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Polarization Induced by a Terahertz Electric Field on a Core-shell Particle
Title
Polarization Induced by a Terahertz Electric Field on a Core-shell Particle
Creator
Li, Yanlin
Date
2018
Description
Interactions of an electromagnetic wave with a sphere that is smaller than the wavelength can be accounted for by studying the dipole moments,...
Show moreInteractions of an electromagnetic wave with a sphere that is smaller than the wavelength can be accounted for by studying the dipole moments, which are the valid explanation for the scattering characteristics in the frequency region known as the Rayleigh region. The semiconductor nanoparticle with a core-shell structure describes a specific geometry yielding tunable plasmon resonance of the nanostructure. This is achieved by varying the thickness of the dielectric material shell layer on a semiconductor core. The polarization of core-shell sphere induced by a dynamic field is studied both analytically and numerically. Dielectric function is used for the description of the response of bound charges to an applied field, resulting in the electric polarization, which has been employed to explicate scattering and absorption properties of particles over the years. However, this traditional model has some limitations in accounting for some aspects of polarization when mobile charges are present. By coupling the transport equations of the charge carriers to the Maxwell’s equations, we can derive the electric field, charge and the total induced dipole moment of a nano-core-shell particle. Results of calculations accomplished for elementary structures such as plates and spheres represented the screening of the internal field while dispersion and absorptions effects are revealed by the complex dipole moments. And the results in static and quasi-static field are shown. Equivalent circuits for the core-shell structures are obtained from carrier transport consideration, which can be employed to guide the synthesis of new nanoparticles with heterogeneous internal structures to achieve novel polarization properties for sensing and terahertz circuitry applications.
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COMMUNICATION-BASED DISTRIBUTED CONTROL IN MICROGRIDS AND NETWORKED MICROGRIDS
Title
COMMUNICATION-BASED DISTRIBUTED CONTROL IN MICROGRIDS AND NETWORKED MICROGRIDS
Creator
Zhou, Quan
Date
2019
Description
Microgrids representing localized small-scale power systems are capable of operating as self-controlled entities, which cluster and manage...
Show moreMicrogrids representing localized small-scale power systems are capable of operating as self-controlled entities, which cluster and manage distributed energy resources (DERs) and other smart devices within a defined electrical boundary. By utilizing locally available resources, microgrids reduce their dependencies on the utility grid, which provide more reliable, resilient, and economic power services to local customers. Geographically close microgrids can be connected for forming a networked microgrid system, which provides additional operational flexibility and further enhances the system reliability and resilience by sharing available DERs.Considering variable and controllable characteristics of DERs, locally available DERs need to be appropriately coordinated and controlled to respond to changing loads. The proliferation of microgrids will make it inevitable to rely on communication systems among microgrids for realizing the coordinated control of participating DERs in networked microgrid systems. The networked microgrid system is considered as a cyber-physical system (CPS) which requires sophisticated network technologies to cope with the massive adaption of communication, computation and control devices. Conventionally, a networked system has been managed by a centralized master controller, which processes the data collected from participating DERs and sends operational set points to each participant.Compared with the centralized control strategies, distributed control is more advantageous for connecting participating DERs. The connectivity of distributed control system (i.e., meshed network) is higher than that of a centralized structure (i.e., star network), in particular when critical circumstances are encountered in which some of the network connections are lost. Also, the distributed control system enables parallel data processing and control, which speeds up the networked system response to variable DERs and loads and promote economic merits. The communication-based distributed control strategies have proven to demonstrate higher reliability, resilience, and scalability while possessing lower implementation costs as compared with centralized control strategies.We have proposed several communication-based distributed control strategies for realizing the coordinated operation of participating microgrids and DERs, which can be applied to achieve various operational objectives, including proportional active power sharing, DER plug-and-play capability, seamless microgrid islanding, and resynchronization operations, and optimal economic operations. The benefits and challenges of communication-based distributed control strategies in networked microgrid systems are discussed and addressed in our work. Extensive case studies have been conducted in this thesis to validate the effectiveness of the proposed communication-based distributed controller design.
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Electric Machine Windings with Reduced Space Harmonic Content
Title
Electric Machine Windings with Reduced Space Harmonic Content
Creator
Tang, Nanjun
Date
2023
Description
The reduction of magnetomotive force (MMF) space harmonic content in electric machine windings can significantly improve the machine's...
Show moreThe reduction of magnetomotive force (MMF) space harmonic content in electric machine windings can significantly improve the machine's electromagnetic performance. Potential benefits include a reduction of torque ripple, a more sinusoidal back EMF, and reduced power losses. With the proposal of a uniform mathematical representation that applies to both distributed windings and fractional-slot concentrated windings (FSCWs), closed-form expressions can be derived for harmonic magnitudes, winding factors, etc. These expressions can then be used to formulate the MMF space harmonic suppression problem for windings, which looks for improved windings with certain harmonic orders reduced or even eliminated, by varying the slot distribution and coil turns. Different solution techniques are explored to gain additional insights about the solution space. The underlying mathematical relations between different harmonic orders are mathematically proved to establish the family phenomenon, which presents clear pictures of the higher order part of the harmonic spectrum and is the foundation for exact calculation of the total harmonic distortion (THD) of windings. The exact THD calculation further indicates how the minimal THD can be achieved for a winding. Windings can also be analyzed and designed from the view of subsets to incorporate distribution and excitation phase shift effects. With reduced or the minimal space harmonic content, new winding designs can help significantly improve the Pareto front when combined with motor geometry optimization. Design examples including a 12-slot 2-pole mixed-layer distributed winding, a 18-slot 2-pole mixed-layer distributed winding, and a four-layer 24-slot 22-pole FSCW with excitation phase shift are presented with finite element analysis (FEA) results to verify the performance improvements.
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Polarization Induced by A Terahertz Electric Field in A Semiconductor Nanodimer in the Overlapping Regime
Title
Polarization Induced by A Terahertz Electric Field in A Semiconductor Nanodimer in the Overlapping Regime
Creator
Wang, Zi
Date
2023
Description
Boltzmann transport equation is a theoretical framework for the description of thermodynamics or charge reactions in a system not in...
Show moreBoltzmann transport equation is a theoretical framework for the description of thermodynamics or charge reactions in a system not in equilibrium, which can be applied to the analysis of the interactions of mobile charges with an electromagnetic wave. When the dimensions of the object are small compared to the wavelength, the induced dipole moment provides a means to characterize the collective response while providing insight to the nature of the charge-field interactions. Semiconductor nanoparticles exhibit surface plasmon resonance in the terahertz frequency range and are of current interest for the development of components and circuits in that part of the electromagnetic spectrum. By changing the plasmon frequencies of doped semiconductors through the change of carrier concentration, new opportunities arise for plasmonic manipulation in terahertz region leading to various promising applications. Despite the Drude model's long-term success and convenience in describing the electrical conductivity of metals in terms of dielectric functions, some aspects of polarization are not accounted for by bulk properties. By incorporating the transport equations of the charge carriers with Maxwell's equations, screening effects of charge carriers can be accounted for, enabling the internal field, space charge and induced dipole moment of a semiconductor nanoparticle to be studied.The computations performed for elementary dimer structures in overlapping cases revealed the internal field screening, while the complex dipole moments show dispersion and absorption effects. The numerical algorithms are implemented using the finite element method to investigate the surface plasmon resonance (SPR) induced on the semiconductor particles. Unique SPR modes evolution is observed as the thickness of the overlap region is varied. The characteristics can be interpreted by the migration of local space charge as the level of overlap is varied. This degree of freedom provided by a semiconductor nanodimer could be employed to control the local field near a simple cluster of nanoparticles, with potential for application in sensing and circuit components in the terahertz frequency range.
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