum (10-7 Torr) followed by an ultrasonic cleaning. The second treatment omits the high temperature bake-out. A low impedance Marx generator, with no intermediate pulse forming apparatus, is used to drive the vircator. For all testing, an aluminum cathode is used. The pressure measurement systems and diagnostics are described, and gas analysis and composition are presented for multiple anode materials.Item Physics of dielectric surface flashover at atmospheric pressures(Texas Tech University, 2003-12) Krile, John T.The limits of the application of dc, ac, or pulsed high voltage are determined by breakdown along insulators or insulating support structures. It is of major technical importance to predict breakdown voltages for given structures, with parameters such as geometry, material, and temporal characteristics of the applied voltage. The impact of atmospheric conditions such as humidity, pressure, temperature, and types of gas present is also important. In order to determine the processes involved in surface flashover, the test setup is designed to produce and closely monitor breakdowns across various gap distances and insulator geometries at atmospheric conditions with varying humidity. Current, voltage, luminosity, and optical emission spectra are measured with nanosecond to sub-nanosecond resolution. Spatially and temporally resolved light emission data is also collected. Results obtained from the light emission data show that a short light emission pulse is first detected at the cathode, a result of electron emission at the cathode. Approximately 100 ns later, significantly more light is detected at the anode when the electron avalanche strikes it. Finally, 50ns later, light is detected at the center of the gap, as the streamer reaches the cathode and the gap closes. The fast imaging data shows a distinct trend for the spark in air to closely follow the surface even if a strong normal electrical field component is present. This tendency is lacking in the presence of gas such as nitrogen, where the spark develops and remains away from the surface. Finally, the breakdown voltage is shown to decrease significantly with an increase in humidity.Item Surface flashover under RF and unipolar excitation at atmospheric conditions(2006-05) Krile, John T.; Neuber, Andreas A.; Krompholz, Hermann G.; Gibson, Thomas; Kristiansen, MagneIn vacuum environments surface flashover events driven by RF (f < 10 GHz) and unipolar excitation have been shown to have virtually identical dominant mechanisms. Similarities between RF (representing high-power microwave window breakdown on the high pressure side) and unipolar surface flashover are expected in an atmospheric environment as well. Extensive testing has already been completed for both DC and pulsed unipolar flashover with several dominant processes identified. Further testing with a setup capable of HPM driven surface flashover are conducted to identify similar dominant processes. The two separate experimental setups, utilized to investigate both unipolar flashover and RF window flashover under atmospheric conditions, enable controlling excitation, temperature, pressure, humidity, and type of gas present, all under similar electric field�surface geometry. In order to ensure the conditions are as matched as possible, the local electric field at the flashover initiating points has been numerically calculated in detail for all test geometries. In addition a Monte Carlo type electron motion simulation program was created to further isolate the individual processes. For both RF and unipolar pulsed excitation, the flashover dynamics are changed by the application of UV light to the dielectric surface. A UV pre-pulse has a distinct impact on the arc�s path and a tendency to decrease the hold-off electric field. The effect of humidity on the hold-off electric field for both pulsed unipolar and RF excitations, along with temporally resolved emission spectroscopy of the flashover event, are discussed.