VTechWorks Repository :: Browsing by Author "Scales, Wayne A."

Browsing by Author "Scales, Wayne A."

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All-fiber Spectral Filters based on LP01 - LP11 Mode Coupling and Applications in Wavelength Division Multiplexing and Dispersion Compensation
Jyothikumar, Jagannathan (Virginia Tech, 1996-06-27)
All-fiber spectral filters have the advantages of providing low coupling loss and being readily integrated into fiber-optic networks. Spectral filters made of single-mode identical or dissimilar core parallel fibers provide 3-dB spectral widths on the order of 1 to 10 nm. A spectral filter made of single-mode and dual-mode fibers and operating based on coupling of power between LP01 and LP11 modes is proposed for applications as narrowband demultiplexers and as broadband mode converters with spectral widths of fraction of 1 nm to few 10 nm, respectively. With appropriate choice of parameters, filters can be designed such that the LP01 mode of the single-mode fiber is phase matched with the LP11 mode of the dual-mode fiber at a desired wavelength. Thus, significant exchange of power between these two modes can occur. The coupled-mode theory of parallel dielectric waveguides is used to analyze the proposed filter. Transmission expressions are derived from the governing coupled-mode equations and evaluated numerically for example cases. Two cases corresponding to maximum power coupling at 1.33 mm and 1.55 mm wavelengths are examined. Design data and transmission characteristics versus wavelength for these two cases are presented. The influence of the distance between fiber cores on peak transmission wavelength, spectral width, and coupling length is investigated. The application of the proposed filter as mode converter, which is required in the implementation of dispersion compensation using LP11 mode, is elucidated.
Analysis of Aperture Radiation Using Computer Visualization and Image-Processing Techniques
Monkevich, James Matthew (Virginia Tech, 1998-05-04)
In order to accurately describe the behavior of an antenna, one needs to understand the radiation mechanisms that govern its operation. One way to gain such an insight is to view the fields and currents present on a radiating structure. Unfortunately, in close proximity to an antenna empirical techniques fail because the measurement probe alters the operation of the radiating structure. Computational methods offer a solution to this problem. By simulating the operation of an antenna, one can obtain electromagnetic field data near (or even internal to) a radiating structure. However, these computationally intense techniques often generate extremely large data sets that cannot be adequately interpreted using traditional graphical approaches. A visualization capability is developed that allows an analysis of the above-mentioned data sets. With this technique, the data is viewed from a unique, global perspective. This format is well suited for analytical investigations as well as debugging during modeling and simulation. An illustrative example is provided in the context of a rectangular microstrip patch antenna. A comparison is performed between the visualized data and the theory of operation for the microstrip patch in order to demonstrate that radiation mechanisms can be obtained visually. An additional analysis tool is developed using Gabor filters and image-processing techniques. This tool allows one to detect and filter electromagnetic waves propagating with different velocities (both speed and direction). By doing so, each mode of an antenna can be analyzed independently. The fields of a multi-moded, open-ended rectangular waveguide are analyzed in order to demonstrate the effectiveness of these techniques.
Analysis of Path Loss from a Transmitter in an Aircraft Cabin to an Exterior Fuselage-Mounted Antenna
Wang-Hurst, Kathy Weiquan (Virginia Tech, 2007-12-04)
It is important to investigate the threat posed to commercial aircraft by on board electronic transmitters in the passenger cabin and the cargo holds of large transport aircraft. These transmitters may be in the form of unintentional use of portable electronic devices or even intentional radio frequency (RF) threat sources from terrorists. Thus, it is of interest to determine the "interference path loss" (IPL) from a transmitting device inside the cabin of such aircraft to the antenna terminals of a potential victim system of the aircraft. Past studies have concentrated on measurements. These efforts to measure IPL directly have demonstrated that accurate and repeatible measurements are difficult to obtain. Very little modeling work has been done successfully to understand the IPL on aircraft. In this thesis, we propose a 3-step methodology to quantify the interference path loss (IPL). We then apply this methodology to a broad class of aircraft and show results. To validate our results, we compare our findings to known measurements and discuss possible sources of errors. Finally we suggest areas of improvement to our analysis and propose future work.
Analysis of Plasma Wave Irregularities Generated during Active Experiments in Near-Earth Space Environment
Bordikar, Maitrayee Ranade (Virginia Tech, 2013-05-26)
This work focuses on the analysis of plasma irregularities generated during two active space experiments: the injection of an artificial dust layer, and high-power radio waves. The objective of the "first experiment is to examine the effects of artificially created dust layers on the scatter of radars from plasma irregularities embedded in dusty plasma in space. This is an alternate approach for understanding the mechanisms of enhanced radar scatter from plasma irregularities embedded in Noctilucent Clouds and Polar Mesospheric Summer Echoes. The second experiment involves a transmission of high power electromagnetic waves into the ionospheric plasma from the ground, which can excite stimulated electromagnetic emissions offset from the transmitter frequency. These stimulated electromagnetic emissions provide diagnostic information of the ionosphere and thus can be used to investigate fundamental physical principles which govern the earth\'s ionosphere, so that present and future transmission technologies may take into account the complexities of the ionosphere. The interaction altitude of the artificial dust layer and high power radio waves is approximately 250 km and 160 km respectively, thus dealing with uniquely different regions of the ionosphere. Each experiment is discussed separately using theoretical, observational and advanced computational methodologies. The study first investigates plasma turbulence associated with the creation of an artificial dust layer in the earth's ionosphere. Two scenarios are considered for plasma irregularity generation as dust is injected at an oblique angle across the geomagnetic field. The first is a shear-driven plasma instability due to inhomogeneities in the boundary layer between the injected charged dust layer and the background plasma. This begins to appear at very early times once the dust is released into the space plasma, which is of the order or less than the dust charging time period. The second mechanism is free streaming of the charged dust relative to the background plasma. This produces irregularities at times much longer than the dust charging period and also longer than the dust plasma period. Although both mechanisms are shown to produce turbulence in the lower hybrid frequency range, the resulting irregularities have important differences in their physical characteristics. A comparison between the processes is made to determine the consequences for upcoming observations. Both processes are shown to have the possibility of generating turbulence after the release of dust for the regimes of upcoming space experiments over a range of timescales. This work also presents the first observations of unique narrowband emissions ordered near the Hydrogen ion (H+) gyro-frequency (fcH) in the Stimulated Electromagnetic Emission (SEE) spectrum when the transmitter is tuned near the second electron gyro-harmonic frequency (2fce), during ionospheric modification experiments. The frequency structuring of these newly discovered emission lines is quite unexpected since H+ is known to be a minor constituent in the interaction region which is near 160 km altitude. The spectral lines are typically shifted from the pump wave frequency by harmonics of a frequency about 10% less than fcH (" 800 Hz) and have a bandwidth of less than 50 Hz which is near the O+ gyro-frequency fcO. A theory is proposed to explain these emissions in terms of a Parametric Decay Instability (PDI) in a multi-ion species plasma due to possible proton precipitation associated with the disturbed conditions during the heating experiment. The observations can be explained by including several percent H+ ions into the background plasma. The implications are new possibilities for characterizing proton precipitation events during ionospheric heating experiments.
Antenna Selection for a Public Safety Cognitive Radio
Hugine, Akilah L. (Virginia Tech, 2006-05-10)
Ever since the dawn of radio communication systems, the antenna has been the key component in the construction and performance of every wireless system. With the proliferation of new radio systems, a cognitive radio is a radio that has the capability to sense, learn, and autonomously adapt to its environment. The hardware components are essential to optimizing performance. Antenna hardware for cognitive radio applications presents distinctive problems, since in theoretical terms, a cognitive radio can operate anywhere in the spectrum. The purpose of this thesis is to investigate a particular type of cognitive radio system and examine the potential affects the antenna will have on the system. The thesis will provide an overview of fundamental antenna properties, the performance characteristics of the particular antenna used in this research, and the system characteristics when the antenna is integrated. This thesis will also illustrate how the antenna and its properties affect the overall public safety cognitive radio performance. This information can be used to establish antenna selection criteria for optimum system performance.
Applications in Remote Sensing Using the Method of Ordered Multiple Interactions
Westin, Benjamin Alexander (Virginia Tech, 2013-04-24)
The Method of Ordered Multiple Interactions provides a numerical solution to the integral
equations describing surface scattering which is both computationally efficient and reliably
convergent. The method has been applied in a variety of ways to solving the electromagnetic
scattering from perfectly-conducting rough surfaces. A desire to more accurately predict
the scattering from natural terrain has led to the representation of the surface material as
penetrable instead of conductive.

For this purpose, the Method of Ordered Multiple Interactions is applied to numerically
solve the electromagnetic scattering from randomly-rough dielectric surfaces. A primary
consequence of the penetrable surface material is the introduction of a pair of coupled integral equations in place of the single integral equation used to solve the problem with a perfectly conducting surface. The method is tested and analyzed by developing independent scattering solutions for canonical cases in a transform domain and by comparing results with solutions from other techniques.

The dielectric implementation of the Method of Ordered Multiple Interactions is used to solve
the electromagnetic scattering from a class of randomly-rough dielectric surfaces. This allows
for the characterization of the effect of a number of transmitter and surface parameters in the
scattering problem, observing bistatically and also specifically in the backscatter direction.

MOMI is then applied as a method to examine subsurface penetration characteristics from
a similar family of rough surfaces. Characteristics of the environment parameters and the
scattered field itself are examined, and the numerical challenges associated with observing
beneath the surface are identified and addressed.

The Method of Ordered Multiple Interactions is then incorporated as a major component of
a larger solution which computes the total scattering when a dielectric object is buried just
beneath the rough surface. This hyrid approach uses MOMI and the Method of Moments to
iteratively account for multiple interactions between the target and the dielectric interface,
enabling the study of scattering from the combined environment of a rough surface and the
embedded object, as well as the individual scattering events which combine to form the
steady-state solution.
Automated Detection and Analysis of Low Latitude Nightside Equatorial Plasma Bubbles
Adkins, Vincent James (Virginia Tech, 2024-06-21)
Equatorial plasma bubbles (EPBs) are large structures consisting of depleted plasma that generally form on the nightside of Earth's ionosphere along magnetic field lines in the upper thermosphere/ionosphere. While referred to as `bubbles', EPBs tend to be longer along magnetic latitudes and narrower along magnetic longitudes which are on the order of thousands and hundreds of kilometers, respectively. EPBs are a well documented occurrence with observations spanning many decades. As such, much is known about their general behavior, seasonal variation of occurrences, increasing/decreasing occurrences with increasing/decreasing solar activity, and their ability to interact and interfere with radio waves such as GPS. This dissertation expands on this understanding by focusing on the detection and tracking of EPBs in the upper thermosphere/ionosphere along equatorial to low latitudes. To do this, far ultraviolet (FUV) emission observations of the recombination of O$^+$ with electrons via the Global-Scale Observations of the Limb and Disk (GOLD) mission are analyzed. GOLD provides consistent data from geostationary orbit with the eastern region of the Americas, Atlantic, and western Africa. The optical data can be used to pick out gradients in brightness along the 135.6 nm wavelength which correlate with the location of EPBs in the nightside ionosphere. The dissertation provides a novel method to look at and analyze 2-dimensional data with inconsistent time-steps for EPB detection and tracking. During development, preprocessing of large scale (multiple years) data proved to be the largest time sync. To that end, this dissertation tests the possibility of using convolution neural networks for detection of EPBs with the end goal of reducing the amount of preprocessing necessary. Further, data from the Ionospheric Connection Explorer's (ICON's) ion velocity meter (IVM) are compared to EPBs detected via GOLD to understand how the ambient plasma around the EPBs behave. Along with the ambient plasma, zonal and meridional thermospheric winds observed by ICON's Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument are analyzed in conjunction with the same EPBs to understand how winds coincident with EPBs behave. An analysis of winds before EPBs form is also done to observe the potential for both zonal and meridional winds' ability to suppress and amplify EPB formation.
Biological Agent Sensing Integrated Circuit (BASIC): A New Complementary Metal-oxide-semiconductor (CMOS) Magnetic Biosensor System
Zheng, Yi (Virginia Tech, 2014-06-10)
Fast and accurate diagnosis is always in demand by modern medical professionals and in the area of national defense. At present, limitations of testing speed, sample conditions, and levels of precision exist under current technologies, which are usually slow and involve testing the specimen under laboratory conditions. Typically, these methods also involve several biochemical processing steps and subsequent detection of low energy luminescence or electrical changes, all of which reduce the speed of the test as well as limit the precision. In order to solve these problems and improve the sensing performance, this project proposes an innovative CMOS magnetic biological sensor system for rapidly testing the presence of potential pathogens and bioterrorism agents (zoonotic microorganisms) both in specimens and especially in the environment. The sensor uses an electromagnetic detection mechanism to measure changes in the number of microorganisms--tagged by iron nanoparticles--that are placed on the surface of an integrated circuit (IC) chip. Measured magnetic effects are transformed into electronic signals that count the number and type of organisms present. This biosensor introduces a novel design of a conical-shaped inductor, which achieves ultra-accuracy of sensing biological pathogens. The whole system is integrated on a single chip based on the fabrication process of IBM 180 nm (CMOS_IBM_7RF), which makes the sensor small-sized, portable, high speed, and low cost. The results of designing, simulating, and fabricating the sensor are reported in this dissertation.
Characterization and Helicopter Flight Test of 3-D Imaging Flash LIDAR Technology for Safe, Autonomous, and Precise Planetary Landing
Roback, Vincent Eric (Virginia Tech, 2012-08-13)
Two flash lidars, integrated from a number of cutting-edge components from industry and NASA, are lab characterized and flight tested under the Autonomous Landing and Hazard Avoidance (ALHAT) project (in its fourth development and field test cycle) which is seeking to develop a guidance, navigation, and control (GNC) and sensing system based on lidar technology capable of enabling safe, precise human-crewed or robotic landings in challenging terrain on planetary bodies under any ambient lighting conditions. The flash lidars incorporate pioneering 3-D imaging cameras based on Indium-Gallium-Arsenide Avalanche Photo Diode (InGaAs APD) and novel micro-electronic technology for a 128 x 128 pixel array operating at 30 Hz, high pulse-energy 1.06 ?m Nd:YAG lasers, and high performance transmitter and receiver fixed and zoom optics. The two flash lidars are characterized on the NASA-Langley Research Center (LaRC) Sensor Test Range, integrated with other portions of the ALHAT GNC system from around the country into an instrument pod at NASA-JPL, integrated onto an Erickson Aircrane Helicopter at NASA-Dryden, and flight tested at the Edwards AFB Rogers dry lakebed over a field of human-made geometric hazards. Results show that the maximum operational range goal of 1000m is met and exceeded up to a value of 1200m, that the range precision goal of 8 cm is marginally met, and that the transmitter zoom optics divergence needs to be extended another eight degrees to meet the zoom goal 6° to 24°. Several hazards are imaged at medium ranges to provide three-dimensional Digital Elevation Map (DEM) information.
Characterization of Collisional Shock Structures Induced by the Stagnation of Railgun-driven Multi-ion-species Plasma-jets
Schneider, Maximilian Kurt (Virginia Tech, 2020-01-22)
The study of shock-waves in supersonic plasma jets is essential to understanding the complex dynamics involved in many physical systems. Specifically, ion-species separation caused by a shock wave propagating through a plasma is an important but not yet well understood phenomenon. In inertial confinement fusion implosions, a shock wave precedes the rapid compression of a fuel pellet to ignition conditions that theory and computational studies suggest may be separating the fuel and reducing the neutron yield. In astrophysics, the shock wave produced when a supernovae explodes has been shown to have an effect on nucleosynthesis as a result of shock heating. In both these cases the time and length scales make them difficult to study experimentally, but experiments on more reasonable scales can shed light on these phenomena. This body of work provides the basis for doing just that. The work begins by describing the development of a small, linear, plasma-armature railgun designed to accelerate plasma jets in vacuum to high-Mach-number. This is followed by discussion of an experimental campaign to establish a plasma parameter space for the jets, in order to predict how effectively the accelerator can be used to study centimeter-scale shock structures in jet collisions. The final section presents an experimental campaign in which jet collisions are induced, and the resultant structures that appear during the collision are diagnosed to assess how conducive the experiment is to the future study of shock-wave induced species separation in laboratory plasmas. This work is a foundation for future experimental studies of ion-separation mechanisms in a multi-ion-species plasma. This research was supported in part by the National Science Foundation under grant number PHY-1903442.
Characterization of multi-scale ionospheric irregularities using ground-based and space-based GNSS observations
Peng, YuXiang; Scales, Wayne A.; Hartinger, Michael D.; Xu, Zhonghua; Coyle, Shane (2021-07-12)
Ionospheric irregularities can adversely affect the performance of Global Navigation Satellite System (GNSS). However, this opens the possibility of using GNSS as an effective ionospheric remote sensing tool. Despite ionospheric monitoring has been undertaken for decades, these irregularities in multiple spatial and temporal scales are still not fully understood. This paper reviews Virginia Tech’s recent studies on multi-scale ionospheric irregularities using ground-based and space-based GNSS observations. First, the relevant background of ionospheric irregularities and their impact on GNSS signals is reviewed. Next, three topics of ground-based observations of ionospheric irregularities for which GNSS and other ground-based techniques are used simultaneously are reviewed. Both passive and active measurements in high-latitude regions are covered. Modelling and observations in mid-latitude regions are considered as well. Emphasis is placed on the increased capability of assessing the multi-scale nature of ionospheric irregularities using other traditional techniques (e.g., radar, magnetometer, high frequency receivers) as well as GNSS observations (e.g., Total-Electron-Content or TEC, scintillation). Besides ground-based observations, recent advances in GNSS space-based ionospheric measurements are briefly reviewed. Finally, a new space-based ionospheric observation technique using GNSS-based spacecraft formation flying and a differential TEC method is demonstrated using the newly developed Virginia Tech Formation Flying Testbed (VTFFTB). Based on multi-constellation multi-band GNSS, the VTFFTB has been developed into a hardware-in-the-loop simulation testbed with external high-fidelity global ionospheric model(s) for 3-satellite formation flying, which can potentially be used for new multi-scale ionospheric measurement mission design.
Co-Channel Interference In Bluetooth Piconets
Lynch, Jamel Pleasant (Virginia Tech, 2002-06-10)
Bluetooth™ is an emerging short-range RF wireless voice and data communication technology expected to spread widely in a couple of years. The open specification technology eliminates the need for cables to connect mobile phones, portable computers and countless other devices to each other from all different manufacturers. This thesis provides an overview of the emerging Bluetooth™ technology and investigates the performance of Bluetooth™ data networks in various network topologies simulated from actual usage scenarios. Using a typical office environment, the study examines the probability and effects of Co-Channel interference as Bluetooth™ ad-hoc networks are formed in adjacent offices. A computer aided simulation tool, MATLAB simulates a low to highly dense interfering Bluetooth™ environment which provides the parameters to evaluate the bluetooth co-channel interference and performance. Several metrics are identified to predict Bluetooth™ performance in a piconet after a collision has occurred: data through put, the probability of frequency collision, transmitter - receiver distance, and power received. Next, to predict Bluetooth performance we also need to define what constitutes a lost packet. Finally, a Bluetooth™ network simulation is developed to measure the metrics, given occurrence of the lost packets.
Comparison and Investigation of Solar Spectral Irradiance with Solar Aspect Monitor
Lin, Ying-Tsen (Virginia Tech, 2014-09-30)
On-board the International Space Station (ISS), the Remote Atmospheric and Ionospheric Detection System (RAIDS) is a suite of limb-scanning monitors taking measurements from the extreme ultraviolet (EUV) to the near infrared (NIR). A single-scattering Rayleigh model is developed to eliminate the scattered brightness below 90 km and an inversion technique is applied on limb-scanned radiance profiles at 236.5 nm, NO (0,1) gamma band. The ISS orbit allows observations from 7:00 to 16:00 local hours over a one-month period from mid-June to mid-July of 2010 and observation of the local-time variation of NO abundance in the lower thermosphere is derived. The uniquely stable solar activity during 2010 allows the local time variation of NO to be observed with limited influence of solar variability. The comparison with a 1D model shows good agreement at altitude above 120 km, suggesting that most of the local time variation of NO is due to solar illumination, radiation, chemistry, and vertical diffusion. Solar soft X-ray is the major driver of the variability observed in the ionospheric and thermospheric constituents at the equatorial region. Over the years measurements in these wavelengths are scarce and discrepancies lie among the existing data. The Solar Aspect Monitor (SAM) is a pinhole camera on the Extreme-ultraviolet Variability Experiment (EVE) flying on the Solar Dynamics Observatory (SDO). Every 10 seconds SAM projects the solar disk onto the CCD through a metallic filter designed to allow only solar photons shortward of 7 nm to pass. Contamination from energetic particles and out-of-band irradiance is, however, present. The broadband (BB) technique is developed for isolating the 0.1 to 7 nm integrated irradiance to produce broadband irradiance. The results agree with the zeroth-order product from the EUV SpectroPhotometer (ESP) with 25% regardless of solar activity level. Active regions in the solar atmosphere are tracked by the Apertural Progression Procedure for Light Estimate (APPLE). The photon event detection (PED) algorithm takes both BB and APPLE results as prior information to extract in-band photons. Applications of the PED products, including solar feature studies and spectral resolved irradiance, are demonstrated.
A Comprehensive Entry, Descent, Landing, and Locomotion (EDLL) Vehicle for Planetary Exploration
Schroeder, Kevin Kent (Virginia Tech, 2017-08-26)
The 2012 Decadal Survey has stated that there is a critical role for a Venus In-situ Explore (VISE) missions to a variety of important sites, specifically the Tessera terrain. This work aims to answer the Decadal Survey's call by developing a new comprehensive Entry, Descent, Landing, and Locomotion (EDLL) vehicle for in-situ exploration of Venus, especially in the Tessera regions. TANDEM, the Tension Adjustable Network for Deploying Entry Membrane, is a new planetary probe concept in which all of EDLL is achieved by a single multifunctional tensegrity structure. The concept uses same fundamental concept as the ADEPT (Adaptable Deployable Entry and Placement Technology) deployable heat shield but replaces the standard internal structure with the structure from the tensegrity-actuated rover to provide a combined aeroshell and rover design. The tensegrity system implemented by TANDEM reduces the mass of the overall system while enabling surface locomotion and mitigating risk associated with landing in the rough terrain of Venus's Tessera regions, which is otherwise nearly inaccessible to surface missions. TANDEM was compared to other state-of-the-art lander designs for an in-situ mission to Venus. It was shown that TANDEM provides the same scientific experimentation capabilities that were proposed for the VITaL mission, with a combined mass reduction for the aeroshell and lander of 52% (1445 kg), while eliminating the identified risks associated with entry loads and very rough terrain. Additionally, TANDEM provides locomotion when on the surface as well as a host of other maneuvers during entry and descent, which was not present in the VITaL design. Based on its unique multifunctional infrastructure and excellent crashworthiness for impact on rough surfaces, TANDEM presents a robust system to address some of the Decadal Survey's most pressing questions about Venus.
Comprehensive Multi-Scale Progressive Failure Analysis for Damage Arresting Advanced Aerospace Hybrid Structures
Horton, Brandon Alexander (Virginia Tech, 2017-08-31)
In recent years, the prevalence and application of composite materials has exploded. Due to the demands of commercial transportation, the aviation industry has taken a leading role in the integration of composite structures. Among the leading concepts to develop lighter, more fuel-efficient commercial transport is the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept. The highly integrated structure of PRSEUS allows pressurized, non-circular fuselage designs to be implemented, enabling the feasibility of Hybrid Wing Body (HWB) aircraft. In addition to its unique fabrication process, the through-thickness stitching utilized by PRSEUS overcomes the low post-damage strength present in typical composites. Although many proof-of-concept tests have been performed that demonstrate the potential for PRSEUS, efficient computational tools must be developed before the concept can be commercially certified and implemented. In an attempt to address this need, a comprehensive modeling approach is developed that investigates PRSEUS at multiple scales. The majority of available experiments for comparison have been conducted at the coupon level. Therefore, a computational methodology is progressively developed based on physically realistic concepts without the use of tuning parameters. A thorough verification study is performed to identify the most effective approach to model PRSEUS, including the effect of element type, boundary conditions, bonding properties, and model fidelity. Using the results of this baseline study, a high fidelity stringer model is created at the component scale and validated against the existing experiments. Finally, the validated model is extended to larger scales to compare PRSEUS to the current state-of-the-art. Throughout the current work, the developed methodology is demonstrated to make accurate predictions that are well beyond the capability of existing predictive models. While using commercially available predictive tools, the methodology developed herein can accurately predict local behavior up to and beyond failure for stitched structures such as PRSEUS for the first time. Additionally, by extending the methodology to a large scale fuselage section drop scenario, the dynamic behavior of PRSEUS was investigated for the first time. With the predictive capabilities and unique insight provided, the work herein may serve to benefit future iteration of PRSEUS as well as certification by analysis efforts for future airframe development.
Contemporary Ionospheric Scintillation Studies: Statistics, 2D Analytical and 3D Numerical Inversion
Conroy, James Patrick (Virginia Tech, 2022-08-31)
The propagation of radiowaves through ionospheric irregularities can lead to random amplitude and phase fluctuations of the signal, otherwise known as scintillation, which can severely impact the performance of Global Navigation Satellite System (GNSS) and communication systems. Research into high latitude scintillation, through statistical analysis and inverse modeling, was completed to provide insight into the temporal and spatial distribution, and irregularity parameters, which can ultimately support the development of impact mitigation techniques, and deepen our understanding of the underlying physics. The work in this dissertation focused on the statistical analysis of Global Positioning System (GPS) scintillation data, data inversion, two-dimensional (2D) and three-dimensional (3D) scintillation modeling. The statistical analysis revealed distinct trends in the distribution of scintillation, while demonstrating that for GPS signals, phase scintillation occurs most frequently and can be treated as stochastic Total Electron Content (TEC); findings which have significant implications for impact mitigation. For the first of two inversion studies, scintillation data associated with a series of Polar Cap Patches (PCPs), which are common large-scale high latitude structures, was inverted to gain insight into the composition of the underlying irregularities. The results of this study suggest that the irregularities can be modeled as rods interbedded with sheets, which is knowledge that is crucial for the anchoring of models used to develop system mitigation techniques. The final study presents the results of modeling and inversion work to identify the conditions under which a 2D analytic version of the 3D numerical Satellite-beacon Ionospheric-scintillation global model of the upper atmosphere (SIGMA) model can be used to perform modeling in high latitude regions. During the study, it was found that the analytic model tends to diverge for electron density variance times irregularity layer thickness values exceeding 2, matched reasonably well for correlation length to thickness ratios up to 0.2, and was incompatible when ratios approached 0.35. An elevation angle limitation was also identified for the 2D model, and inflated values for the electron density variance were observed overall, which are thought to result from the weak scatter limits of the analytic model. These inflated values were particularly acute in the auroral zone during elevated conditions and suggest that the analytic model used in the study is not well suited for modeling the highly elongated irregularities associated with auroral precipitation.
A Continuum Kinetic Investigation into the Role of Transport Physics in the Bohm Speed formulation
Krishna Kumar, Vignesh (Virginia Tech, 2023-10-26)
When plasmas come in contact with the boundaries that confine them, various complex processes occur between the plasma and the materials in the boundary. These processes, called plasma-material interactions (PMI) lead to physical and chemical modifications in the materials and in the plasma. In the case of a tokamak, a magnetic confinement fusion reactor, the interactions between the plasma and the material in the bounding walls can negatively impact the performance and service life of the reactor. Furthermore, PMI are also found in other areas of significant engineering interest, such as plasma-based spacecraft propulsion engines, where interactions affect the transport properties of the plasma and consequently the performance of the engine. Therefore, gaining a fundamental understanding of the various plasma-material interactions is necessary for the development and improvement of these devices. PMI are dictated by the plasma sheath, a layer of net positive charge that forms at the plasma-boundary interface. The sheath regulates the energy and particle fluxes to the boundary, mediating the interactions. Sheaths, however, are only stable and well-developed when the ions enter the sheath with a speed equal to or greater than the `Bohm speed'. The Bohm speed is a landmark result in sheath theory and various mathematical expressions for it have been derived from fluid and kinetic treatment of plasmas. Although these models are widely used, they are only accurate in cases where the thickness of the sheath is negligible when compared to the scale length of the plasma in consideration. These cases are said to satisfy the `asymptotic limit'. To resolve this, a new Bohm speed model that considers the effects of transport terms such as the electron heat flux, thermal force, and temperature isotropization has been recently proposed [Y. Li et al., Physical Review Letters (2022)]. The model is verified using particle-in-cell (PIC) kinetic simulations and is shown to accurately predict the Bohm speed in cases away from the asymptotic limit. This thesis investigates the new model using the continuum kinetic approach on the Gkeyll software framework. The continuum kinetic approach numerically solves the Vlasov-Maxwell equations using the discontinuous Galerkin method and captures the kinetic phenomena of the plasma without needing to track individual particles. Multiple collisional cases ranging from a Knudsen number of 20 to 5000 are considered in a 1X3V simulation domain using the Lenard-Bernstein collisional operator. The results of the continuum kinetic simulations are benchmarked to the PIC simulation results. It is concluded that across a wide range of collisionalities, the continuum kinetic method captures much of the same physics as the PIC method while offering noise-free results. However, there is a discrepancy between the Bohm speed prediction and the simulation results in the continuum kinetic case. This discrepancy is explored and significant error in the collisional integral derived transport terms between the continuum kinetic method and PIC method is found, suggesting that the difference in collisional operator may be the source of the discrepancy. Nevertheless, the sheath profiles developed in the PIC simulations and the continuum kinetic simulations are in reasonable agreement.
Continuum Kinetic Simulations of Plasma Sheaths and Instabilities
Cagas, Petr (Virginia Tech, 2018-09-07)
A careful study of plasma-material interactions is essential to understand and improve the operation of devices where plasma contacts a wall such as plasma thrusters, fusion devices, spacecraft-environment interactions, to name a few. This work aims to advance our understanding of fundamental plasma processes pertaining to plasma-material interactions, sheath physics, and kinetic instabilities through theory and novel numerical simulations. Key contributions of this work include (i) novel continuum kinetic algorithms with novel boundary conditions that directly discretize the Vlasov/Boltzmann equation using the discontinuous Galerkin method, (ii) fundamental studies of plasma sheath physics with collisions, ionization, and physics-based wall emission, and (iii) theoretical and numerical studies of the linear growth and nonlinear saturation of the kinetic Weibel instability, including its role in plasma sheaths. The continuum kinetic algorithm has been shown to compare well with theoretical predictions of Landau damping of Langmuir waves and the two-stream instability. Benchmarks are also performed using the electromagnetic Weibel instability and excellent agreement is found between theory and simulation. The role of the electric field is significant during nonlinear saturation of the Weibel instability, something that was not noted in previous studies of the Weibel instability. For some plasma parameters, the electric field energy can approach magnitudes of the magnetic field energy during the nonlinear phase of the Weibel instability. A significant focus is put on understanding plasma sheath physics which is essential for studying plasma-material interactions. Initial simulations are performed using a baseline collisionless kinetic model to match classical sheath theory and the Bohm criterion. Following this, a collision operator and volumetric physics-based source terms are introduced and effects of heat flux are briefly discussed. Novel boundary conditions are developed and included in a general manner with the continuum kinetic algorithm for bounded plasma simulations. A physics-based wall emission model based on first principles from quantum mechanics is self-consistently implemented and demonstrated to significantly impact sheath physics. These are the first continuum kinetic simulations using self-consistent, wall emission boundary conditions with broad applicability across a variety of regimes.
Control Design for an Inertially Stabilized Rifle
White, Alejandro Porter (Virginia Tech, 2007-12-12)
An alternate method for mitigating the depredating physiological affects of a soldiers marksmanship due to combat stressors can be achieved through the design and implementation of a active stabilization system for small arms weapons. The INSTAR system is an innovative active stabilization system designed to decouple the shooter's disturbance effects from the barrel movement. The INSTAR system uses an piezoelectric actuator separating the barrel of the rifle from its stock to stabilize barrel movement. This paper uses various control techniques to develop control algorithms for simulation. The level of performance for each control algorithm is based on how well each they measure up to the criteria developed from the INSTAR system. This paper furthers research on INSTAR by developing and comparing four control designs that may be implemented within the INSTAR system.
Coupling Computationally Expensive Radiative Hydrodynamic Simulations with Machine Learning for Graded Inner Shell Design Optimization in Double Shell Capsules
Vazirani, Nomita Nirmal (Virginia Tech, 2022-12-29)
High energy density experiments rely heavily on predictive physics simulations in the design process. Specifically in inertial confinement fusion (ICF), predictive physics simulations, such as in the radiation-hydrodynamics code xRAGE, are computationally expensive, limiting the design process and ability to find an optimal design. Machine learning provides a mechanism to leverage expensive simulation data and alleviate limitations on computational time and resources in the search for an optimal design. Machine learning efficiently identifies regions of design space with high predicted performance as well as regions with high uncertainty to focus simulations, which may lead to unexpected designs with great potential. This dissertation focuses on the application of Bayesian optimization to design optimization for ICF experiments conducted by the double shell campaign at Los Alamos National Lab (LANL). The double shell campaign is interested in implementing graded inner shell layers to their capsule geometry. Graded inner shell layers are expected to improve stability in the implosions with fewer sharp density jumps, but at the cost of lower yields, in comparison to the nominal bilayer inner shell targets. This work explores minimizing hydrodynamic instability and maximizing yield for the graded inner shell targets by building and coupling a multi-fidelity Bayesian optimization framework with multi-dimensional xRAGE simulations for an improved design process.

Browsing by Author "Scales, Wayne A."

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