Browsing by Subject "Techniques"
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Item A theory of lock-on and electrical breakdown(Texas Tech University, 2003-12) Kambour, Kenneth EIn this dissertation, a theory of electrical breakdown of insulators is developed. This theory is based on collective impact ionization, which includes both the electric field dependence and the carrier density dependence of impact ionization. This theory is applied to photoconductive semiconductor switches (PCSS's) and is used to explain the lock-on effect, an optically triggered breakdown that occurs in GaAs PCSS's. The basic principle of collective impact ionization theory is that, at high carrier densities, carrier-carrier scattering will enhance the impact ionization rate. This occurs because these interactions increase the number of carriers with energies above the impact ionization threshold. This generalized breakdown theory uses a rate equation approach to obtain the carrier density or densities which, at a given electric field, result in a steady state or a zero net carrier growth rate. In this approach, the competition between carrier generation (by impact ionization) and carrier recombination (by Auger and defect mechanisms) leads to a steady state condition for the net carrier growth rate. It is the existence of this steady state that governs whether or not electrical breakdown occurs. This approach leads to a definition of the bulk breakdown field as the lowest field for which the injection of an infinitesimally small carrier density will result in a steady state with a large carrier density. It also leads to the definition of the lock-on field as the lowest field for which a stable, steady state carrier density is possible. To implement this theory for PCSS materials, the Ensemble Monte Carlo (EMC) method is used to calculate the carrier distribution function, including the effects of carrier-carrier scattering. This distribution function is used to calculate the impact ionization and Auger recombination rates and thus the steady state carrier growth rate. Since the EMC calculations which include cc-scattering are computationally intense and time consuming, this theory is also implemented using both low and high density approximations for the distribution function. The low density limit is obtained using the EMC method without including cc-scattering. The high density limit is obtained by approximating the distribution function as a steady state Maxwellian. Using this theory, predictions are made for both the lock-on field and the bulk breakdown field in several materials and the results are compared, where possible, with experiment. In this theory, the lock-on effect is a type of carrier-density dependent electrical breakdown which occurs in all insulating materials. Further, it is the difference between the predicted lock-on and the breakdown fields which determines whether or not the lock-on effect will observable as a phenomenon distinct from ordinary breakdown. If the two fields are sufficiently distinct, it is likely that the two phenomena can be distinguished. However, if they are similar, it is likely that they will be difficult to distinguish experimentally.Item Big idea patterns of the advertising creative process(2011-05) Lindsay, Cabe Erin; Cunningham, Isabella C. M.; Sung, YongjunThe analysis of creative processes reveals that there are behaviors, techniques, and resources that have proven to be indispensable when embraced by advertising creatives in order to achieve big ideas. There are specific behaviors that clearly define successful creatives, and there are techniques and resources that creatives commonly use to arrive at big ideas. Some of these behaviors, techniques, and resources are well-known and time-tested, while others are proposed here for the first time, backed by research. This report aims to improve the productivity of creativity.Item Empowering silenced voices : counseling techniques for Black rape survivors(2014-05) Roberts, Davia; Cokley, Kevin O. (Kevin O'Neal), 1969-In the United States, Black women are more likely to be sexually victimized than any other ethnic group. Despite the higher rates among Black women, there is little research that provides therapists with approaches and techniques that are culturally specific to Black women. Instead, most literature has focused on women as a whole, rather than examining the different cultural values and forms of support that are needed for recovery in different ethnic groups. Therefore, this report will provide therapists with a sociocultural lens to increase awareness of the Black woman's identity, while suggesting a Womanist theoretical approach to group psychotherapy as an ideal method of treatment.Item Reflecting pools(2011-05) Payavis, Daniel David; Stoney, John; Brauntuch, Troy; Hubbard, Teresa; Perzynski, BogdanThis report sketches a series of considerations that operate within my art practice. They are intended to elaborate on and open up, rather than simplify, the processes by which I arrive at art works. The ideas discussed roughly fit into, or are products of, three efforts which run parallel and independent of one another: my studio practice and production of art works; the study of artistic practice in a general sense, not always dependent on my particular art practice; and the study of a broad range of image types. The serial arrangement of the sections is meant to convey the non-hierarchical relationships of the ideas. Selected art works I have produced are explicated as they relate to the ideas; some art works appear in more than one section.Item Spin-orbit misalignments, planet candidate validation, and nodal precession via Doppler tomography(2016-08) Johnson, Marshall Caleb; Cochran, William D.; Collier Cameron, Andrew; Endl, Michael; Robinson, Edward L; Sneden, ChrisTransiting planets around intermediate-mass stars (1.5 MSun < M* < 2.5MSun , or 6500 K < Teff < 10000 K on the main sequence) occupy a largely unexplored part of parameter space; these stars tend to rotate rapidly, and thus are unsuitable for the precise radial velocity observations that are typically used to follow up and confirm transiting planet candidates. In this dissertation I demonstrate the use of Doppler tomography, where I spectroscopically resolve the perturbation to the rotationally broadened stellar line profile during the transit due to the Rossiter-McLaughlin effect, to confirm transiting planet candidates around rapidly-rotating stars and to measure the (mis)alignment of the planetary orbit with respect to the stellar rotation. The demographics of planets around intermediate-mass stars are important for constraining models of planet formation as a function of stellar mass, while the distribution of spin-orbit misalignments of transiting planets is a powerful statistical tracer of planet migration processes. Using Doppler tomography, I show that the hot Jupiter Kepler-13 Ab has a misaligned but prograde orbit; I improve upon the precision of the misalignment measurement for the hot Jupiter WASP-79 b, which has a nearly polar orbit; and I measure an aligned orbit for the warm Jupiter Kepler-448 b, and two degenerate solutions (both prograde) for the hot Jupiter HAT-P-41 b. I also analyze observations of several Kepler planet candidates–KOI-366.01, KOI-368.01, and KOI-972.01–in an attempt to validate them as bona fide planets. Unfortunately, due to small planetary radii and/or limited amounts of data, I am able to achieve only a ∼ 2σ detection of KOI-368.01, and am unable to detect KOI-366.01 or KOI-972.01. I also analyze two epochs of Doppler tomographic observations of the highly inclined hot Jupiter WASP-33 b taken six years apart, and show that the path of the transit across the stellar disk has changed between the two epochs due to nodal precession of the planetary orbit. This allows me to measure the precession rate, and constrain the stellar gravitational quadrupole moment J2. Overall, I show that Doppler tomography is an effective tool for confirming and characterizing transiting planets around rapidly-rotating stars, where typical methods have difficulty.