A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Modeling and validation of dosimetry measurement assumptions within the Armed Forces Radiobiology Research Institute TRIGA Mark F reactor and associated exposure facilities using Monte Carlo techniques(2009) Hall, Donald Edward; Modarres, Mohammad; Al-Sheikhly, Mohamad; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The TRIGA Mark F reactor at the Armed Forces Radiobiology Research Institute in Bethesda Maryland is a 1 megawatt steady state reactor which can also be operated in pulse mode at a power of up to 2500 megawatts. It is characterized by a moveable core and two large exposure rooms, rather than a thermal column or beam ports, as found in most research reactors. A detailed model of the reactor and the associated exposure facilities was developed using the Monte Carlo N-Particle (MCNP) and Monte Carlo N-Particle Extended (MCNPX) software programs. The model was benchmarked against operational data from the reactor, to include total core excess reactivity, control rod worths, and nominal fuel element worths. The model was then used to model burnup within individual fuel elements within the core to determine the effect of core movement within the reactor pool on individual element burnup. Movement of the core with respect to the two exposure rooms was modeled to determine the effect of movement of the core on the radiation fields and gamma and neutron fluxes within the exposure rooms. Additionally, the model was used to demonstrate the effectiveness of gadolinium paint used within the exposure rooms to reduce thermal neutron production and subsequent Ar41 production within the exposure rooms. The model showed a good approximation to measured benchmark data across all applied metrics, and additionally provided confirmation of data on dose rates within the exposure rooms. It also showed that, while there was some variation of burnup within individual fuel elements based on core position within the reactor pool, the overall effect was negligible for effective fuel management within the core. Finally, the model demonstrated explicitly that the use of gadolinium paint within the exposure rooms was, and remains, an effective way of reducing the thermal flux, and subsequent Ar-41 production within the exposure rooms. It also demonstrated that the gadolinium paint also resulted in a much steeper neutron flux gradient within the exposure rooms than would have been obtained had neutrons been allowed to thermalize within the wood walls lining the rooms and then reenter the exposure facilities.Item Electromagnetic Interference Reduction using Electromagnetic Bandgap Structures in Packages, Enclosures, Cavities, and Antennas(2007-11-26) Mohajer Iravani, Baharak; Ramahi, Omar M.; Granatstein, Victor L.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electromagnetic interference (EMI) is a source of noise problems in electronic devices. The EMI is attributed to coupling between sources of radiation and components placed in the same media such as package or chassis. This coupling can be either through conducting currents or through radiation. The radiation of electromagnetic (EM) fields is supported by surface currents. Thus, minimizing these surface currents is considered a major and critical step to suppress EMI. In this work, we present novel strategies to confine surface currents in different applications including packages, enclosures, cavities, and antennas. The efficiency of present methods of EM noise suppression is limited due to different drawbacks. For example, the traditional use of lossy materials and absorbers suffers from considerable disadvantages including mechanical and thermal reliability leading to limited life time, cost, volume, and weight. In this work, we consider the use of Electromagnetic Band Gap (EBG) structures. These structures are suitable for suppressing surface currents within a frequency band denoted as the bandgap. Their design is straight forward, they are inexpensive to implement, and they do not suffer from the limitations of the previous methods. A new method of EM noise suppression in enclosures and cavity-backed antennas using mushroom-type EBG structures is introduced. The effectiveness of the EBG as an EMI suppresser is demonstrated using numerical simulations and experimental measurements. To allow integration of EBGs in printed circuit boards and packages, novel miniaturized simple planar EBG structures based on use of high-k dielectric material (r > 100) are proposed. The design consists of meander lines and patches. The inductive meander lines serve to provide current continuity bridges between the capacitive patches. The high-k dielectric material increases the effective capacitive load substantially in comparison to commonly used material with much lower dielectric constant. Meander lines can increase the effective inductive load which pushes down the lower edge of bandgap, thus resulting in a wider bandgap. Simulation results are included to show that the proposed EBG structures provide very wide bandgap (~10GHz) covering the multiple harmonics of of currently available microprocessors and its harmonics. To speed up the design procedure, a model based on combination of lumped elements and transmission lines is proposed. The derived model predicts accurately the starting edge of bandgap. This result is verified with full-wave analysis. Finally, another novel compact wide band mushroom-type EBG structure using magneto-dielectric materials is designed. Numerical simulations show that the proposed EBG structure provides in-phase reflection bandgap which is several times greater than the one obtained from a conventional EBG operating at the same frequency while its cell size is smaller. This type of EBG structure can be used efficiently as a ground plane for low-profile wideband antennas.Item The ADI-FDTD Method for High Accuracy Electrophysics Applications(2006-11-24) Haeri Kermani, Mohammad; Ramahi, Omar M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Finite-Difference Time-Domain (FDTD) is a dependable method to simulate a wide range of problems from acoustics, to electromagnetics, and to photonics, amongst others. The execution time of an FDTD simulation is inversely proportional to the time-step size. Since the FDTD method is explicit, its time-step size is limited by the well-known Courant-Friedrich-Levy (CFL) stability limit. The CFL stability limit can render the simulation inefficient for very fine structures. The Alternating Direction Implicit FDTD (ADI-FDTD) method has been introduced as an unconditionally stable implicit method. Numerous works have shown that the ADI-FDTD method is stable even when the CFL stability limit is exceeded. Therefore, the ADI-FDTD method can be considered an efficient method for special classes of problems with very fine structures or high gradient fields. Whenever the ADI-FDTD method is used to simulate open-region radiation or scattering problems, the implementation of a mesh-truncation scheme or absorbing boundary condition becomes an integral part of the simulation. These truncation techniques represent, in essence, differential operators that are discretized using a distinct differencing scheme which can potentially affect the stability of the scheme used for the interior region. In this work, we show that the ADI-FDTD method can be rendered unstable when higher-order mesh truncation techniques such as Higdon's Absorbing Boundary Condition (ABC) or Complementary Derivatives Method (COM) are used. When having large field gradients within a limited volume, a non-uniform grid can reduce the computational domain and, therefore, it decreases the computational cost of the FDTD method. However, for high-accuracy problems, different grid sizes increase the truncation error at the boundary of domains having different grid sizes. To address this problem, we introduce the Complementary Derivatives Method (CDM), a second-order accurate interpolation scheme. The CDM theory is discussed and applied to numerical examples employing the FDTD and ADI-FDTD methods.Item Precise SAR Measurements In The Near-Field of RF Antenna Systems(2006-04-27) Hakim, Bandar M; Davis, Christopher C; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wireless devices must meet specific safety radiation limits, and in order to assess the health affects of such devices, standard procedures are used in which standard phantoms, tissue-equivalent liquids, and miniature electric field probes are used. The accuracy of such measurements depend on the precision in measuring the dielectric properties of the tissue-equivalent liquids and the associated calibrations of the electric-field probes. This thesis describes work on the theoretical modeling and experimental measurement of the complex permittivity of tissue-equivalent liquids, and associated calibration of miniature electric-field probes. The measurement method is based on measurements of the field attenuation factor and power reflection coefficient of a tissue-equivalent sample. A novel method, to the best of the author's knowledge, for determining the dielectric properties and probe calibration factors is described and validated. The measurement system is validated using saline at different concentrations, and measurements of complex permittivity and calibration factors have been made on tissue-equivalent liquids at 900MHz and 1800MHz. Uncertainty analysis have been conducted to study the measurement system sensitivity. Using the same waveguide to measure tissue-equivalent permittivity and calibrate e-field probes eliminates a source of uncertainty associated with using two different measurement systems. The measurement system is used to test GSM cell-phones at 900MHz and 1800MHz for Specific Absorption Rate (SAR) compliance using a Specific Anthropomorphic Mannequin phantom (SAM).Item Analyses of advanced concepts in multi-stage gyro-amplifiers and startup in high-power gyro-oscillators(2005-12-06) Sinitsyn, Oleksandr V; Granatstein, Victor L; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gyrotrons are well recognized sources of high-power coherent electromagnetic radiation. The power that gyrotrons can radiate in the millimeter- and submillimeter-wavelength regions exceeds the power of classical microwave tubes by many orders of magnitude. In this work, the author considers some problems related to the operation of gyro-devices and methods of their solution. In particular, the self-excitation conditions for parasitic backward waves and effect of distributed losses on the small-signal gain of gyro-TWTs are analyzed. The corresponding small-signal theory describing two-stage gyro-traveling-wave tubes (gyro-TWTs) with the first stage having distributed losses is presented. The theory is illustrated by using it for the description of operation of a Ka-band gyro-TWT designed at the Naval Research Laboratory. Also, the results of nonlinear studies of this tube are presented and compared with the ones obtained by the use of MAGY, a multi-frequency, self-consistent code developed at the University of Maryland. An attempt to build a large signal theory of gyro-TWTs with tapered geometry and magnetic field profile is made and first results are obtained for a 250 GHz gyro-TWT. A comparative small-signal analysis of conventional four-cavity and three-stage clustered-cavity gyroklystrons is performed. The corresponding point-gap models for these devices are presented. The efficiency, gain, bandwidth and gain-bandwidth product are analyzed for each scheme. Advantages of the clustered-cavity over the conventional design are discussed. The startup scenarios in high-power gyrotrons and the most important physical effects associated with them are considered. The work presents the results of startup simulations for a 140 GHz, MW-class gyrotron developed by Communications and Power Industries (CPI) for electron-cyclotron resonance heating (ECRH) and current drive experiments on the "Wendelstein 7-X" stellarator plasma. Also presented are the results for a 110 GHz, 1.5 MW gyrotron currently being developed at CPI. The simulations are carried out for six competing modes and with the effects of electron velocity spread and voltage depression taken into account. Also, the slow stage of the startup in long-pulse gyrotrons is analyzed and attention is paid to the effects of ion compensation of the beam space charge, frequency deviation due to the cavity wall heating and beam current decrease due to cathode cooling. These effects are modeled with a simple nonlinear theory and the code MAGY.Item LASER SWITCHED ELECTRON BEAM MODULATION WITH TERAHERTZ APPLICATIONS(2005-04-19) Neumann, Jonathan; O'Shea, Patrick G; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes the exploration of relativistic electron beams modulated at terahertz frequencies using laser driven photoemission. It is divided into three distinct areas: laser beam modulation; electron beam dynamics; and an application of electron beam modulation, the generation of terahertz radiation. The laser modulation portion covers the development of an interferometer system used to control the 266 nm drive laser modulation and the experimental results. The laser pulse is delivered to the photocathode of the accelerator, and is used as a switch that induces an initial electron beam modulation at frequencies between 0.5 and 1.6 terahertz. The electron beam dynamics section includes measurements of the electron beam longitudinal distribution after acceleration to relativistic energy as well as the results obtained from a numerical simulation using the code PARMELA. Both the experimental and numerical results indicate that some of the initial density modulation imposed by the drive laser modulation is retained on the electron beam, although the density modulation that remains, and the frequency of the modulation, falls as a function of increasing charge. Electron beam modulation is achieved between 0.712 and 1.66 terahertz. One application of the deliberate modulation of an electron beam is the generation of coherent radiation, as is seen in many devices ranging from the klystron to the free electron laser. The third section of this work discusses terahertz light generated by transition radiation when a mirror intercepts the modulated electron beam. In this section, transition radiation measured by a bolometric detector is compared to expected results based on the longitudinal electron beam distributions predicted by the PARMELA simulation as well as the measurements from the accelerator system. This dissertation demonstrates that it is possible for an electron beam pre-modulated at the cathode on a subpicosecond time scale to be accelerated to relativistic energy and used for the production of tunable terahertz radiation.