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 Quasi-phasematched acceleration of electrons in a density modulated plasma waveguide(2014) Yoon, Sung Jun; Milchberg, Howard M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two quasi-phasematching schemes are proposed for efficient acceleration of electrons to relativistic energies using moderate intensity laser pulses. In the first scheme, Direct Laser Acceleration (DLA) in a corrugated plasma waveguide is proposed for acceleration of relativistic electrons with sub-terawatt laser systems, using the laser field directly as the accelerating field. The second scheme uses the fact that a plasma wakefield generated by an intense guided pulse in a corrugated plasma waveguide can accelerate relativistic electrons significantly beyond the well-known dephasing limit. In each case, particle-in-cell (PIC) simulations are used to validate the acceleration concept, demonstrating linear acceleration by either the phase matched laser field or phase-matched wakefield. In the phase matched wakefield case, theory and PIC simulations demonstrate a significant increase in energy gain compared to the standard laser wakefield acceleration (LWFA) scheme. Corrugated plasma waveguides can be generated by the interaction between an ionizing laser pulse and an atomic cluster flow interrupted by an array of thin wires,. When the collisional mean free path of the clusters is greater than the wire diameter, shadows of the periodically located wires are imparted on the cluster flow, leading to the production of axially modulated plasma waveguides after laser heating of the flow. This occurs when the population ratio of clusters to monomers in the gas is high. At other limit, dominated by gas monomer flow, shock waves generated off the wires by the supersonic gas flow disrupts modulated waveguide generation. Lastly, we experimentally demonstrate LWFA with ionization injection in a N5+ plasma waveguide. It is first shown that the plasma waveguide is almost completely composed of He-like nitrogen (N5+). It is then shown that intense pulse channeling in the plasma waveguide drives stronger wakefields, while the ionization injection process is critical to lowering the laser intensity threshold for self-trapping.Item Small-Scale Testing to Study Mitigation of Acceleration on Simulated Vehicles(2013) Bonsmann, Jarrod; Fourney, William L; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates various means for mitigating acceleration experienced by passengers on vehicles subjected to blast loading. In order to complete this study, small-scale testing of simulated vehicles was used. The explosives designated for this research are exclusively buried in saturated sand, which will act as the loading media for the simulated vehicles. In addition to explosive testing, various tests were performed dynamically using a high-pressure gas gun. Initially, tests were performed to better understand the effects of vehicle mass and stand-off distance on vehicle acceleration due to blast loads; after which, studies were conducted to mitigate the acceleration. Test plates used in this study vary in both size and geometry. When necessary, simple plate geometries are employed to investigate various mitigation parameters. Ultimately, much of the testing was conducted on simplified scaled versions of vehicles likely to be subjected to attack. This paper focuses mainly on mitigation through crushing of thin-walled cylinders, but also investigates the advantages of applying polymeric coatings to dynamically loaded structures. Piezoelectric accelerometers are used in conjunction with high speed videography to collect test data. In addition to acceleration, impulse and kinetic energy of each test plate is examined. This research, though funded by the US Army, will be of use to all branches of the armed forces utilizing Mine-Resistant Ambush-Protected vehicles. The ultimate goal of this research is to help create a vehicle that will increase the probability that the passengers will survive a blast event with minimal long-term damage to the brain.Item Mitigation of Frame Acceleration Induced by a Buried Charge(2010) Brodrick, Thomas James; Foruney, William; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, methods to mitigate acceleration delivered to the frame of a vehicle with an attached v-shaped hull are investigated. The frame of a vehicle represents an alternative location for crew seating, as opposed to seats being secured to the floorboard. Mitigation techniques were investigated for three test setups: aluminum frame with a downwardly convex aluminum hull, steel frame with a downwardly convex steel hull, and a steel frame with a downwardly concave steel hull. Accelerations of the frame were measured using piezoelectric accelerometers placed at three different locations on the frame. These acceleration measurements were verified against video recorded by high speed cameras. Each test was intended to reduce peak accelerations experienced by the frame, and to reduce the width of the acceleration envelope at large g levels. Mitigation techniques focused on reducing the initial hull-frame interactions, while damping subsequent responses of the system. Mitigation systems and hull orientation were compared for their ability to reduce blast effects experienced by the frame.Item EFFICIENT SIMULATION OF ELECTRON TRAPPING IN LASER AND PLASMA WAKEFIELD ACCELERATION(2009) Morshed, Sepehr; Antonsen, Thomas M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma based laser Wakefield accelerators (LWFA) have been a subject of interest in the plasma community for many years. In LWFA schemes the laser pulse must propagate several centimeters and maintain its coherence over this distance, which corresponds to many Rayleigh lengths. These Wakefields and their effect on the laser can be simulated in the quasistatic approximation. The 2D, cylindrically symmetric, quasistatic simulation code, WAKE is an efficient tool for the modeling of short-pulse laser propagation in under dense plasmas [P. Mora & T.M. Antonsen Phys. Plasmas 4, 1997]. The quasistatic approximation, which assumes that the driver and its wakefields are undisturbed during the transit time of plasma electrons, through the pulse, cannot, however, treat electron trapping and beam loading. Here we modify WAKE to include the effects of electron trapping and beam loading by introducing a population of beam electrons. Background plasma electrons that are beginning to start their oscillation around the radial axis and have energy above some threshold are removed from the background plasma and promoted to "beam" electrons. The population of beam electrons which are no longer subject to the quasistatic approximation, are treated without approximation and provide their own electromagnetic field that acts upon the background plasma. The algorithm is benchmarked to OSIRIS (a standard particle in cell code) simulations which makes no quasistatic approximation. We also have done simulation and comparison of results for centimeter scale GeV electron accelerator experiments from LBNL. These modifications to WAKE provide a tool for simulating GeV laser or plasma wakefield acceleration on desktop computers.Item Seismic Response of Acceleration-sensitive Nonstructural Components Mounted on Moment-resisting Frame Structures(2007-02-20) Sankaranarayanan, Ragunath; Medina, Ricardo A; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A statistical analysis of the peak acceleration demands for nonstructural components (NSCs) supported on elastic and inelastic regular moment-resisting frame structures is presented. The response of a variety of stiff and flexible frame structures (with 3, 6, 9, 12, 15, and 18 stories) subjected to a set of 40 far-field ground motions are evaluated. The NSCs under consideration are those that can be represented by single-degree-of-freedom systems with masses that are small as compared to the total mass of the supporting structure. The study evaluates and quantifies the dependence of peak component accelerations on the location of the nonstructural component in the structure, the damping ratio of the component, and the properties of the supporting structure such as its modal periods, height, stiffness distribution, and strength. The results show that current seismic code provisions will not always provide an adequate characterization of peak component accelerations especially when the period of the NSCs fall in the higher modal period region of the supporting structure and the provisions do not address the inelastic action of the supporting structure. A parameter called as acceleration response modification factor (Racc) is proposed to quantify the reduction in component amplification factors and inelastic FRS that is achieved due to the inelastic behavior of the building. A methodology that makes use of the Racc factor to estimate the acceleration demands on NSCs mounted on inelastic supporting structures from that of elastic buildings is outlined. Separate Racc factors are proposed for long-period, fundamental-period and short-period regions of the FRS at three different locations in the building namely roof, mid-height, and bottom-third location. A comparison of the proposed Racc factors to that of results obtained from real multi-bay buildings show that the recommendations fall within 20% error range for both fundamental-period and short-period regions of FRS.