Browsing by Subject "Microwave"
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Item Effects of diode gap closure and bipolar flow on vircator microwave generation(Texas Tech University, 1996-05) Young, Douglas T.High power microwaves are useful in a wide variety of applications in science, industry and in the military. The virtual cathode oscillator, or vircator, has been seen to produce very high output power over a widely tunable range compared with other high power microwave sources. Many experiments and numerical simulations have been done to study various aspects of vircators and the radiation produced by them. Two effects which have not been previously studied are the effects of diode gap closure and bipolar flow on the microwave radiation produced in vircators. In this work, these effects are studied using the PIC code MAGIC. One- and two-dimensional diode simulations agree well with known analytical results formulated for these diode cases. In both one- and two-dimensional vircator simulations, diode gap closure and bipolar flow were studied in detail. The geometry used in the two-dimensional vircator was made to be similar to that of an experiment done at the Himeji Institute of Technology in Japan. This was done so that comparisons between the simulations and an actual experiment could be made. The primary conclusion of this thesis is that neither bipolar flow alone nor diode gap closure alone can generate electron beam pinching in a two-dimensional vircator in which the experiment did show electron beam pinching. The second conclusion that is drawn in this work is that diode gap closure is the mechanism that allows vircators to achieve higher output powers. Finally, comparisons between the one- and two-dimensional vircators with diode gap closure or bipolar flow show a dramatic difference in the frequency spectrum of the output microwaves. Suggestions for further work, which include the creation of a model which combines both diode gap closure and bipolar flow, are also included.Item Experimentation and modeling of pulse sharpening and gyromagnetic precession within a nonlinear transmission line(2011-08) Vaselaar, Andrew; Bayne, Stephen B.; Mankowski, John J.A computer model is developed to predict the output of a NonLinear Transmission Line (NLTL) based upon an input pulse, material properties and physical dimensions. Additionally, a test bed is developed with the goal of validating the computer model with regards to its level of accuracy in comparison with a real world system. The test bed is also designed to provide the apparatus for evaluating ferrite materials for their performance characteristics as components for NLTL applications. The resulting model provides two computation algorithms that successfully simulate aspects of NLTL operation. They require further development to improve their computational algorithms and to take into account more material properties in order to serve as a better tool for design of NLTL systems. The NLTL testbed results in a working pulser and diagnostic system, however the NLTL suffers from high voltage breakdown that prevents it from functioning as designed.Item High-power microwave breakdown of dielectric interfaces(Texas Tech University, 1991-08) Calico, Steve EugeneA project to study the electrical breakdown of microwave windows due to high-power pulsed microwave fields was undertaken at Texas Tech University. The pulsed power equipment was acquired from the Air Force Weapons Laboratory (now Phillips Laboratory) in Albuquerque, NM, refurbished and redesigned as necessary, and serves as the high-power microwave source. The microwaves are used to test various vacuum to atmosphere interfaces (windows) in an attempt to isolate the mechanisms governing the electrical breakdown at the window. Windows made of three different materials and of three basic geometrical designs were tested in this experiment. Additionally, the surfaces of two windows were sanded with different grit sandpapers to determine the effect the surface texture has on the breakdown. The windows were tested in atmospheric pressure air, argon, helium, and to a lesser extent sulfur-hexafluoride. Estimates of the breakdown threshold in air and argon on a Lexan window were obtained as a consequence of these tests and were found to be considerably lower than that reported for pulsed microwave breakdown in gases. A hypothesis is presented in an attempt to explain the lower breakdown electric field threshold. A discussion of the comparative performance of the windows and an explanation as to the enhanced performance of some windows is given.Item High-power, coaxial vircator geometries(Texas Tech University, 1998-05) Woolverton, Kevin ScottHigh-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.Item Investigation of trace amounts of gas on microvave water-cut measurement(Texas A&M University, 2006-08-16) Liu, JinIn recent years, the upstream oil and gas industry has dealt with some of the most challenging metering applications. One of these is the measurement of water percentage at the point of allocation. It is an essential requirement when test separators or the newly developed full multiphase meters are utilized for oil well production testing. Water-cut can be obtained from measurement of differential pressure, capacitance/conductance, gamma rays absorption, absorption of infrared light, coriolis mass measurement, or microwave permittivity. The use of microwave permittivity has been shown to be very effective with the added benefit of not requiring a nuclear source, as is the case with a gamma ray densitometers. A common problem encountered in oil well production testing is that of gas ??carry-under?? into the liquid stream exiting the test separator. This results in a trace amount of gas entering the water-cut meter, producing errors in the water-cut reading. Gas carry-under may be caused by high liquid viscosity, improper separator operation, or poor separator design. Gas carry-under is believed to be one of the major causes of large allocation factors in oil and gas operations. Problems in clearly defining the three-phase stream as to flow regime and actual gas bubble size have been described in the technical literature. Pertinent references are discussed and compared. The issues in trying to perform such tests in the laboratory and the correlation of the data are disclosed and the difficulties in trying to correlate the effects of the entrained gas are described. Field testing and experience by at least one manufacturer of equipment has verified the effect of entrained gas, but little quantitative data relating gas-cut to increased error of measurement has been published. The objective of this work was to investigate the performance of a microwave water-cut analyzer under three-phase flow conditions to determine the impact of the presence of gas in the liquid stream. Experiments were performed that investigated the effects of entrained gas on a commercial water-cut analyzer. These tests were conducted at the Texas A&M Tommie E. Lohman Fluid Measurement Laboratory at low pressure conditions (< 40 psig). The test fluids were air, water and two types of oil: mineral oil and hydraulic oil. These experiments investigated oil continuous emulsion conditions with the Gas Volume Fraction (GVF) ranging from 0-25% and the water-cut ranging from 5-30%. Liquid flow rates were between 500-3,700 bbl/day. A 2-inch water-cut full range meter was utilized for these tests. The error in water-cut was seen to increase with increasing GVF ranging from 0% to 25%. However, the measurement remained stable over the entire range of tests. A correction was developed to correct water-cut meter readings based on the amount of gas in the liquid stream.Item Microwave assisted generation and trapping of acetylketene(2007-12) Gudipati, Indra Sena Reddy; Birney, David M.; Mayer, Michael F.Acetylketene is a reactive intermediate, which is generally generated by the reflux of 2,2,6-trimethyl-4H-1,3-dioxin-4-one in the presence of a trapping agent in toluene. In this work acetylketene is generated using microwave radiation. Microwave-assisted generation and trapping of acetylketene has several advantages. The heating time is only 20 minutes and a small amount of solvent (10 mL) is used. The yields are good when alcohols and aldehydes are used as trapping agents. In the case of ketones as trapping agents, there was no reaction with acetophenone. However acetylketene reacted to some extent with trifluoroacetophenone.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 Modeling and design of compact microwave components and systems for wireless communications and power transmission(Texas A&M University, 2004-09-30) Zepeda, PaolaThe contribution of the work here presented involves three main topics: Wireless Power Transmission (WPT) technology, phased array systems, and microwave components design and modeling. The first topic presents the conceptual design of a WPT system at 2.45GHz with 90% efficiency and 1MW of DC output power. Second, a comparative study between 2.45 and 35GHz WPT operation is provided. Finally, the optimization of a taper distribution with reduced thermal constraints on a sandwich transmitter is realized. For a 250- and 375-m antenna radius, 89.7% of collection efficiency with 29% reduction in maximum power density (compared to the Gaussian), and 93% collection efficiency with 39% reduction of maximum power density, are obtained respectively with two split tapers. The reduction in maximum power density and the use of split taper are important to alleviate the thermal problems in high power transmission. For the phased array project, the conceptual design of a small-scale system and in-depth analysis using two main approaches (statistical and field analysis) is realized. Practical aspects are addressed to determine the phased array main design features. The statistical method provides less accurate results than the field analysis since it is intended for large arrays. Careful theoretical analysis led to good correlation between statistical, field analysis and experimental results. In the components chapter, efficient loop transitions used in a patch antenna array are designed at K- and W-band. Measured insertion loss (IL) K-band loop is under 0.4dB. The K- and W-band antenna array measured broadside gains are 23.6dB at 24.125GHz and 25dB at 76.5GHz with return loss under 9.54dB from 24 to 24.4GHz and 12 dB from 75.1 to 77.3GHz, respectively. Also, a multilayer folded line filter is designed at 5.8GHz and compared to planar ring filters. Improved measured bandwidth from 2GHz to 7.5GHz and IL of 1.2dB are obtained with approximately half the size of a planar ring resonator. Thirdly, a simplified switch model is implemented for use in broadband phased-shifters. The model presents very good fit to the measured results with an overall total error under 3%, magnitude error less than 8%, and phase errors less than ?0.4?.Item Reflex-triode geometry of the virtual cathode oscillator(Texas Tech University, 2003-12) Lara, Matthew BAn eight-stage four-hundred kilovolt Marx generator, in connection with a 60 nanosecond pulse-forming line, is constructed and utilized as a pulsed source to power a triode version of the Virtual Cathode Oscillator (Vircator). Eight . I uF capacitors, charged to 50kV each, are switched in series by dry-air pressurized spark gaps. The energy from the bank charges a 23 ohm oil transmission line, breaking a peaking gap when the maximum voltage is reached, delivering a 60 ns-400kV pulse to the diode. The design of the "Reflex-Triode" geometry Vircator is based upon claims of high efficiency by Didenko et al.[l] A previously constructed TTU Vircator includes a unique E-beam source, the "brush" cathode; in which a circular array of pins is used as an explosive field emission source to produce relatively high beam currents. The anode consists of a round wire mesh through which the E-beam passes, generating a dense cloud of negative charge known as a "Virtual Cathode." This initial phase of testing is composed of basic operation of the entire system and baseline output power and efficiency measurements.Item RF/microwave absorbing nanoparticles and hyperthermia(2009-12) Cook, Jason Ray; Emelianov, Stanislav Y.; Pearce, John A.The primary purpose of this work was to evaluate the capability of nanoparticles to transform electromagnetic energy at microwave frequencies into therapeutic heating. Targeted nanoparticles, in conjunction with microwave irradiation, can increase the temperatures of the targeted area over the peripheral region. Therefore, to become clinically viable, microwave absorbing nanoparticles must first be identified, and a system to monitor the treatment must be developed. In this study, ultrasound temperature imaging was used to monitor the temperature of deep lying structures. First, a material-dependent quantity to correlate the temperature induced changes in ultrasound images (i.e. apparent time shifts) to differential temperatures was gathered for a tissue-mimicking phantom, porcine longissimus dorsi muscle, and porcine fat. Then microwave nanoabsorbers were identified using an infrared radiometer. The determined nanoabsorbers were then injected into ex-vivo porcine longissimus dorsi muscle tissue. Ultrasound imaging frames were gathered during microwave treatment of the inoculated tissue. Finally, the ultrasound frames were analyzed using the correlation between temperature and apparent shifts in ultrasound for porcine muscle tissue. The outcome was depth-resolved temperature profiles of the ex-vivo porcine muscle during treatment. The results of this study show that magnetite is a microwave nanoabsorber that increases the targeted temperature of microwave hyperthermia treatments. Overall, there is clinical potential to use microwave nanoabsorbers to increase the efficiency of microwave hyperthermia treatments.Item Testing and Simulation of the SRF Wafer Test Cavity for the Characterization of Superconductors and Heterostructures(2014-08-07) Comeaux, JustinThe wafer test cavity, designed at Texas A&M University, has been constructed and tested at Thomas Jefferson National Accelerator Facility. The mode structure, quality factor and coupling methods have been investigated. The TE_(011) mode has been located at 1.886492 GHz and the quality factor has been measured from room temperature to 2K. The measured quality factor at 2.7K is found to be 3.96x10^(8). Simulations of the cavity have been performed using Comsol Multiphysics and Mathematica to demonstrate the cavities potential as a test bed for superconducting materials, such as Heterostructures. The results of the experimental measurements are incorporated into the simulations to show that a Radio Frequency magnetic field of 162 mT can be placed on a sample of superconducting material, giving 81% of the BCS critical magnetic field for niobium.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.