VTechWorks Repository :: Browsing by Author "Srinivasan, Bhuvana"

Browsing by Author "Srinivasan, Bhuvana"

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Additive Manufacturing in Spacecraft Design and In-Space Robotic Fabrication of Large Structures
Spicer, Randy Lee (Virginia Tech, 2023-08-31)
Additive Manufacturing (AM, 3D printing) has made significant advancements over the past decade and has become a viable alternative to traditional machining techniques. AM offers several advantages over traditional manufacturing techniques including improved geometric freedom, reduction in part lead time, cost savings, enhanced customization, mass reduction, part elimination, and remote production. There are many different AM processes with the most commonly used process being Fused Filament Fabrication (FFF). Small satellites have also made significant advancements over the past two decades with the number of missions launched annually increased by orders of magnitude over that time span. Small satellites offer several advantages compared to traditional spacecraft architectures including increased access to space, lower development costs, and disaggregated architectures. On-orbit manufacturing and assembly have become major research and development topics for government and commercial entities seeking the capability to build very large structures in space. AM is well suited on-orbit manufacturing since the process is highly automated, produces little material waste, and allows for a large degree of geometric freedom. This dissertation seeks to address three major research objectives regarding applications of additive manufacturing in space systems: demonstrate the feasibility of 3D printing an ESPA class satellite using FFF, develop a FFF 3D printer that is capable of operating in high vacuum and characterize its performance, and analyze the coupled dynamics between a satellite and a robot arm used for 3D printing in-space. This dissertation presents the design, finite element analysis, dynamic testing, and model correlation of AdditiveSat, an additively manufactured small satellite fabricated using FFF. This dissertation also presents the design, analysis, and test results for a passively cooled FFF 3D printer capable of manufacturing parts out of engineering grade thermoplastics in the vacuum of space. Finally, this dissertation presents a numerical model of a free-flying small satellite with an attached robotic arm assembly to simulate 3D printing structures on-orbit with analysis of the satellite controls required to control the dynamics of the highly coupled system.
Characterization and Modeling of Solar Flare Effects in the Ionosphere Observed by HF Instruments
Chakraborty, Shibaji (Virginia Tech, 2021-06-08)
The ionosphere is the conducting part of the upper atmosphere that plays a significant role in trans-ionospheric high frequency (HF, 3-30 MHz) radiowave propagation. Solar activities, such as solar flares, radiation storms, coronal mass ejections (CMEs), alter the state of the ionosphere, a phenomenon known as Sudden Ionospheric Disturbance (SID), that can severely disrupt HF radio communication links by enhancing radiowave absorption and altering signal frequency and phase. The Super Dual Auroral Radar Network (SuperDARN) is an international network of low-power HF coherent scatter radars distributed across the globe to probe the ionosphere and its relation to solar activities. In this study, we used SuperDARN HF radar measurements with coordinated spacecraft and riometer observations to investigate statistical characteristics and the driving mechanisms of various manifestations of solar flare-driven SIDs in HF observations. We begin in Chapter 2 with a statistical characterization of various effects of solar flares on SuperDARN observations. Simultaneous observations from GOES spacecraft and SuperDARN radars confirmed flare-driven HF absorption depends on solar zenith angle, operating frequency, and intensity of the solar flare. The study found flare-driven SID also affects the SuperDARN backscatter signal frequency, which produces a sudden rise in Doppler velocity observation, referred to as the ``Doppler flash'', which occurs before the HF absorption effect. In Chapter 3, we further investigate the HF absorption effect during successive solar flares and those co-occurring with other geomagnetic disturbances during the 2017 solar storm. We found successive solar flares can extend the ionospheric relaxation time and the variation of HF absorption with latitude is different depending on the type of disturbance. In Chapter 4, we looked into an inertial property of the ionosphere, sluggishness, its variations with solar flare intensity, and made some inferences about D-region ion-chemistry using a simulation study. Specifically, we found solar flares alter the D-region chemistry by enhancing the electron detachment rate due to a sudden rise in molecular vibrational and rotational energy under the inﬂuence of enhanced solar radiation. In Chapter 5, we describe a model framework that reproduces HF absorption observed by riometers. This chapter compares different model formulations for estimating HF absorption and discusses different driving influences of HF absorption. In Chapter 6, we have investigated different driving mechanisms of the Doppler flash observed by SuperDARN radars. We note two particular findings: (i) the Doppler flash is predominantly driven by a change in the F-region refractive index and (ii) a combination of solar flare-driven enhancement in photoionization, and changes in the zonal electric field and(or) ionospheric conductivity reduces upward ion-drift, which produces a lowering effect in the F-region HF radiowave reflection height. Collectively, these research findings provide a statistical characterization of various solar flare effects on the ionosphere seen in the HF observations, and insights into their driving mechanisms and impacts on ionospheric dynamics.
Characterization of a Low Current LaB6 Heaterless Hollow Cathode with Krypton Propellant
Jain, Prachi Lalit (Virginia Tech, 2020-06-25)
A first-generation LaB6 heaterless hollow cathode with a flat-plate anode is experimentally investigated. The cathode is characterized using krypton as propellant at varying flow rates, discharge currents and cathode-anode distances. Voltage probes, used to make direct voltage measurements in the ignition circuit, are the only diagnostic tool used experimentally. A plasma model is used to infer plasma parameters in the cathode emitter region. The cathode characterization results are consistent with those obtained during previous investigations of 1 A-class LaB6 hollow cathode with krypton. A peak-to-peak anode voltage criterion is used to identify the discharge modes and the occurrence of mode transition. Fourier analysis of the keeper and anode voltage waveforms carried out to study the discharge mode behavior reveals resonant frequencies ranging from 40 to 150 kHz. Lastly, post-test visual observations of the cathode components show signs of emitter poisoning and keeper erosion.
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 Liquid Metal Free Surface Response to an Electromagnetic Impulse and Implications for Future Nuclear Fusion Devices
Weber, Daniel Perry (Virginia Tech, 2024-01-10)
Liquid metals (LMs) are compelling candidates for use as plasma facing components (PFCs) in fusion devices to mitigate heat loading, limit damage due to erosion, and possibly breed tritium. When used as electrodes, such as in z-pinch devices, PFCs are subject to large current and magnetic flux densities resulting in large Lorentz forces. Furthermore, if the PFCs are LM, the forces excite wave behavior that has not previously been investigated. The work presented here first characterizes the response of LMs to current pulses which peak between 50 and 200 kA and generate magnetic pressures between 0.5 and 5 MPa. High-speed videography records the liquid metal free surface during and after the current pulse and captures a fast moving, annular jet of LM emerging from the main body. The vertical velocities of the jet range from 0.6 to 5.3 m/s which is consistent with hydrodynamic predictions. Ejection of small droplets is observed from the LM immediately after the current pulse, preceding the LM jet, with velocities ranging from −3.1 to 18.9 m/s in the vertical direction and −14.3 to 6.3 m/s in the radial. A statistical model is developed to predict the likelihood of certain LM PFC material contaminating a core plasma and the severity in such an event. Lastly, effectiveness of bulk wave movement mitigation is investigated with two solid barrier designs, a cylindrical and conical baffle. These designs were fabricated after an iterative design process with assistance from hydrodynamic simulations. A cylindrical baffle design is shown to be preferable for integration into future fusion devices for the reduced likelihood of interference with plasma column formation.
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.
CPU/GPU Code Acceleration on Heterogeneous Systems and Code Verification for CFD Applications
Xue, Weicheng (Virginia Tech, 2021-01-25)
Computational Fluid Dynamics (CFD) applications usually involve intensive computations, which can be accelerated through using open accelerators, especially GPUs due to their common use in the scientific computing community. In addition to code acceleration, it is important to ensure that the code and algorithm are implemented numerically correctly, which is called code verification. This dissertation focuses on accelerating research CFD codes on multi-CPUs/GPUs using MPI and OpenACC, as well as the code verification for turbulence model implementation using the method of manufactured solutions and code-to-code comparisons. First, a variety of performance optimizations both agnostic and specific to applications and platforms are developed in order to 1) improve the heterogeneous CPU/GPU compute utilization; 2) improve the memory bandwidth to the main memory; 3) reduce communication overhead between the CPU host and the GPU accelerator; and 4) reduce the tedious manual tuning work for GPU scheduling. Both finite difference and finite volume CFD codes and multiple platforms with different architectures are utilized to evaluate the performance optimizations used. A maximum speedup of over 70 is achieved on 16 V100 GPUs over 16 Xeon E5-2680v4 CPUs for multi-block test cases. In addition, systematic studies of code verification are performed for a second-order accurate finite volume research CFD code. Cross-term sinusoidal manufactured solutions are applied to verify the Spalart-Allmaras and k-omega SST model implementation, both in 2D and 3D. This dissertation shows that the spatial and temporal schemes are implemented numerically correctly.
Deceleration Stage Rayleigh-Taylor Instability Growth in Inertial Confinement Fusion Relevant Configurations
Samulski, Camille Clement (Virginia Tech, 2021-06-08)
Experimental results and simulations of imploding fusion concepts have identified the Rayleigh-Taylor (RT) instability as one of the largest inhibitors to achieving fusion. Understanding the origin and development of the RT instability will allow for the development of mitigating measures to dampen the instability growth, thus improving the chance that fusion concepts such as inertial confinement fusion (ICF) are successful. A study of 1D and 2D simulations are presented for investigating RT instability growth in deceleration stage of imploding geometries. Two cases of laser-driven implosion geometry, Cartesian and cylindrical, are used to study late stage deceleration-phase RT instability development on the interior surface of imploding targets. FLASH's hydrodynamic (HD) and magnetohydrodynamic (MHD) modeling capabilities are used for different laser and target parameters in order to study the RT instability and the impact of externally applied magnetic fields on their evolution. Several simulation regimes have been identified that provide novel insight into the impact that a seeded magnetic field can have on RT instability growth and the conditions under which magnetic field stabilization of the RT instability is observable. Finally, future work and recommendations are made.
Development and Modelling of a Low Current LaB₆ Heaterless Hollow Cathode
Nikrant, Alex Warner (Virginia Tech, 2019-09-20)
The presented research discusses the design, analysis, and testing of a low current, LaB6 heaterless hollow cathode for space propulsion applications. A heaterless design using LaB6 is chosen to reduce complexity and increase electrical power efficiency and robustness. Argon propellant is used due to its more favorable breakdown voltage characteristics compared to xenon. An original model for the insert region plasma is derived by combining several analyses in literature. This model allows the simultaneous calculation of many plasma and thermal parameters in the cathode using only two completely unobtrusive measurements, and requires several assumptions which are common in hollow cathode research. The design of the cathode and its subsystems are presented in detail. No diagnostics were used in the cathode except direct voltage measurements in the power circuit. A discussion of emitter poisoning and ignition behavior is presented. The cathode is characterized by measuring anode and keeper voltages as a function of anode current and propellant flow rate, with the cathode discharging directly to a flat metal anode. Results are consistent with those obtained by previous investigations of argon hollow cathodes. This data is used with the derived plasma model to calculate the dependence of various parameters on current and flow rate. A discussion of the spot-plume transition behavior is presented. Finally, insights and design improvements are discussed based on the experimental results.
Development of Magnetic Nozzle Simulations for Space Propulsion Applications
Glesner, Colin Christopher (Virginia Tech, 2017-02-06)
A means of space propulsion using the channeling of plasma by a divergent magnetic field, referred to as a magnetic nozzle has been explored by a number of research groups. This research develops the capability to apply the high order accurate Runge-Kutta discontinuous Galerkin numerical method to the simulation of magnetic nozzles. The resistive magnetohydrodynamic model of plasma behavior is developed for these simulations. To facilitate this work, several modeling capabilities are developed, including the implementation of appropriate inflow and far-field boundary conditions, the application of a technique for correcting errors that develop in the divergence of the magnetic field, and a split formulation for the magnetic field between the applied and the perturbed component. This model is then applied to perform a scaling study of the performance of magnetic nozzles over a range of Bk and Rm. In addition, the effect of the choice of simulation domain size is investigated. Finally, recommendations for future work are made.
Discontinuous Galerkin Studies of Collisional Dynamics in Continuum-Kinetic Plasma
Rodman, John Morgan (Virginia Tech, 2025-01-24)
Numerical investigations of collisional physics have historically been impeded by the issue of computational expense. While the continuum-kinetic Vlasov-Maxwell-Fokker-Planck system is well-established in theory and has been used as the basis for many approximate fluid equations, simulations utilizing the distribution function are relatively uncommon, due primarily to the high dimensionality of the problem. However, advances in numerical methods are steadily making these models more accessible. In this work, we utilize the Gkeyll framework, which applies a novel, highly efficient discontinuous Galerkin (DG) finite element method to the Vlasov-Maxwell-Fokker-Planck system. We first investigate the Rayleigh-Taylor (RT) instability in a neutral gas in regimes of finite collisionality which are inaccessible to the fluid codes that are traditionally applied to this instability. Utilizing a spatially constant, finite collision frequency, we demonstrate the ability of the Vlasov-Boltzmann model to approach the fluid result at high collision frequency while also accessing a regime of intermediate collisionality in which the RT instability deviates greatly from classic fluid behavior. We then extend upon this finding by choosing a collision frequency that varies spatially, resulting in new dynamics with asymmetric diffusion affecting the development of the RT instability. Having demonstrated the utility of collisional kinetic modeling even in the simple case of a neutral gas with a basic collision operator, we transition to development and implementation of a fully-conservative, recovery-based DG algorithm for the full nonlinear Rosenbluth/Fokker-Planck collision operator (FPO). Details of the novel recovery scheme for the cross-derivatives and conservation enforcement are presented, and we show that the scheme converges and exhibits stability criteria as expected. Finally, the FPO is applied to test cases that demonstrate the importance of accurate handling of the velocity-dependent collision frequency as compared to an approximate model.
Discretization Error Estimation Using the Error Transport Equations for Computational Fluid Dynamics Simulations
Wang, Hongyu (Virginia Tech, 2021-06-11)
Computational Fluid Dynamics (CFD) has been widely used as a tool to analyze physical phenomena involving fluids. To perform a CFD simulation, the governing equations are discretized to formulate a set of nonlinear algebraic equations. Typical spatial discretization schemes include finite-difference methods, finite-volume methods, and finite-element methods. Error introduced in the discretization process is called discretization error and defined as the difference between the exact solution to the discrete equations and the exact solution to the partial differential or integral equations. For most CFD simulations, discretization error accounts for the largest portion of the numerical error in the simulation. Discretization error has a complicated nonlinear relationship with the computational grid and the discretization scheme, which makes it especially difficult to estimate. Thus, it is important to study the discretization error to characterize numerical errors in a CFD simulation. Discretization error estimation is performed using the Error Transport Equations (ETE) in this work. The original nonlinear form of the ETE can be linearized to formulate the linearized ETE. Results of the two types of the ETE are compared. This work implements the ETE for finite-volume methods and Discontinuous Galerkin (DG) finite-element methods. For finite volume methods, discretization error estimates are obtained for both steady state problems and unsteady problems. The work on steady-state problems focuses on turbulent flow modelled by the Spalart-Allmaras (SA) model and Menter's $k-omega$ SST model. Higher-order discretization error estimates are obtained for both the mean variables and the turbulence working variables. The type of pseudo temporal discretization used for the steady-state problems does not have too much influence on the final converged solution. However, the temporal discretization scheme makes a significant difference for unsteady problems. Different temporal discretizations also impact the ETE implementation. This work discusses the implementation of the ETE for the 2-step Backward Difference Formula (BDF2) and the Singly Diagonally Implicit Runge-Kutta (SDIRK) methods. Most existing work on the ETE focuses on finite-volume methods. This work also extends ETE to work with the DG methods and tests the implementation with steady state inviscid test cases. The discretization error estimates for smooth test cases achieve the expected order of accuracy. When the test case is non-smooth, the truncation error estimation scheme fails to generate an accurate truncation error estimate. This causes a reduction of the discretization error estimate to first-order accuracy. Discussions are made on how accurate truncation error estimates can be found for non-smooth test cases.
The Effect of Anomalous Resistivity on the Electrothermal Instability
Masti, Robert Leo (Virginia Tech, 2021-06-09)
The current driven electrothermal instability (ETI) forms when the material resistivity is temperature dependent, occurring in nearly all Z-pinch-like high energy density platforms. ETI growth for high-mass density materials is predominantly striation form which corresponds to magnetically perpendicular mode growth. The striation form is caused by a resistivity that increases with temperature, which is often the case for high-mass density materials. In contrast, low-density ETI growth is mainly filamentation form, magnetically aligned modes, because the resistivity tends to decrease with temperature. Simulating ETI is challenging due to the coupling of magnetic field transport to equation of state over a large region of state space spanning solids to plasmas. This dissertation presents a code-code verification study to effectively model the ETI. Specifically, this study provides verification cases which ensure the unit physics components essential to modeling ETI are accurate. This provides a way for fluid-based codes to simulate linear and nonlinear ETI. Additionally, the study provides a sensitivity analysis of nonlinear ETI to equation of state, vacuum resistivity, and vacuum density. Simulations of ETI typically use a collisional form of the resistivity as provided, e.g., in a Lee-More Desjarlais conductivity table. In regions of low-mass density, collision-less transport needs to be incorporated to properly simulate the filamentation form of ETI growth. Anomalous resistivity (AR) is an avenue by which these collision-less micro-turbulent effects can be incorporated into a collisional resistivity. AR directly changes the resistivity which will directly modify the linear growth rate of ETI, so a new linear growth rate is derived which includes AR's added dependency on current density. This linear growth rate is verified through a filamentation ETI simulation using an ion acoustic based AR model. Kinetically based simulations of vacuum contaminant plasmas provide a physical platform to study the use of AR models in pulsed-power platforms. Using parameters from the Z-machine pulsed-power device, the incorporation of AR can increase a collisional-based resistivity by upwards of four orders of magnitude. The presence of current-carrying vacuum contaminant plasmas can indirectly affect nonlinear ETI growth through modification of the magnetic diffusion wave. The impact of AR on nonlinear ETI is explored through pulsed-power simulations of a dielectrically coated solid metallic liner surrounded by a low-density vacuum contaminant plasma.
The Effects of Collisions on Plasma-Sheath Transition
Li, Yuzhi (Virginia Tech, 2023-05-05)
The plasma sheath is essential for understanding the plasma-material interaction (PMI) since it regulates the plasma particle and energy fluxes to the wall. The key concept in sheath theory is the Bohm criterion that gives the lower bound of the plasma exit flow speed, also known as the Bohm speed. Traditionally, the Bohm speed is evaluated in the asymptotic limit of an infinitely thin sheath and ignores the transport physics in the plasma-sheath transition problem. Whereas in practical applications, the sheath has a finite thickness and the transport in the neighborhood of the sheath entrance is complicated. The focus of this thesis is on performing Bohm speed analysis for different applications that are away from the asymptotic limits, with an emphasis on the critical role of transport physics on the Bohm speed formulation. The classical sheath problem with a wide range of Coulomb collisionality is revisited. Here, we derive an expression for the Bohm speed from a set of anisotropic plasma transport equations. The thermal force, temperature isotropization and heat flux enter into the eval- uation of the Bohm speed. Away from the asymptotic limit, it is shown that there exists a plasma-sheath transition region, rather than a single point at the sheath entrance. In the transition region, the quasineutrality is weakly perturbed and the Bohm speed is predicted for the entire transition region. By comparison with kinetic simulation results, the Bohm speed model in our work is shown to be accurate in the sheath transition region over a broad range of collisionality. The Bohm speed analysis developed above can be applied to plasma-sheath transition prob- lems with more complex transport physics, such as a high recycling divertor in a fusion reactor. In the high recycling regime, the plasma particles hitting on the divertor surface will be recycled through reflection or desorption and return to the plasma in the form of neutrals. The plasma will interact with the recycled neutrals through atomic collisions such as ionization, excitation, or ion charge-exchange collision, complicating the plasma transport in the transition layer. A new Bohm speed model is proposed to account for the effect of the anisotropic transport and atomic collisions in the transition layer. A first principle ki- netic code VPIC with the atomic collision package is used to investigate a 1D self-consistent slab plasma with a high recycling boundary for tungsten and carbon divertors. The results demonstrate the accuracy of the Bohm speed model in predicting the ion exit flow speed in the transition region, as well as the reduction of the Bohm speed due to the ion-neutral friction.
Emitting Wall Boundary Conditions in Continuum Kinetic Simulations: Unlocking the Effects of Energy-Dependent Material Emission on the Plasma Sheath
Bradshaw, Kolter Austen (Virginia Tech, 2024-02-23)
In a wide variety of applications such as the Hall thruster and the tokamak, understanding the plasma-material interactions which take place at the wall is important for improving performance and preventing failure due to material degradation. In the region near a surface, the plasma sheath forms and regulates the electron and ion fluxes into the material. Emission from the material has the potential to change sheath structure drastically, and must be modeled rigorously to produce accurate predictions of the fluxes into the wall. Continuum kinetic codes offer significant advantages for the modeling of sheath physics, but the complexity of emission physics makes it difficult to implement accurately. This difficulty results in major simplifications which often neglect important energy-dependent physics. A focus of the work is on proper simulation of the sheath. The implementation of source and collision terms is discussed, alongside a brief study of the Weibel instability in the sheath demonstrating the necessity of proper collision implementation to avoid missing relevant physics. A novel implementation of semi-empirical models for electron-impact secondary electron emission into the boundary conditions of a continuum kinetic code is presented here. The features of both high and low energy regimes of emission are represented self-consistently, and the underlying algorithms are flexible and can be easily extended to other emission mechanisms, such as ion-impact secondary electron emission. The models are applied to simulations of oxidized and clean lithium for fusion-relevant plasma regimes. Oxidized lithium has a high emission coefficent and the sheath transitions into space-charge limited and inverse modes for different parameters. The breakdown of the classical sheath results in an increase of energy fluxes to the surface, with potential ramification for applications.
Empirical Ionospheric Models: The Road To Conductivity
Edwards, Thomas Raymond (Virginia Tech, 2019-04-15)
The Earth's polar ionosphere is a highly dynamic region of the upper atmosphere, and acts as the closure of the greater magnetospheric current system. This region plays host to many electrodynamic effects that impact terrestrial systems, such as power grids, railroads, and pipelines. These effects are fundamentally related to the currents, electric fields, and conductivity present in the polar ionosphere. Understanding and predicting the electrodynamics of this region is vital to being able to determine the physical impacts on terrestrial systems and provide predictions to government and commercial entities. Empirical models play a key role in the research and forecasting of the solar wind and interplanetary magnetic field's impact on the polar ionosphere, and is an active area of development and research. Recent interest in polar ionospheric conductivity has led to a community-wide campaign to develop our understanding of this portion of the electrodynamic system. Characterizing the interactions between the solar wind and the polar ionosphere is a difficult task, as the region of interest is highly data starved in many respects. In particular, satellite based data products are scarce due to being costly and logistically difficult. Recent advancements in data sources (such as the Swarm and CHAMP satellite missions) as well as continued research into the physical relationships between solar wind and interplanetary magnetic field drivers have provided the opportunity to develop new, novel tools to study this region of interest. In this dissertation, two polar ionosphere models are described in Chapters 3 and 4, along with the original research that their construction has produced in Chapter 1. These two models are combined to provide a foundation for future research in this area, which is described in Chapter 5.
Examining Plasma Instabilities as Ionospheric Turbulence Generation Mechanisms Using Pseudo-Spectral Methods
Rathod, Chirag (Virginia Tech, 2021-03-30)
Turbulence in the ionosphere is important to understand because it can negatively affect communication signals. This work examines different scenarios in the ionosphere in which turbulence may develop. The two main causes of turbulence considered in this work are the gradient drift instability (GDI) and the Kelvin-Helmholtz instability (KHI). The likelihood of the development of the GDI during the August 17, 2017 total solar eclipse is studied numerically. This analysis uses the ``Sami3 is Also a Model of the Ionosphere" (SAMI3) model to study the effect of the eclipse on the plasma density. The calculated GDI growth rates are small compared to how quickly the eclipse moves over the Earth. Therefore, the GDI is not expected to occur during the solar eclipse. A novel 2D electrostatic pseudo-spectral fluid model is developed to study the growth of these two instabilities and the problem of ionospheric turbulence in general. To focus on the ionospheric turbulence, a set of perturbed governing equations are derived. The model accurately captures the GDI growth rate in different limits; it is also benchmarked to the evolution of instability development in different collisional regimes of a plasma cloud. The newly developed model is used to study if the GDI is the cause of density irregularities observed in subauroral polarization streams (SAPS). Data from Global Positioning System (GPS) scintillations and the Super Dual Auroral Radar Network (SuperDARN) are used to examine the latitudinal density and velocity profiles of SAPS. It is found that the GDI is stabilized by velocity shear and therefore will only generate density irregularities in regions of low velocity shear. Furthermore, the density irregularities cannot extend through regions of large velocity shear. In certain cases, the turbulence cascade power laws match observation and theory. The transition between the KHI and the GDI is studied by understanding the effect of collisions. In low collisionality regimes, the KHI is the dominant instability. In high collisionality regimes, the GDI is the dominant instability. Using nominal ionospheric parameters, a prediction is provided that suggests that there exists an altitude in the upper textit{F} region ionosphere above which the turbulence is dominated by the KHI.
Experimental Characterization of Diffusive Phenomena in Multi-Ion-Species Plasma Shocks Formed During Railgun-Driven Plasma Jet Collision Events
Mohammed, Ameer Insaf (Virginia Tech, 2024-02-23)
Gradient-driven mass diffusion, or species separation, is a transport process which can occur in plasma shocks. Experimental observations of this phenomena are difficult to make, but are of interest to ongoing Inertial Confinement Fusion efforts. This body of work describes the results of two major experimental campaigns conducted at Virginia Tech's Experimental Plasma and Propulsion Laboratory to identify species separation in multi-component plasma shocks. A linear plasma-armature railgun forms and accelerates low temperature, high density, supersonic plasma jets, with the collision between two of these jets shown to induce a collisional plasma shock in the first campaign. The second campaign leverages this experimental setup while employing spatially resolved emission spectroscopy alongside collisional radiative modeling to identify species separation within multi-ion-species plasma shocks consisting of argon, aluminum, and nitrogen. These results are some of the first to be performed in a plasma shock with more than two ion species, and can be used for verification and validation of physics models of fusion plasmas. This body of work was supported by the National Science Foundation under Grant No. PHY-1903442.
Innovative Platform Design for In Vitro Primary Blast Injury Research
Showalter, Noah Wade (Virginia Tech, 2023-07-10)
One of the principal challenges of primary blast injury research is imitation of shock waves accurately and consistently in a safe and tunable platform. Existing simulators have been effective in these goals but have not been conducive for in vitro models due to their large size and air-mediated wave propagation. In this thesis, a redesigned benchtop shock wave generator (SWG) has provided a platform for in vitro models. A pulsed power generator charges a capacitor and discharges the capacitor through a bridge wire. The discharge causes the bridge wire to experience phase changes, momentarily becoming a gas or plasma. In this moment, the bridge wire expands radially and creates a pressure wave in the surrounding water. As the wave propagates, it forms a shock wave and strikes the cell platform at the far end of the conical tank. Current design efforts are focused on the tunability of the SWG, by varying the bridge wire material and diameter. Five materials at three bridge wire diameters have been tested. Each bridge wire was inserted into the SWG via a pinching mechanism. Either side of the pinching mechanism was connected to either terminal of the capacitor. When the pulsed power generator was cycled, the bridge wire was vaporized and generated a shock wave. A piezoelectric sensor near the wide end of the tank recorded the passing of the shock wave, which was used to derive various pressure metrics that correlate to injury. The sample size for each combination of diameter and material was five, with a grand total of seventy-five samples run. Two-way ANOVAs measuring the impacts of bridge wire material and diameter on a variety of shock wave metrics found that the diameter played a significant role in determining the peak overpressure and positive impulse generated while the main effect of material played a much smaller role. The interaction between material and diameter was also found to be significant. The tunable benchtop SWG provides a platform for exploration of primary blast injury using in vitro models. By adjusting the bridge wire diameter, the SWG can generate waves with a variety of shock wave metrics, providing an opportunity for researchers to address various degrees of injury. With the addition of this technology to the efforts to understand primary blast injury, development of treatments and protective equipment can be expedited.

Browsing by Author "Srinivasan, Bhuvana"

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